Subject: Assistance Required for Modbus RTU Communication using S7-1500 with ET200SP CM PTP Module

Dear Team,

I am currently working on a setup involving a Siemens S7-1500 PLC integrated with ET200SP remote I/O. Within the ET200SP, I have installed the CM PTP communication module (6ES7 137-6AA00-0BA0), mounted on a black base unit.

Wiring Details:

• I have connected the Modbus RTU slave device using 2-wire RS-485.
• Pin 12 (TxD+/RxD+) of the CM PTP → A terminal of the Modbus device
• Pin 14 (TxD−/RxD−) → B terminal of the Modbus device

Configuration Details:

• Modbus slave address: 12
• Baud rate: 9600
• Stop bits: 1
• Parity: None
• These settings are identical in both the Modbus device and the PLC configuration.

Software Setup:

• I have implemented the MB_COMM_LOAD and MB_MASTER function blocks within the TIA Portal.
• The blocks compile successfully and run without any CPU faults.
• However, I am unable to receive any data from the Modbus device.

Questions and Concerns:

1. Black Base Unit Compatibility
   Can the CM PTP module function correctly when installed on a black base unit, while the first (leftmost) module in the ET200SP rack is on a white base unit?
   Is there any restriction on communication modules requiring white base units only?
2. Wiring Validation
   Is my wiring correct for 2-wire half-duplex RS-485 communication as described above (Pin 12 = A, Pin 14 = B)?
   Should I consider swapping A/B lines in case the slave has inverse labeling?
3. Communication Failure Troubleshooting
   Since both hardware and software configurations seem aligned, what could be potential causes for data read failure?
   Could it be due to lack of termination resistors, incorrect function block parameters, or port settings?
4. Experience Sharing
   Has anyone successfully used the CM PTP module (6ES7 137-6AA00-0BA0) for reading data via half-duplex 2-wire RS-485 Modbus RTU?
   If so, could you kindly share any working configuration, wiring diagram, or function block sample that helped establish stable communication?

I would highly appreciate any guidance, practical tips, or shared experience regarding this setup. Please let me know if further information (such as screenshots or diagnostic logs) would help in troubleshooting.

Thank you in advance for your support.

Best regards,
M.Ali

So, What should i do next. I am stuck on this point from last three days.

Post screenshots of your code blocks and data blocks. Also there should be some error values being returned by the instructions if they fail.

Hi.. 

Here i have attached the screenshots of the block and the setting + configuration  of the CM-PTP.

Also modbus master status showing continuous "7002" 

Hi...

I had visited the same link a few days ago but i found it no useful. Please provide me some sample program so can can save my time.

Thank you.

Stuff from screenshots seems to be correct except maybe the slave address. In the initial post you mentioned slave address 12, yet on the mb_master instruction you have 41 ?

humperdoo

Post screenshots of your code blocks and data blocks. Also there should be some error values being returned by the instructions if they fail.

What @humperdoo said earlier is of importance. Note that the error state and its reporting, are held only for 1 cycle. It is a must that you record the instruction's status value when the error bit goes high. This is true for the communication initialization and the master block.

I find it strange that you are attempting to send the slave device some REAL (datatype) data. I would expect some WORD format there. The sequence of bytes may be an issue. Consider reading from the slave device some simple data that is known (serial number?). That way you can check the data addressing too (start at zero or one).

This forum is manned by volunteers. We are blind to most of what matters since we are not there at the equipment and is uninformed to what the slave device if. If we are not able to supply you with answers, then consider contacting Siemens directly. 

Hi 

Initially the address was 12 now i have changed it to 41. but still i am unable to trace what i am doing wrong.