Clarification on how to poll registers on SP-C using Modbus.


Documentation is not completely clear, require further clarification on exactly what can be polled.

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SmartStruxure Solution, Other


Security Expert



Other than the Application Note on Modbus there is no additional documentation available for this Modbus interface.


Below are further clarifications from the Application Note.

1. 16 PGM output Expander Coils - as per the note on page 15 these support the first 128 registered modules (128 modules * 16 coils = 2048 coils).

2. Register address map - These support all 512 modules (512 * 16 bit registers) . Each register contains 16 bits and is therefore a much more efficient way of expressing the information.

3. Register 50001 is the first Output Expander, 50002 is the second, etc...

4. Each register contains 16 bits an is able to express all 16 PGM outputs of the Expander.

5. You can poll register groups using function code 03, see below.

The register read function code 03 will allow you to specify the number of registers. If you are using Citect or a modbus driver of any type for SBO AS then you should get an exception if the controller does not support the function.

Functions supported are 1,2,3,4,5,6,7,15,16,17