Modbus RTU using RS232 to Micrologix 1400 chan 2

sophickj · May 31, 2016

I am trying to communicate from a Micrologix 1400 chan 2 to a SC2124M scanner via modbus rtu. I have taken a straight through cable with RS232 on both ends and cut to connect to another cable based on distance. Sending and receiving but not seeing any data in holding registers. There is also an Error code 2 counter counting.

Mickey · May 31, 2016

I have taken a straight through cable with RS232 on both ends

I would think a null-modem cable would be the one to use.

Pin 2 to 3
Pin 3 to 2
Pin 5 to 5
__________________

Firejo · May 31, 2016

Mickey said:
I would think a null-modem cable would be the one to use.

Pin 2 to 3
Pin 3 to 2
Pin 5 to 5
__________________

Actually the SC2124M has a DCE port (according to its manual) and the MicroLogix’s port is a DTE. A straight cable should work.

sophickj · Jun 1, 2016

I am seeing where in the master (PLC) the messages are being sent and received. But no data is in the table of the holding registers. Error code 2 is counting in the channel status window for channel 2.

Ken Roach · Jun 1, 2016

Error code 2 is counting in the channel status window

This is probably the most important troubleshooting fact; Exception Code 02 in Modbus means "Illegal Address".

This is consistent with everything working fine on the MicroLogix side, because the messages are getting through and the slave device is responding to them. It's just rejecting their contents because it doesn't have a memory address to put that data in.

What are the Modbus function codes you are using and what are the Modbus addresses ? Are you certain that you are using the correct initial address and length ?

sophickj · Jun 1, 2016

The Modbus Command is Function code 03 Read Holding Registers
Size in elements 20 Data Type 16 bit MB Data Address 47001-47024. See additional info attached.

sophickj · Jun 1, 2016

Thanks Ken. Really want to resolve this issue. I am reading the addresses and storing it in Holding register N file. I have tried modbus address with and without the 4 at the front and still no success.

Firejo · Jun 1, 2016

sophickj said:
The Modbus Command is Function code 03 Read Holding Registers
Size in elements 20 Data Type 16 bit MB Data Address 47001-47024. See additional info attached.

There are two addressing schemes for Modbus, Base 0 and Base 1 (I won’t go into all of the different vernacular that gets used for this). Some PLC’s start “0” and some start at “1”. You might try adding a “1” or subtracting a “1” and see if you get any results. The other thing you could do is download Modbus Poll and make it work with the SC2124 so that you can confirm the addressing and that the device responds correctly.

Ken Roach · Jun 1, 2016

Definitely omit the "4" from the Modbus Address field when you type it in RSLogix 500.

This confuses everybody when they start with Modbus: they look at "40001" and think "Forty Thousand and 1" when it's really "Data Area 4, Register Offset 0".

I like to use Simply Modbus or Modscan to test whenever I can because those make it very easy to see the raw data going back and forth.

Modbus RTU using RS232 to Micrologix 1400 chan 2

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