Modbus TCP Server connection is ESTABLISHED but all holding registers are value 0?

egr99 — Wed, 08 Apr 2026 12:17:21 -0000

You're absolutely right. Sorry for that.

In the meantime I wrote a Function block to handle the reads and writes to the different attached RTU-devices.
It processes the parameter like (str_Modbus:=(usiSlave:=1, uiFC:=5, uiLEN:=1, uiOFFSET:=0, uiInterval:=500)) and the corresponding output, waits for the response and processes next request in a 5-state procedure. It's working for 2 gateways (stable without errors, although error recovery is included).

But nevertheless it should be possible with the Channel definitions of Codesys. Much easier to maintain :)

Modbus TCP Server connection is ESTABLISHED but all holding registers are value 0?

egr99 — Sun, 05 Apr 2026 20:10:38 -0000

Hi there,

Read the above description and experienced the same: Reading coil/discrete input/input register/holding register doesn't fill the mapped I/O variable.

Any suggestions?

Defined (see attachment):
- Ethernet (with parameters according to the used Ethernet adapter of my Windows 11 laptop)
- -Modbus TCP Client
- - -LOGO231 as Modbus TCP Server
- - -WS242 as Modbus TCP Server (WaveShare Modbus TCP <-> RTU converter)
- - - -WS242_01 as Modbus Server, COM port (slave-id 1 RTU)
- - - -WS242_03 idem (slave-id 3 RTU)
- - - -WS242_11 idem (slave-id 11 RTU)

Writing coil/holding register works from a mapped I/O variable works fine.

Verification of Modbus traffic with WireShark:
QUERY to and the RESPONSE of all slave-ids are correct

Status of all defined is CONNECTED, RESPONSE_SUCCESS, RUNNING etc.

Request counters increase with expected frequency. Error counter = 0.
(To have all counters works fine I had to change Modbus TCP Client's parameter OptimizationON to FALSE).

LOGO231 works fine: values in mapped I/O conform expectation
WS242 without Modbus Server, COM port and
with defined channels (for only 1 slave-id possible) works fine.

PLC Settings:
Update I/O while in stop: I see xBusy changing from FALSE to TRUE and vice versa.
Always update variables: Enabled 1

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Modbus TCP Server connection is ESTABLISHED but all holding registers are value 0?

Modbus TCP Server connection is ESTABLISHED but all holding registers are value 0?