Problem with MODBUS Polling for the MICROLOGIX 1400

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Thread Starter

DineshChauhaan

I have a problem with Micrologix 1400. Channel 2 of the PLC is configured as MODBUS SLAVE. Data memory kept as N101 (Contains 250 registers inside it)

I am not able to poll more than 123 registers at a time.

Starting address is 0 and no of register 123: PLC is replying normally

Starting address is 0 and no of register 124: Replying with error code 03.

Starting address is 25 and no of register 123: PLC is replying normally

Any solution?

Dinesh Chauhan
 
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Bob Peterson

I think this is a limit in Modbus itself. I seem to recall you are limited to a 255 byte packet. 123 registers is 246 bytes. by the time you add in packet data maybe 123 registers is the max.

--
Bob
http://ilbob.blogspot.com/
 
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Lynn August Linse

I don't have an answer for you, but I know the old DF1 serial was limited 122 registers ... maybe some code internally is mapping MODBUS through the PCCC task? You should check with Rockwell.

MODBUS NEVER promises that you can read 125 registers, only promises that 125 is the max. Many devices support less than 125. Some due to less data, like if they only have 17 registers, that's the max you can read. Plus I've seen other devices which limit reads to 1, 4, or 10 registers despite having more data merely to limit the max serial buffer sizes in micro-P code.
 
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