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[modbus] Properly support client-mode broadcast sends (#17467)

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Co-authored-by: J. Nick Koston <nick@koston.org>
This commit is contained in:
Bonne Eggleston
2026-08-11 08:31:15 -05:00
committed by GitHub
co-authored by J. Nick Koston
parent eefd2a00c7
commit 8ee3c8d41d
7 changed files with 405 additions and 15 deletions
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esphome/components/modbus/modbus.cpp
+26 -1
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@@ -815,6 +815,16 @@ void ModbusClientHub::send_next_frame_() {
}
cmd->sent();
if (cmd->frame.address() == BROADCAST_ADDRESS) {
// A broadcast (address 0) is never answered (Modbus 4.1), so it is fire-and-forget: on_sent above
// reports the transmission, and the entry then retires with no terminal callback instead of
// occupying the waiting slot until the send-wait timeout expires. The turnaround delay already
// spaces the next frame; the following sweep erases the entry.
ESP_LOGV(TAG, "Broadcast to address 0 sent; no reply expected (fire-and-forget)");
cmd->complete_broadcast();
this->sweep_needed_ = true;
return;
}
this->waiting_for_response_ = true;
}
@@ -1033,9 +1043,24 @@ bool ModbusClientHub::queue_pdu(uint8_t address, std::span<const uint8_t> pdu, M
ESP_LOGE(TAG, "Frame too large, refused: %" PRIu8 ":%zu bytes", address, pdu.size());
return false;
}
// classify() drives both the broadcast guard and the continuous check below; compute it once.
const CommandPriority priority = ModbusDeviceCommand::classify(pdu[0]);
// A broadcast (address 0) is never answered (Modbus 4.1), so it is only meaningful for a command that
// changes state. Refuse a broadcast that expects a reply - anything but a write or a custom/vendor code -
// as it could never deliver a result, so the caller learns via the false return (and on_not_sent).
// 0x17 (read/write multiple) is a knowing inclusion: classify() treats it as a write, so its write half
// lands on every server and its unanswerable read half is simply discarded. An exception-flagged custom
// code (0x80 bit set) is refused: is_function_code_custom() masks that bit away, so exclude it explicitly
// here to match classify()'s exception-first handling of the write side.
if (address == BROADCAST_ADDRESS && priority != CommandPriority::WRITE &&
(!helpers::is_function_code_custom(pdu[0]) || helpers::is_function_code_exception(pdu[0]))) {
ESP_LOGW(TAG, "Broadcast refused for function 0x%X: a broadcast (address 0) is never answered", pdu[0]);
return false;
}
// continuous is ignored for every mutating code (re-writing a value forever is never intended).
const bool mutates = ModbusDeviceCommand::classify(pdu[0]) == CommandPriority::WRITE;
const bool mutates = priority == CommandPriority::WRITE;
bool continuous = false;
if (options.continuous) {
if (mutates) {