Experimental Physics and
Industrial Control System

Hi Marco,

This is an ao record and one of the links is P.ASLO.  This means that depending on whether the link is broken the actual integer value being written to the device could change, since ASLO affects that.

According to page 48 of the Modbus reference document

https://epics-modbus.readthedocs.io/en/latest/_downloads/c210fb586108fe21da5a09d202ab86d1/Modbus_Application_Protocol_V1_1b.pdf

Modbus exception 3 is “ILLEGAL DATA VALUE”.  This means:

A value contained in the query data field is not an allowable value for server (or slave). This indicates a fault in the structure of the remainder of a complex request, such as that the implied length is incorrect. It specifically does NOT mean that a data item submitted for storage in a register has a value outside the expectation of the application program, since the MODBUS protocol is unaware of the significance of any particular value of any particular register.

So according to the specification exception 3 should only be returned when the command syntax is wrong, not if the data value is outside the allowed range.  However, it is always possible that Eurotherm has not following the spec, and it is returning exception 3 for a data value outside the allowed range.

Please send the complete error message when you get the exception.

It may also be helpful to put the following 2 commands in your startup script:

asynSetTraceIOMask Eurotherm_EMODBUS_1_1_Wr_P -1 HEX

asynSetTraceMask Eurotherm_EMODBUS_1_1_Wr_P -1 ERROR|DRIVER

That will show the Modbus messages sent to and received from the device.

Mark

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