diff --git a/softioc/__init__.py b/softioc/__init__.py
index 7c6f6360..b9c186b0 100644
--- a/softioc/__init__.py
+++ b/softioc/__init__.py
@@ -5,6 +5,11 @@
 from epicscorelibs.ioc import \
     iocshRegisterCommon, registerRecordDeviceDriver, pdbbase
 
+# Do this as early as possible, in case we happen to use cothread
+# This will set the CATOOLS_LIBCA_PATH environment variable in case we use
+# cothread.catools. It works even if we don't have cothread installed
+import epicscorelibs.path.cothread  # noqa
+
 # This import will also pull in the extension, which is needed
 # before we call iocshRegisterCommon
 from .imports import dbLoadDatabase
diff --git a/softioc/asyncio_dispatcher.py b/softioc/asyncio_dispatcher.py
index c85ceefb..aa10fe0e 100644
--- a/softioc/asyncio_dispatcher.py
+++ b/softioc/asyncio_dispatcher.py
@@ -3,23 +3,18 @@
 import threading
 
 
-class AsyncioDispatcher(threading.Thread):
-    """A dispatcher for `asyncio` based IOCs. Means that `on_update` callback
-    functions can be async. Will run an Event Loop in a thread when
-    created.
-    """
-    def __init__(self):
-        """Create an AsyncioDispatcher suitable to be used by
-        `softioc.iocInit`."""
-        # Docstring specified to suppress threading.Thread's docstring, which
-        # would otherwise be inherited by this method and be misleading.
-        super().__init__()
+class AsyncioDispatcher:
+    def __init__(self, loop=None):
+        """A dispatcher for `asyncio` based IOCs, suitable to be passed to
+        `softioc.iocInit`. Means that `on_update` callback functions can be
+        async. If loop is None, will run an Event Loop in a thread when created.
+        """
         #: `asyncio` event loop that the callbacks will run under.
-        self.loop = asyncio.new_event_loop()
-        self.start()
-
-    def run(self):
-        self.loop.run_forever()
+        self.loop = loop
+        if loop is None:
+            # Make one and run it in a background thread
+            self.loop = asyncio.new_event_loop()
+            threading.Thread(target=self.loop.run_forever).start()
 
     def __call__(self, func, *args):
         async def async_wrapper():
diff --git a/softioc/builder.py b/softioc/builder.py
index 0fa1447d..964d651e 100644
--- a/softioc/builder.py
+++ b/softioc/builder.py
@@ -11,7 +11,6 @@
 PythonDevice = pythonSoftIoc.PythonDevice()
 
 
-
 # ----------------------------------------------------------------------------
 #  Wrappers for PythonDevice record constructors.
 #
diff --git a/softioc/softioc.py b/softioc/softioc.py
index 27d08f2b..843a4a3a 100644
--- a/softioc/softioc.py
+++ b/softioc/softioc.py
@@ -1,6 +1,7 @@
 import os
 import sys
 from ctypes import *
+from tempfile import NamedTemporaryFile
 
 from epicsdbbuilder.recordset import recordset
 
@@ -245,33 +246,43 @@ def dbLoadDatabase(database, path = None, substitutions = None):
     '''Loads a database file and applies any given substitutions.'''
     imports.dbLoadDatabase(database, path, substitutions)
 
-def _add_records_from_file(dir, file, macros):
-    # This is very naive, for instance macros are added to but never removed,
-    # but it works well enough for devIocStats
-    with open(os.path.join(dir, file)) as f:
+
+def _add_records_from_file(dirname, file, substitutions):
+    # This is very naive, it loads all includes before their parents which
+    # possibly can put them out of order, but it works well enough for
+    # devIocStats
+    with open(os.path.join(dirname, file)) as f:
+        lines, include_subs = [], ""
         for line in f.readlines():
             line = line.rstrip()
             if line.startswith('substitute'):
-                # substitute "QUEUE=scanOnce, QUEUE_CAPS=SCANONCE
-                for sub in line.split('"')[1].split(','):
-                    k, v = sub.split('=')
-                    macros[k.strip()] = v.strip()
+                # substitute "QUEUE=scanOnce, QUEUE_CAPS=SCANONCE"
+                # keep hold of the substitutions
+                include_subs = line.split('"')[1]
             elif line.startswith('include'):
                 # include "iocQueue.db"
-                _add_records_from_file(dir, line.split('"')[1], macros)
+                subs = substitutions
+                if substitutions and include_subs:
+                    subs = substitutions + ", " + include_subs
+                else:
+                    subs = substitutions + include_subs
+                _add_records_from_file(dirname, line.split('"')[1], subs)
             else:
                 # A record line
-                for k, v in macros.items():
-                    line = line.replace('$(%s)' % k, v)
-                recordset.AddBodyLine(line)
+                lines.append(line)
+        # Write a tempfile and load it
+        with NamedTemporaryFile(suffix='.db', delete=False) as f:
+            f.write(os.linesep.join(lines).encode())
+        dbLoadDatabase(f.name, substitutions=substitutions)
+        os.unlink(f.name)
 
 
 def devIocStats(ioc_name):
     '''This will load a template for the devIocStats library with the specified
     IOC name. This should be called before `iocInit`'''
-    macros = dict(IOCNAME=ioc_name, TODFORMAT='%m/%d/%Y %H:%M:%S')
+    substitutions = 'IOCNAME=' + ioc_name + ', TODFORMAT=%m/%d/%Y %H:%M:%S'
     iocstats_dir = os.path.join(os.path.dirname(__file__), 'iocStatsDb')
-    _add_records_from_file(iocstats_dir, 'ioc.template', macros)
+    _add_records_from_file(iocstats_dir, 'ioc.template', substitutions)
 
 
 def interactive_ioc(context = {}, call_exit = True):
diff --git a/tests/conftest.py b/tests/conftest.py
index 382e0951..11d5afff 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -1,5 +1,67 @@
+import atexit
+import os
+import random
+import string
+import subprocess
 import sys
 
+import pytest
+
 if sys.version_info < (3,):
     # Python2 has no asyncio, so ignore these tests
-    collect_ignore = ["test_asyncio.py", "sim_asyncio_ioc.py"]
+    collect_ignore = [
+        "test_asyncio.py", "sim_asyncio_ioc.py", "sim_asyncio_ioc_override.py"
+    ]
+
+class SubprocessIOC:
+    def __init__(self, ioc_py):
+        self.pv_prefix = "".join(
+            random.choice(string.ascii_uppercase) for _ in range(12)
+        )
+        sim_ioc = os.path.join(os.path.dirname(__file__), ioc_py)
+        cmd = [sys.executable, sim_ioc, self.pv_prefix]
+        self.proc = subprocess.Popen(
+            cmd, stdin=subprocess.PIPE,
+            stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+
+    def kill(self):
+        if self.proc.returncode is None:
+            # still running, kill it and print the output
+            self.proc.kill()
+            out, err = self.proc.communicate()
+            print(out.decode())
+            print(err.decode())
+
+
+@pytest.fixture
+def cothread_ioc():
+    ioc = SubprocessIOC("sim_cothread_ioc.py")
+    yield ioc
+    ioc.kill()
+
+
+def aioca_cleanup():
+    from aioca import purge_channel_caches, _catools
+    # Unregister the aioca atexit handler as it conflicts with the one installed
+    # by cothread. If we don't do this we get a seg fault. This is not a problem
+    # in production as we won't mix aioca and cothread, but we do mix them in
+    # the tests so need to do this.
+    atexit.unregister(_catools._catools_atexit)
+    # purge the channels before the event loop goes
+    purge_channel_caches()
+
+
+@pytest.fixture
+def asyncio_ioc():
+    ioc = SubprocessIOC("sim_asyncio_ioc.py")
+    yield ioc
+    ioc.kill()
+    aioca_cleanup()
+
+
+@pytest.fixture
+def asyncio_ioc_override():
+    ioc = SubprocessIOC("sim_asyncio_ioc_override.py")
+    yield ioc
+    ioc.kill()
+    aioca_cleanup()
diff --git a/tests/expected_records.db b/tests/expected_records.db
index 977e2407..290c5c82 100644
--- a/tests/expected_records.db
+++ b/tests/expected_records.db
@@ -1,568 +1,7 @@
-# This file was automatically generated on Thu 29 Apr 2021 14:11:13 BST.
-#
+# This file was automatically generated on Fri 13 Aug 2021 16:49:03 BST.
+# 
 # *** Please do not edit this file: edit the source file instead. ***
-#
-
-# Used by Channel Access Security to determine access to this IOC.
-record(mbbo, "TS-DI-TEST-01:ACCESS")
-{
-  field(DESC, "TS-DI-TEST-01 Acc Mode")
-  field(PINI, "YES")
-  field(ZRST, "Running")
-  field(ZRSV, "NO_ALARM")
-  field(ONST, "Maintenance")
-  field(ONSV, "MINOR")
-  field(TWST, "Test")
-  field(TWSV, "MINOR")
-  field(THST, "OFFLINE")
-  field(THSV, "MAJOR")
-  info(autosaveFields, "VAL")
-}
-record(stringin, "TS-DI-TEST-01:STARTTOD")
-{
-    field(DESC, "Time and date of startup")
-    field(DTYP, "Soft Timestamp")
-    field(PINI, "YES")
-    field(INP, "@%m/%d/%Y %H:%M:%S")
-}
-record(stringin, "TS-DI-TEST-01:TOD")
-{
-    field(DESC, "Current time and date")
-    field(DTYP, "Soft Timestamp")
-    field(SCAN, "1 second")
-    field(INP, "@%m/%d/%Y %H:%M:%S")
-}
-record(calcout, "TS-DI-TEST-01:HEARTBEAT")
-{
-    field(DESC, "1 Hz counter since startup")
-    field(CALC, "(A<2147483647)?A+1:1")
-    field(SCAN, "1 second")
-    field(INPA, "TS-DI-TEST-01:HEARTBEAT")
-}
-# if autosave is working, START_CNT creates a running count of
-# number of times the IOC was started.
-record(calcout, "TS-DI-TEST-01:START_CNT")
-{
-    field(DESC, "Increments at startup")
-    field(CALC, "A+1")
-    field(PINI, "YES")
-    field(INPA, "TS-DI-TEST-01:START_CNT")
-    info(autosaveFields_pass0, "VAL")
-}
-#
-# Using an existing internal set of subroutines, this
-# PV updates the Access Security mechanism dynamically.
-# The .acf file is re-read.
-#
-record( sub, "TS-DI-TEST-01:READACF")
-{
-    field( DESC, "TS-DI-TEST-01 ACF Update")
-    field( INAM, "asSubInit")
-    field( SNAM, "asSubProcess")
-    field( BRSV, "INVALID")
-}
-record(sub, "TS-DI-TEST-01:SYSRESET")
-{
-    alias("TS-DI-TEST-01:SysReset")
-    field(DESC, "IOC Restart" )
-    field(SNAM, "rebootProc")
-    field(BRSV,"INVALID")
-    field(L,"1")
-}
-
-record(ai, "TS-DI-TEST-01:CA_CLNT_CNT") {
-  field(DESC, "Number of CA Clients")
-  field(SCAN, "I/O Intr")
-  field(DTYP, "IOC stats")
-  field(INP, "@ca_clients")
-  field(HOPR, "200")
-  field(HIHI, "175")
-  field(HIGH, "100")
-  field(HHSV, "MAJOR")
-  field(HSV, "MINOR")
-  info(autosaveFields_pass0, "HOPR LOPR HIHI HIGH LOW LOLO HHSV HSV LSV LLSV")
-}
-
-record(ai, "TS-DI-TEST-01:CA_CONN_CNT") {
-  field(DESC, "Number of CA Connections")
-  field(SCAN, "I/O Intr")
-  field(DTYP, "IOC stats")
-  field(INP, "@ca_connections")
-  field(HOPR, "5000")
-  field(HIHI, "4500")
-  field(HIGH, "4000")
-  field(HHSV, "MAJOR")
-  field(HSV, "MINOR")
-  info(autosaveFields_pass0, "HOPR LOPR HIHI HIGH LOW LOLO HHSV HSV LSV LLSV")
-}
-
-record(ai, "TS-DI-TEST-01:RECORD_CNT") {
-  field(DESC, "Number of Records")
-  field(PINI, "YES")
-  field(DTYP, "IOC stats")
-  field(INP, "@records")
-}
-
-record(ai, "TS-DI-TEST-01:FD_MAX") {
-  field(DESC, "Max File Descriptors")
-  field(PINI, "YES")
-  field(DTYP, "IOC stats")
-  field(INP, "@maxfd")
-}
-
-record(ai, "TS-DI-TEST-01:FD_CNT") {
-  field(DESC, "Allocated File Descriptors")
-  field(SCAN, "I/O Intr")
-  field(DTYP, "IOC stats")
-  field(FLNK, "TS-DI-TEST-01:FD_FREE  PP MS")
-  field(INP, "@fd")
-}
-
-record(calc, "TS-DI-TEST-01:FD_FREE") {
-  field(DESC, "Available FDs")
-  field(CALC, "B>0?B-A:C")
-  field(INPA, "TS-DI-TEST-01:FD_CNT  NPP MS")
-  field(INPB, "TS-DI-TEST-01:FD_MAX  NPP MS")
-  field(INPC, "1000")
-  field(HOPR, "150")
-  field(LOLO, "5")
-  field(LOW, "20")
-  field(LLSV, "MAJOR")
-  field(LSV, "MINOR")
-  info(autosaveFields_pass0, "HOPR LOPR LOW LOLO LSV LLSV")
-}
-
-record(ai, "TS-DI-TEST-01:SYS_CPU_LOAD") {
-  field(DESC, "System CPU Load")
-  field(SCAN, "I/O Intr")
-  field(DTYP, "IOC stats")
-  field(INP, "@sys_cpuload")
-  field(EGU, "%")
-  field(PREC, "1")
-  field(HOPR, "100")
-  field(HIHI, "80")
-  field(HIGH, "70")
-  field(HHSV, "MAJOR")
-  field(HSV, "MINOR")
-  info(autosaveFields_pass0, "HOPR LOPR HIHI HIGH LOW LOLO HHSV HSV LSV LLSV")
-}
-
-record(ai, "TS-DI-TEST-01:IOC_CPU_LOAD") {
-  alias("TS-DI-TEST-01:LOAD")
-  field(DESC, "IOC CPU Load")
-  field(SCAN, "I/O Intr")
-  field(DTYP, "IOC stats")
-  field(INP, "@ioc_cpuload")
-  field(EGU, "%")
-  field(PREC, "1")
-  field(HOPR, "100")
-  field(HIHI, "80")
-  field(HIGH, "70")
-  field(HHSV, "MAJOR")
-  field(HSV, "MINOR")
-  info(autosaveFields_pass0, "HOPR LOPR HIHI HIGH LOW LOLO HHSV HSV LSV LLSV")
-}
-
-record(ai, "TS-DI-TEST-01:CPU_CNT") {
-  field(DESC, "Number of CPUs")
-  field(DTYP, "IOC stats")
-  field(INP, "@no_of_cpus")
-  field(PINI, "YES")
-}
-
-record(ai, "TS-DI-TEST-01:SUSP_TASK_CNT") {
-  field(DESC, "Number Suspended Tasks")
-  field(SCAN, "I/O Intr")
-  field(DTYP, "IOC stats")
-  field(INP, "@suspended_tasks")
-  field(HIHI, "1")
-  field(HHSV, "MAJOR")
-  info(autosaveFields_pass0, "HOPR LOPR HIHI HIGH LOW LOLO HHSV HSV LSV LLSV")
-}
-
-record(ai, "TS-DI-TEST-01:MEM_USED") {
-  field(DESC, "Allocated Memory")
-  field(SCAN, "I/O Intr")
-  field(DTYP, "IOC stats")
-  field(INP, "@allocated_bytes")
-  field(EGU, "byte")
-}
-
-record(ai, "TS-DI-TEST-01:MEM_FREE") {
-  field(DESC, "Free Memory")
-  field(SCAN, "I/O Intr")
-  field(DTYP, "IOC stats")
-  field(INP, "@free_bytes")
-  field(EGU, "byte")
-  field(LLSV, "MAJOR")
-  field(LSV, "MINOR")
-  info(autosaveFields_pass0, "HOPR LOPR LOW LOLO LSV LLSV")
-}
-
-record(ai, "TS-DI-TEST-01:MEM_MAX") {
-  field(DESC, "Maximum Memory")
-  field(SCAN, "I/O Intr")
-  field(DTYP, "IOC stats")
-  field(INP, "@total_bytes")
-  field(EGU, "byte")
-}
-
-record(ao, "TS-DI-TEST-01:CA_UPD_TIME") {
-  field(DESC, "CA Check Update Period")
-  field(DTYP, "IOC stats")
-  field(OUT, "@ca_scan_rate")
-  field(EGU, "sec")
-  field(DRVH, "60")
-  field(DRVL, "1")
-  field(HOPR, "60")
-  field(VAL, "15")
-  field(PINI, "YES")
-}
-
-record(ao, "TS-DI-TEST-01:FD_UPD_TIME") {
-  field(DESC, "FD Check Update Period")
-  field(DTYP, "IOC stats")
-  field(OUT, "@fd_scan_rate")
-  field(EGU, "sec")
-  field(DRVH, "60")
-  field(DRVL, "1")
-  field(HOPR, "60")
-  field(VAL, "20")
-  field(PINI, "YES")
-}
-
-record(ao, "TS-DI-TEST-01:LOAD_UPD_TIME") {
-  field(DESC, "CPU Check Update Period")
-  field(DTYP, "IOC stats")
-  field(OUT, "@cpu_scan_rate")
-  field(EGU, "sec")
-  field(DRVH, "60")
-  field(DRVL, "1")
-  field(HOPR, "60")
-  field(VAL, "10")
-  field(PINI, "YES")
-}
-
-record(ao, "TS-DI-TEST-01:MEM_UPD_TIME") {
-  field(DESC, "Memory Check Update Period")
-  field(DTYP, "IOC stats")
-  field(OUT, "@memory_scan_rate")
-  field(EGU, "sec")
-  field(DRVH, "60")
-  field(DRVL, "1")
-  field(HOPR, "60")
-  field(VAL, "10")
-  field(PINI, "YES")
-}
-
-record(stringin, "TS-DI-TEST-01:ST_SCRIPT1") {
-  field(DESC, "Startup Script Part1")
-  field(DTYP, "IOC stats")
-  field(INP, "@startup_script_1")
-  field(PINI, "YES")
-}
-
-record(stringin, "TS-DI-TEST-01:ST_SCRIPT2") {
-  field(DESC, "Startup Script Part2")
-  field(DTYP, "IOC stats")
-  field(INP, "@startup_script_2")
-  field(PINI, "YES")
-}
-
-record(waveform, "TS-DI-TEST-01:ST_SCRIPT") {
-  field(DESC, "Startup Script")
-  field(DTYP, "IOC stats")
-  field(INP, "@startup_script")
-  field(NELM, "120")
-  field(FTVL, "CHAR")
-  field(PINI, "YES")
-}
-
-record(stringin, "TS-DI-TEST-01:KERNEL_VERS") {
-  field(DESC, "Kernel Version")
-  field(DTYP, "IOC stats")
-  field(INP, "@kernel_ver")
-  field(PINI, "YES")
-}
-
-record(stringin, "TS-DI-TEST-01:EPICS_VERS") {
-  field(DESC, "EPICS Version")
-  field(DTYP, "IOC stats")
-  field(INP, "@epics_ver")
-  field(PINI, "YES")
-}
-
-record(waveform, "TS-DI-TEST-01:EPICS_VERSION") {
-  field(DESC, "EPICS Version")
-  field(DTYP, "IOC stats")
-  field(INP, "@epics_ver")
-  field(NELM, "120")
-  field(FTVL, "CHAR")
-  field(PINI, "YES")
-}
-
-record(stringin, "TS-DI-TEST-01:HOSTNAME") {
-  field(DESC, "Host Name")
-  field(DTYP, "IOC stats")
-  field(INP, "@hostname")
-  field(PINI, "YES")
-}
-
-record(stringin, "TS-DI-TEST-01:APP_DIR1") {
-  field(DESC, "Application Directory Part 1")
-  field(DTYP, "IOC stats")
-  field(INP, "@pwd1")
-  field(PINI, "YES")
-}
-
-record(stringin, "TS-DI-TEST-01:APP_DIR2") {
-  field(DESC, "Application Directory Part 2")
-  field(DTYP, "IOC stats")
-  field(INP, "@pwd2")
-  field(PINI, "YES")
-}
-
-record(waveform, "TS-DI-TEST-01:APP_DIR") {
-  field(DESC, "Application Directory")
-  field(DTYP, "IOC stats")
-  field(INP, "@pwd")
-  field(NELM, "160")
-  field(FTVL, "CHAR")
-  field(PINI, "YES")
-}
-
-record(stringin, "TS-DI-TEST-01:UPTIME") {
-  field(DESC, "Elapsed Time since Start")
-  field(SCAN, "1 second")
-  field(DTYP, "IOC stats")
-  field(INP, "@up_time")
-  field(PINI, "YES")
-}
-
-record(stringin, "TS-DI-TEST-01:ENGINEER") {
-  field(DESC, "Engineer")
-  field(DTYP, "IOC stats")
-  field(INP, "@engineer")
-  field(PINI, "YES")
-}
-
-record(stringin, "TS-DI-TEST-01:LOCATION") {
-  field(DESC, "Location")
-  field(DTYP, "IOC stats")
-  field(INP, "@location")
-  field(PINI, "YES")
-}
-
-record(ai, "TS-DI-TEST-01:PROCESS_ID") {
-  field(DESC, "Process ID")
-  field(PINI, "YES")
-  field(DTYP, "IOC stats")
-  field(INP, "@proc_id")
-}
-
-record(ai, "TS-DI-TEST-01:PARENT_ID") {
-  field(DESC, "Parent Process ID")
-  field(PINI, "YES")
-  field(DTYP, "IOC stats")
-  field(INP, "@parent_proc_id")
-}
-
-record(ai, "TS-DI-TEST-01:SCANONCE_Q_SIZE") {
-  field(DESC, "max # entries in IOC scanOnce queue")
-  field(DTYP, "IOC stats")
-  field(INP,  "@scanOnceQueueSize")
-  field(PINI, "YES")
-}
-record(ai, "TS-DI-TEST-01:CB_Q_SIZE") {
-  field(DESC, "max # entries in IOC callback queues")
-  field(DTYP, "IOC stats")
-  field(INP,  "@cbQueueSize")
-  field(PINI, "YES")
-}
-
-record(ai, "TS-DI-TEST-01:SCANONCE_Q_HIGH") {
-  field(DESC, "max # of elmts in IOC's scanOnce queue")
-  field(SCAN, "I/O Intr")
-  field(DTYP, "IOC stats")
-  field(INP,  "@scanOnceQueueHiWtrMrk")
-  field(PINI, "YES")
-  field(FLNK, "TS-DI-TEST-01:SCANONCE_Q_HIGHPER")
-}
-
-record(calc, "TS-DI-TEST-01:SCANONCE_Q_HIGHPER") {
-  field(DESC, "Max. usage of IOC's scanOnce queue")
-  field(INPA, "TS-DI-TEST-01:SCANONCE_Q_HIGH NPP MS")
-  field(INPB, "TS-DI-TEST-01:SCANONCE_Q_SIZE NPP MS")
-  field(CALC, "100*A/B")
-  field(LOPR, "0")
-  field(HOPR, "100")
-  field(EGU,  "%")
-}
-
-record(ai, "TS-DI-TEST-01:SCANONCE_Q_USED") {
-  field(DESC, "# of entries in IOC's scanOnce queue")
-  field(SCAN, "I/O Intr")
-  field(DTYP, "IOC stats")
-  field(INP,  "@scanOnceQueueUsed")
-  field(PINI, "YES")
-  field(FLNK, "TS-DI-TEST-01:SCANONCE_Q_USEDPER")
-}
-
-record(calc, "TS-DI-TEST-01:SCANONCE_Q_USEDPER") {
-  field(DESC, "Percentage of IOC's scanOnce queue used")
-  field(INPA, "TS-DI-TEST-01:SCANONCE_Q_USED NPP MS")
-  field(INPB, "TS-DI-TEST-01:SCANONCE_Q_SIZE NPP MS")
-  field(CALC, "100*A/B")
-  field(LOPR, "0")
-  field(HOPR, "100")
-  field(EGU,  "%")
-}
-
-record(ai, "TS-DI-TEST-01:SCANONCE_Q_OVERRUNS") {
-  field(DESC, "# of overruns of IOC's scanOnce queue")
-  field(SCAN, "I/O Intr")
-  field(DTYP, "IOC stats")
-  field(INP,  "@scanOnceQueueOverruns")
-  field(PINI, "YES")
-}
-
-record(ai, "TS-DI-TEST-01:CBLOW_Q_HIGH") {
-  field(DESC, "max # of elmts in IOC's cbLow queue")
-  field(SCAN, "I/O Intr")
-  field(DTYP, "IOC stats")
-  field(INP,  "@cbLowQueueHiWtrMrk")
-  field(PINI, "YES")
-  field(FLNK, "TS-DI-TEST-01:CBLOW_Q_HIGHPER")
-}
-
-record(calc, "TS-DI-TEST-01:CBLOW_Q_HIGHPER") {
-  field(DESC, "Max. usage of IOC's cbLow queue")
-  field(INPA, "TS-DI-TEST-01:CBLOW_Q_HIGH NPP MS")
-  field(INPB, "TS-DI-TEST-01:CB_Q_SIZE NPP MS")
-  field(CALC, "100*A/B")
-  field(LOPR, "0")
-  field(HOPR, "100")
-  field(EGU,  "%")
-}
-
-record(ai, "TS-DI-TEST-01:CBLOW_Q_USED") {
-  field(DESC, "# of entries in IOC's cbLow queue")
-  field(SCAN, "I/O Intr")
-  field(DTYP, "IOC stats")
-  field(INP,  "@cbLowQueueUsed")
-  field(PINI, "YES")
-  field(FLNK, "TS-DI-TEST-01:CBLOW_Q_USEDPER")
-}
-
-record(calc, "TS-DI-TEST-01:CBLOW_Q_USEDPER") {
-  field(DESC, "Percentage of IOC's cbLow queue used")
-  field(INPA, "TS-DI-TEST-01:CBLOW_Q_USED NPP MS")
-  field(INPB, "TS-DI-TEST-01:CB_Q_SIZE NPP MS")
-  field(CALC, "100*A/B")
-  field(LOPR, "0")
-  field(HOPR, "100")
-  field(EGU,  "%")
-}
-
-record(ai, "TS-DI-TEST-01:CBLOW_Q_OVERRUNS") {
-  field(DESC, "# of overruns of IOC's cbLow queue")
-  field(SCAN, "I/O Intr")
-  field(DTYP, "IOC stats")
-  field(INP,  "@cbLowQueueOverruns")
-  field(PINI, "YES")
-}
-
-record(ai, "TS-DI-TEST-01:CBMEDIUM_Q_HIGH") {
-  field(DESC, "max # of elmts in IOC's cbMedium queue")
-  field(SCAN, "I/O Intr")
-  field(DTYP, "IOC stats")
-  field(INP,  "@cbMediumQueueHiWtrMrk")
-  field(PINI, "YES")
-  field(FLNK, "TS-DI-TEST-01:CBMEDIUM_Q_HIGHPER")
-}
-
-record(calc, "TS-DI-TEST-01:CBMEDIUM_Q_HIGHPER") {
-  field(DESC, "Max. usage of IOC's cbMedium queue")
-  field(INPA, "TS-DI-TEST-01:CBMEDIUM_Q_HIGH NPP MS")
-  field(INPB, "TS-DI-TEST-01:CB_Q_SIZE NPP MS")
-  field(CALC, "100*A/B")
-  field(LOPR, "0")
-  field(HOPR, "100")
-  field(EGU,  "%")
-}
-
-record(ai, "TS-DI-TEST-01:CBMEDIUM_Q_USED") {
-  field(DESC, "# of entries in IOC's cbMedium queue")
-  field(SCAN, "I/O Intr")
-  field(DTYP, "IOC stats")
-  field(INP,  "@cbMediumQueueUsed")
-  field(PINI, "YES")
-  field(FLNK, "TS-DI-TEST-01:CBMEDIUM_Q_USEDPER")
-}
-
-record(calc, "TS-DI-TEST-01:CBMEDIUM_Q_USEDPER") {
-  field(DESC, "Percentage of IOC's cbMedium queue used")
-  field(INPA, "TS-DI-TEST-01:CBMEDIUM_Q_USED NPP MS")
-  field(INPB, "TS-DI-TEST-01:CB_Q_SIZE NPP MS")
-  field(CALC, "100*A/B")
-  field(LOPR, "0")
-  field(HOPR, "100")
-  field(EGU,  "%")
-}
-
-record(ai, "TS-DI-TEST-01:CBMEDIUM_Q_OVERRUNS") {
-  field(DESC, "# of overruns of IOC's cbMedium queue")
-  field(SCAN, "I/O Intr")
-  field(DTYP, "IOC stats")
-  field(INP,  "@cbMediumQueueOverruns")
-  field(PINI, "YES")
-}
-
-record(ai, "TS-DI-TEST-01:CBHIGH_Q_HIGH") {
-  field(DESC, "max # of elmts in IOC's cbHigh queue")
-  field(SCAN, "I/O Intr")
-  field(DTYP, "IOC stats")
-  field(INP,  "@cbHighQueueHiWtrMrk")
-  field(PINI, "YES")
-  field(FLNK, "TS-DI-TEST-01:CBHIGH_Q_HIGHPER")
-}
-
-record(calc, "TS-DI-TEST-01:CBHIGH_Q_HIGHPER") {
-  field(DESC, "Max. usage of IOC's cbHigh queue")
-  field(INPA, "TS-DI-TEST-01:CBHIGH_Q_HIGH NPP MS")
-  field(INPB, "TS-DI-TEST-01:CB_Q_SIZE NPP MS")
-  field(CALC, "100*A/B")
-  field(LOPR, "0")
-  field(HOPR, "100")
-  field(EGU,  "%")
-}
-
-record(ai, "TS-DI-TEST-01:CBHIGH_Q_USED") {
-  field(DESC, "# of entries in IOC's cbHigh queue")
-  field(SCAN, "I/O Intr")
-  field(DTYP, "IOC stats")
-  field(INP,  "@cbHighQueueUsed")
-  field(PINI, "YES")
-  field(FLNK, "TS-DI-TEST-01:CBHIGH_Q_USEDPER")
-}
-
-record(calc, "TS-DI-TEST-01:CBHIGH_Q_USEDPER") {
-  field(DESC, "Percentage of IOC's cbHigh queue used")
-  field(INPA, "TS-DI-TEST-01:CBHIGH_Q_USED NPP MS")
-  field(INPB, "TS-DI-TEST-01:CB_Q_SIZE NPP MS")
-  field(CALC, "100*A/B")
-  field(LOPR, "0")
-  field(HOPR, "100")
-  field(EGU,  "%")
-}
-
-record(ai, "TS-DI-TEST-01:CBHIGH_Q_OVERRUNS") {
-  field(DESC, "# of overruns of IOC's cbHigh queue")
-  field(SCAN, "I/O Intr")
-  field(DTYP, "IOC stats")
-  field(INP,  "@cbHighQueueOverruns")
-  field(PINI, "YES")
-}
+# 
 
 record(ai, "TS-DI-TEST-01:AI")
 {
diff --git a/tests/hw_records.db b/tests/hw_records.db
new file mode 100644
index 00000000..c4287ec1
--- /dev/null
+++ b/tests/hw_records.db
@@ -0,0 +1,8 @@
+record(bo, "$(device):GAIN") {
+  field(DTYP, "Hy8001")
+  field(OMSL, "supervisory")
+  field(OUT, "#C1 S0 @")
+  field(DESC, "Gain bit 1")
+  field(ZNAM, "Off")
+  field(ONAM, "On")
+}
diff --git a/tests/sim_asyncio_ioc_override.py b/tests/sim_asyncio_ioc_override.py
new file mode 100644
index 00000000..99aab3c9
--- /dev/null
+++ b/tests/sim_asyncio_ioc_override.py
@@ -0,0 +1,37 @@
+from argparse import ArgumentParser
+
+import asyncio
+import os
+import re
+from pathlib import Path
+from tempfile import NamedTemporaryFile
+
+from softioc import softioc, builder, asyncio_dispatcher
+
+
+if __name__ == "__main__":
+    # Being run as an IOC, so parse args and set prefix
+    parser = ArgumentParser()
+    parser.add_argument('prefix', help="The PV prefix for the records")
+    parsed_args = parser.parse_args()
+    builder.SetDeviceName(parsed_args.prefix)
+
+    # Load the base records without DTYP fields
+    with open(Path(__file__).parent / "hw_records.db", "rb") as inp:
+        with NamedTemporaryFile(suffix='.db', delete=False) as out:
+            for line in inp.readlines():
+                if not re.match(rb"\s*field\s*\(\s*DTYP", line):
+                    out.write(line)
+    softioc.dbLoadDatabase(
+        out.name, substitutions=f"device={parsed_args.prefix}")
+    os.unlink(out.name)
+
+    # Override DTYPE and OUT, and provide a callback
+    gain = builder.boolOut("GAIN", on_update=print)
+    softioc.devIocStats(parsed_args.prefix)
+
+    # Run the IOC
+    builder.LoadDatabase()
+    event_loop = asyncio.get_event_loop()
+    softioc.iocInit(asyncio_dispatcher.AsyncioDispatcher(event_loop))
+    event_loop.run_forever()
diff --git a/tests/test_asyncio.py b/tests/test_asyncio.py
index d689201e..82c89c20 100644
--- a/tests/test_asyncio.py
+++ b/tests/test_asyncio.py
@@ -1,88 +1,83 @@
 # Will be ignored on Python2 by conftest.py settings
 
-import random
-import string
-import subprocess
-import sys
-import os
-import atexit
 import pytest
-import time
-
-PV_PREFIX = "".join(random.choice(string.ascii_uppercase) for _ in range(12))
-
-
-@pytest.fixture
-def asyncio_ioc():
-    sim_ioc = os.path.join(os.path.dirname(__file__), "sim_asyncio_ioc.py")
-    cmd = [sys.executable, sim_ioc, PV_PREFIX]
-    proc = subprocess.Popen(
-        cmd, stdin=subprocess.PIPE,
-        stdout=subprocess.PIPE, stderr=subprocess.PIPE)
-    yield proc
-    # purge the channels before the event loop goes
-    from aioca import purge_channel_caches
-    purge_channel_caches()
-    if proc.returncode is None:
-        # still running, kill it and print the output
-        proc.kill()
-        out, err = proc.communicate()
-        print(out.decode())
-        print(err.decode(), file=sys.stderr)
+import sys
 
 
 @pytest.mark.asyncio
 async def test_asyncio_ioc(asyncio_ioc):
     import asyncio
-    from aioca import caget, caput, camonitor, CANothing, _catools, FORMAT_TIME
-    # Unregister the aioca atexit handler as it conflicts with the one installed
-    # by cothread. If we don't do this we get a seg fault. This is not a problem
-    # in production as we won't mix aioca and cothread, but we do mix them in
-    # the tests so need to do this.
-    atexit.unregister(_catools._catools_atexit)
+    from aioca import caget, caput, camonitor, CANothing, FORMAT_TIME
 
     # Start
-    assert (await caget(PV_PREFIX + ":UPTIME")).startswith("00:00:0")
+    pre = asyncio_ioc.pv_prefix
+    assert (await caget(pre + ":UPTIME")).startswith("00:00:0")
     # WAVEFORM
-    await caput(PV_PREFIX + ":SINN", 4, wait=True)
+    await caput(pre + ":SINN", 4, wait=True)
     q = asyncio.Queue()
-    m = camonitor(PV_PREFIX + ":SIN", q.put, notify_disconnect=True)
+    m = camonitor(pre + ":SIN", q.put, notify_disconnect=True)
     assert len(await asyncio.wait_for(q.get(), 1)) == 4
     # AO
-    ao_val = await caget(PV_PREFIX + ":ALARM", format=FORMAT_TIME)
+    ao_val = await caget(pre + ":ALARM", format=FORMAT_TIME)
     assert ao_val == 0
     assert ao_val.severity == 3  # INVALID
     assert ao_val.status == 17  # UDF
-    await caput(PV_PREFIX + ":ALARM", 3, wait=True)
+    await caput(pre + ":ALARM", 3, wait=True)
     await asyncio.sleep(0.1)
-    ai_val = await caget(PV_PREFIX + ":AI", format=FORMAT_TIME)
+    ai_val = await caget(pre + ":AI", format=FORMAT_TIME)
     assert ai_val == 23.45
     assert ai_val.severity == 0
     assert ai_val.status == 0
     await asyncio.sleep(0.8)
-    ai_val = await caget(PV_PREFIX + ":AI", format=FORMAT_TIME)
+    ai_val = await caget(pre + ":AI", format=FORMAT_TIME)
     assert ai_val == 23.45
     assert ai_val.severity == 3
     assert ai_val.status == 7  # STATE_ALARM
     # Check pvaccess works
     from p4p.client.asyncio import Context
     with Context("pva") as ctx:
-        assert await ctx.get(PV_PREFIX + ":AI") == 23.45
+        assert await ctx.get(pre + ":AI") == 23.45
     # Wait for a bit longer for the print output to flush
     await asyncio.sleep(2)
     # Stop
-    out, err = asyncio_ioc.communicate(b"exit\n", timeout=5)
+    out, err = asyncio_ioc.proc.communicate(b"exit\n", timeout=5)
     out = out.decode()
     err = err.decode()
     # Disconnect
     assert isinstance(await asyncio.wait_for(q.get(), 10), CANothing)
     m.close()
     # check closed and output
-    assert "%s:SINN.VAL 1024 -> 4" % PV_PREFIX in out
+    assert "%s:SINN.VAL 1024 -> 4" % pre in out
     assert 'update_sin_wf 4' in out
-    assert "%s:ALARM.VAL 0 -> 3" % PV_PREFIX in out
-    assert 'on_update %s:AO : 3.0' % PV_PREFIX in out
+    assert "%s:ALARM.VAL 0 -> 3" % pre in out
+    assert 'on_update %s:AO : 3.0' % pre in out
     assert 'async update 3.0 (23.45)' in out
     assert 'Starting iocInit' in err
     assert 'iocRun: All initialization complete' in err
     assert '(InteractiveConsole)' in err
+
+
+@pytest.mark.asyncio
+@pytest.mark.skipif(
+    sys.platform.startswith("darwin"),
+    reason="devIocStats reboot doesn't work on MacOS")
+async def test_asyncio_ioc_override(asyncio_ioc_override):
+    from aioca import caget, caput
+
+    # Gain bo
+    pre = asyncio_ioc_override.pv_prefix
+    assert (await caget(pre + ":GAIN")) == 0
+    await caput(pre + ":GAIN", "On", wait=True)
+    assert (await caget(pre + ":GAIN")) == 1
+
+    # Stop
+    await caput(pre + ":SYSRESET", 1)
+    # check closed and output
+    out, err = asyncio_ioc_override.proc.communicate(timeout=5)
+    out = out.decode()
+    err = err.decode()
+    # check closed and output
+    assert '1' in out
+    assert 'Starting iocInit' in err
+    assert 'iocRun: All initialization complete' in err
+    assert 'IOC reboot started' in err
diff --git a/tests/test_cothread.py b/tests/test_cothread.py
index a722edab..06ccd93b 100644
--- a/tests/test_cothread.py
+++ b/tests/test_cothread.py
@@ -1,70 +1,48 @@
-import random
-import string
-import subprocess
 import sys
-import os
 import signal
 import pytest
 
-PV_PREFIX = "".join(random.choice(string.ascii_uppercase) for _ in range(12))
-
 
 if sys.platform.startswith("win"):
     pytest.skip("Cothread doesn't work on windows", allow_module_level=True)
 
 
-@pytest.fixture
-def cothread_ioc():
-    sim_ioc = os.path.join(os.path.dirname(__file__), "sim_cothread_ioc.py")
-    cmd = [sys.executable, sim_ioc, PV_PREFIX]
-    proc = subprocess.Popen(
-        cmd, stdin=subprocess.PIPE,
-        stdout=subprocess.PIPE, stderr=subprocess.PIPE)
-    yield proc
-    if proc.returncode is None:
-        # still running, kill it and print the output
-        proc.kill()
-        out, err = proc.communicate()
-        print(out.decode())
-        print(err.decode())
-
-
 
 def test_cothread_ioc(cothread_ioc):
-    import epicscorelibs.path.cothread
     import cothread
     from cothread.catools import ca_nothing, caget, caput, camonitor
 
+    pre = cothread_ioc.pv_prefix
     # Start
-    assert caget(PV_PREFIX + ":UPTIME").startswith("00:00:0")
+    assert caget(pre + ":UPTIME").startswith("00:00:0")
     # WAVEFORM
-    caput(PV_PREFIX + ":SINN", 4, wait=True)
+    caput(pre + ":SINN", 4, wait=True)
     q = cothread.EventQueue()
-    m = camonitor(PV_PREFIX + ":SIN", q.Signal, notify_disconnect=True)
+    m = camonitor(pre + ":SIN", q.Signal, notify_disconnect=True)
     assert len(q.Wait(1)) == 4
     # STRINGOUT
-    assert caget(PV_PREFIX + ":STRINGOUT") == "watevah"
-    caput(PV_PREFIX + ":STRINGOUT", "something", wait=True)
-    assert caget(PV_PREFIX + ":STRINGOUT") == "something"
+    assert caget(pre + ":STRINGOUT") == "watevah"
+    caput(pre + ":STRINGOUT", "something", wait=True)
+    assert caget(pre + ":STRINGOUT") == "something"
     # Check pvaccess works
     from p4p.client.cothread import Context
     with Context("pva") as ctx:
-        assert ctx.get(PV_PREFIX + ":STRINGOUT") == "something"
+        assert ctx.get(pre + ":STRINGOUT") == "something"
     # Wait for a bit longer for the print output to flush
     cothread.Sleep(2)
     # Stop
-    cothread_ioc.send_signal(signal.SIGINT)
+    cothread_ioc.proc.send_signal(signal.SIGINT)
     # Disconnect
     assert isinstance(q.Wait(10), ca_nothing)
     m.close()
     # check closed and output
-    out, err = cothread_ioc.communicate()
+    out, err = cothread_ioc.proc.communicate()
     out = out.decode()
     err = err.decode()
     # check closed and output
-    assert "%s:SINN.VAL 1024 -> 4" % PV_PREFIX in out
+    assert "%s:SINN.VAL 1024 -> 4" % pre in out
     assert 'update_sin_wf 4' in out
-    assert "%s:STRINGOUT.VAL watevah -> something" % PV_PREFIX in out
+    assert "%s:STRINGOUT.VAL watevah -> something" % pre in out
     assert 'on_update \'something\'' in out
     assert 'Starting iocInit' in err
     assert 'iocRun: All initialization complete' in err