Modbus Serial Master v1.0 8-14-2006 Reading Holding Register Limit

PAC Project
1

I am using the Modbus Serial Master v1.0 8-14-2006 sample code provided by Opto 22 to interface with a Siemens GC. I can successfully read 125 consecutive holding registers but when I try to increase the number above 125 I get errors and no data is received. Any ideas?

Modbus Watch.jpg700×139 24.5 KB

2

125 is the max for function 3 & 4 according to MODBUS Application Protocol Specification V1.1b

3

Thank you.

What would you do if you needed more than 125 registers?

4

Also, do the registers have to be consecutive? If not, how would you code to get the non-contiguous registers?

Thanks,
Craig

5

As you have probably found, you frequently need to read registers at non-consecutive addresses. You just need to make multiple requests with a different start register and then copy the results out of your Modbus holding registers into a new variable/table each time. The same goes for reads with more than 125 registers.

See the attached image for the pattern I typically use when reading from non-contiguous addresses.


6

I understand.

Thanks Phillip