Sandbox::https://github.com/pymodbus-dev/pymodbus/pull/3081

Bingdom · 2026-11-04T04:41:19Z

Some serial devices respond successfully but become temporarily unresponsive to an immediate follow-up request. For example, writing a relay and immediately reading its state can cause intermittent timeouts, even when normal polling succeeds.

Add inter_request_delay to ModbusSerialClient and AsyncModbusSerialClient. The value is in seconds and defaults to 0.0, preserving existing timing.

from pymodbus.client import ModbusSerialClient

with ModbusSerialClient(
    "/dev/ttyUSB0",
    baudrate=115200,
    inter_request_delay=0.020,
) as client:
    client.write_coil(0, True, device_id=1)
    client.read_coils(0, count=1, device_id=1)

Here, readback waits until at least 20 ms after the write transaction finishes. The appropriate interval depends on the device.

Spacing is enforced under the transaction lock, including retries after timeouts. The first request is immediate, existing idle time counts toward the interval, and async waits yield to the event loop.

Tests cover both clients, default behavior, response timing, retries, failures, concurrent callers, cancellation, and parameter validation.

janiversen · 2026-11-04T06:51:32Z

Before I dive into details, I want to point out that your example seems incorrect, if you write a value, the resulting value is returned directly in the response:

Captura de pantalla 2026-11-04 a las 7 48 29

However it is true that some devices need a delay between calls.

janiversen

The idea is quite ok, but the implementation have a side effect on other connected devices, which cannot be what you intended.

The PR also includes some general (but fundamental) changes, which for regression test reasons should be submitted as separate PRs.

Finally monkeypatch() is introduced, adding to the maintenance load, if really needed please explain why mock() is insufficient.

janiversen · 2026-11-04T06:52:33Z

 from __future__ import annotations

 import contextlib
+import math


It seems very strange that the math is needed.

Hi @janiversen thank you for reviewing the code.

For the moment, we need to check if the input is a finite number. We can strip out this check if you consider this level of input validation is out of scope. However, I would suggest the lower-bound check should remain.

janiversen · 2026-11-04T06:54:53Z

            )
        if framer not in [FramerType.ASCII, FramerType.RTU]:
            raise TypeError("Only FramerType RTU/ASCII allowed.")
+        if not math.isfinite(inter_request_delay) or inter_request_delay < 0:


inter_request_delay is defined as a float, so just check it is a float, which probably will cause problems with the type checker.

If it was defined as "float | None", this check would make sense.

"< 0" is comparing against an int not a float ?

I will switch it out to float comparisons for consistency, no problem

Both NaN and infinity are valid float types. So we have the finite check there for that.

janiversen · 2026-11-04T06:55:33Z

        if framer not in [FramerType.ASCII, FramerType.RTU]:
            raise TypeError("Only FramerType RTU/ASCII allowed.")
+        if not math.isfinite(inter_request_delay) or inter_request_delay < 0:
+            raise ValueError("inter_request_delay must be finite and non-negative")


No it must be a float (which includes int) according to the typing

janiversen · 2026-11-04T06:57:03Z

            )
        if framer not in [FramerType.ASCII, FramerType.RTU]:
            raise TypeError("Only RTU/ASCII allowed.")
+        if not math.isfinite(inter_request_delay) or inter_request_delay < 0:


inter_request_delay is defined as a float, so just check it is a float, which probably will cause problems with the type checker.

If it was defined as "float | None", this check would make sense.

"< 0" is comparing against an int not a float ?

janiversen · 2026-11-04T06:58:47Z

+            or self._last_transaction_end is None
+        ):
+            return 0.0
+        return max(


It is a lot cheaper to calculate the difference (as you do) store it in a variable and add it the the if above)

janiversen · 2026-11-04T07:26:04Z

+async def test_no_response_and_independent_clients(clock, monkeypatch, sync):
+    """No-response sends are paced without sharing deadlines between clients."""
+    _, waits = clock
+    first, second = manager(sync, 0.02), manager(sync, 0.02)


this is not a bad test, now add a test with 1 serial device, with 2 devices connected.

janiversen · 2026-11-04T07:27:03Z

+async def test_async_concurrent_callers(clock, monkeypatch):
+    """Queued callers cannot send during another caller's idle interval."""
+    now, waits = clock
+    transact = manager(False, 0.02)


This test is wrong, different callers should be able to send to other devices.

janiversen · 2026-11-04T07:27:31Z

+async def test_sync_concurrent_callers(clock, monkeypatch):
+    """Threads sharing a client retain spacing while queued on its lock."""
+    now, waits = clock
+    transact = manager(True, 0.02)


sync pr definition cannot have concurrent callers !

janiversen · 2026-11-04T07:28:48Z

+        sleeping.set()
+        await asyncio.Future()
+
+    monkeypatch.setattr(transaction_module.asyncio, "sleep", sleep)


We do NOT, patch system libraries.

janiversen · 2026-11-04T07:30:27Z

+    monkeypatch.setattr(transaction_module.asyncio, "sleep", finish_sleep)
+    await execute(transact, False, True)
+    assert send.call_count == 2
+    assert waits == pytest.approx([0.01])


A general comment, all other tests uses mock(), not monkeypatch() so you need some convincing arguments why this is needed.

I am not against monkeypatch() but having 2 different mocks adds to the maintenance burden.

Understood. I've made it similar to existing tests.

janiversen

Removed a non correct comment.

Bingdom · 2026-11-05T03:31:22Z

Tests are failing because monotonic() in <3.13 is imprecise in windows environments. I'll move my implementation to perf_counter(), and leave the existing codebase with monotonic().

I can migrate the rest of the code to perf_counter() in a separate PR if you prefer, as of right now it solves a separate issue.

Bingdom added 5 commits November 3, 2026 16:22

Fix missed buffered serial replies after a delayed wakeup

7780602

Added inter-request delay to sync and async serial clients

e7e437c

Merge remote-tracking branch 'upstream/dev' into dev

adba94f

Updated transaction test to include inter request delay

8bcba26

Fixed python3.10 failed tests

4eed3ff

janiversen requested changes Nov 4, 2026

View reviewed changes

Bingdom added 7 commits November 5, 2026 10:55

Changed to float comparison

d0f5fe2

Updated test cases based on feedback

ac9a0d3

Updated examples

e9330a2

Track inter-request delay by slave ID

2ae8ee7

Release async transaction lock during inter-request delays

277d913

Merge upstream/dev preserving per-slave request delays

d1e50e1

Pylint fixes

8eccdc4

Bingdom added 2 commits November 5, 2026 14:32

Fixed issue due to imprecise monotonic clock with <3.13 on windows

338dfd9

Merge branch 'dev' into dev

91e2ba2

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