Add Slave and SlaveContext

Author: flosse

CHANGELOG.md

@@ -3,6 +3,10 @@
 
 # Changelog
 
+## v0.3.0 (unpublished)
+
+- Added `Slave` and `SlaveContext`
+
 ## v0.2.0 (2025-09-30)
 
 - Added TCP client implementation

src/codec/rtu/mod.rs

@@ -4,6 +4,7 @@
 //! Modbus RTU
 
 use super::*;
+use crate::SlaveId;
 use byteorder::{BigEndian, ByteOrder};
 
 pub mod client;

src/frame/rtu.rs

@@ -1,10 +1,9 @@
 // SPDX-FileCopyrightText: Copyright (c) 2018-2025 slowtec GmbH <post@slowtec.de>
 // SPDX-License-Identifier: MIT OR Apache-2.0
 
-use super::*;
+use crate::SlaveId;
 
-/// Slave ID
-pub type SlaveId = u8;
+use super::*;
 
 /// RTU header
 #[cfg_attr(all(feature = "defmt", target_os = "none"), derive(defmt::Format))]

src/lib.rs

@@ -16,12 +16,17 @@
 #![allow(clippy::similar_names)] // TODO
 #![allow(clippy::wildcard_imports)]
 
+#[cfg(feature = "std")]
+extern crate std;
+
 mod codec;
 mod error;
 mod frame;
+mod slave;
 
 pub use codec::rtu;
 pub use codec::tcp;
 pub use codec::*;
 pub use error::*;
 pub use frame::*;
+pub use slave::*;

src/slave.rs

@@ -0,0 +1,157 @@
+// SPDX-FileCopyrightText: Copyright (c) 2018-2025 slowtec GmbH <post@slowtec.de>
+// SPDX-License-Identifier: MIT OR Apache-2.0
+
+//! Modbus devices
+
+use core::{fmt, num::ParseIntError, str::FromStr};
+
+/// Slave identifier
+pub type SlaveId = u8;
+
+/// A single byte for addressing Modbus slave devices.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
+pub struct Slave(pub SlaveId);
+
+impl Slave {
+    /// The special address for sending a broadcast message to all
+    /// connected Modbus slave devices at once. Broadcast messages
+    /// are one-way and sent from the master to all slaves, i.e.
+    /// a request without a response.
+    ///
+    /// Some devices may use a custom id from the reserved range
+    /// 248-255 for broadcasting.
+    #[must_use]
+    pub const fn broadcast() -> Self {
+        Slave(0)
+    }
+
+    /// The minimum address of a single Modbus slave device.
+    #[must_use]
+    pub const fn min_device() -> Self {
+        Slave(1)
+    }
+
+    /// The maximum address of a single Modbus slave device.
+    #[must_use]
+    pub const fn max_device() -> Self {
+        Slave(247)
+    }
+
+    /// The reserved address for sending a message to a directly
+    /// connected Modbus TCP device, i.e. if not forwarded through
+    /// a TCP/RTU gateway according to the unit identifier.
+    ///
+    /// [Modbus Messaging on TCP/IP Implementation Guide](http://www.modbus.org/docs/Modbus_Messaging_Implementation_Guide_V1_0b.pdf), page 23
+    /// "On TCP/IP, the Modbus server is addressed using its IP address; therefore,
+    /// the Modbus Unit Identifier is useless. The value 0xFF has to be used."
+    #[must_use]
+    pub const fn tcp_device() -> Self {
+        Slave(255)
+    }
+
+    /// Check if the [`SlaveId`] is used for broadcasting
+    #[must_use]
+    pub fn is_broadcast(self) -> bool {
+        self == Self::broadcast()
+    }
+
+    /// Check if the [`SlaveId`] addresses a single device
+    #[must_use]
+    pub fn is_single_device(self) -> bool {
+        self >= Self::min_device() && self <= Self::max_device()
+    }
+
+    /// Check if the [`SlaveId`] is reserved
+    #[must_use]
+    pub fn is_reserved(self) -> bool {
+        self > Self::max_device()
+    }
+}
+
+impl From<SlaveId> for Slave {
+    fn from(from: SlaveId) -> Self {
+        Slave(from)
+    }
+}
+
+impl From<Slave> for SlaveId {
+    fn from(from: Slave) -> Self {
+        from.0
+    }
+}
+
+impl FromStr for Slave {
+    type Err = ParseIntError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let slave_id = match s.parse::<u8>() {
+            Ok(slave_id) => Ok(slave_id),
+            Err(err) => {
+                if let Some(stripped) = s.strip_prefix("0x") {
+                    u8::from_str_radix(stripped, 16)
+                } else {
+                    Err(err)
+                }
+            }
+        }?;
+        Ok(Slave(slave_id))
+    }
+}
+
+impl fmt::Display for Slave {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{} (0x{:0>2X})", self.0, self.0)
+    }
+}
+
+/// Stateful management of the currently active device.
+///
+/// RTU devices are addressed by their assigned *slave id*.
+///
+/// TCP devices are either addressed directly (= implicitly) by using the
+/// reserved *unit id* `Slave::tcp_device() = 0xFF` (default) or indirectly
+/// through an TCP/RTU gateway by setting the *unit id* to the desired
+/// *slave id*.
+///
+/// The names *slave id* and *unit id* are used synonymously depending
+/// on the context. This library consistently adopted the term *slave*.
+pub trait SlaveContext {
+    /// Select a slave device for all subsequent outgoing requests.
+    fn set_slave(&mut self, slave: Slave);
+}
+
+#[cfg(all(test, feature = "std"))]
+mod tests {
+    use super::*;
+
+    use std::format;
+
+    #[test]
+    fn parse_dec() {
+        assert_eq!(Slave(0), Slave::from_str("0").unwrap());
+        assert_eq!(Slave(123), Slave::from_str("123").unwrap());
+        assert_eq!(Slave(255), Slave::from_str("255").unwrap());
+        assert!(Slave::from_str("-1").is_err());
+        assert!(Slave::from_str("256").is_err());
+    }
+
+    #[test]
+    fn parse_hex() {
+        assert_eq!(Slave(0), Slave::from_str("0x00").unwrap());
+        assert_eq!(Slave(123), Slave::from_str("0x7b").unwrap());
+        assert_eq!(Slave(123), Slave::from_str("0x7B").unwrap());
+        assert_eq!(Slave(255), Slave::from_str("0xff").unwrap());
+        assert_eq!(Slave(255), Slave::from_str("0xFF").unwrap());
+        assert!(Slave::from_str("0X00").is_err());
+        assert!(Slave::from_str("0x100").is_err());
+        assert!(Slave::from_str("0xfff").is_err());
+        assert!(Slave::from_str("0xFFF").is_err());
+    }
+
+    #[test]
+    fn format() {
+        assert!(format!("{}", Slave(123)).contains("123"));
+        assert!(format!("{}", Slave(0x7B)).contains("0x7B"));
+        assert!(!format!("{}", Slave(0x7B)).contains("0x7b"));
+    }
+}