I have an S7-1500 with a CM PtP RS422/485 HF module. The PLC is the Modbus master.

I'm trying to read a single register on a slave simulator on my PC, via a ADAM USB-RS485 converter. I've successfully used the converter for both master and slave applications earlier.

For my current program, I'm trying to read the holding register addr = 1 (translated 40002). My slave is interpreting the Modbus frame correctly and responds, but on the PLC I get error for the Modbus_Master block: 16#81E2, which according to doc has to do with baudrate, parity, stop and start bit. I am pretty sure that the baudrate, parity and stop-bit settings between the master and slave correspond. However, for the start-bit I am not so sure. The CM PtP modules' configurations for message receipt is kept default, that is

• Frame start detection
  • Start on any character
• End of frame detection
  • Character delay time 288 bit-times

Please give me a hand to solve this !

Thanks !
Ronny

HI Duris. Thanks for responding.
I'm not able to take screenshots of the program until tomorrow because this PC does not have TIA installed. I'm pretty sure that the settings for the Modbus_Comm_Load and Modbus_Master blocks are ok. My program is based on an application example from Siemens.

The ADAM converter has RS485 going into a CM PtP RS422/R85 HF module, with a 120 Ohms resistor between the A/B lines, and a USB going into the computer. There is no need for a RS232 adapter. link-to-ADAM-4561 

FYI, I have no terminating resistor between the lines on the CM module.

HI Kaulquappe.

Thanks for your input.

I've sucessfully used the ADAM-USB/RS485 for communicating with a Delta power conditioning system, an Eltek battery controller, with an FTDI USB/RS485 cable and PC, and also a Raspberry PI running MinimalModbus. I don't have much experience using RS485. Are there other configurations than parity, baudrate, nr stopbits and flow-control ? I always set flow-control to "none" because I know to little about it. 

The distance between the nodes are about 2 m. I don't have a terminating resistor between the A/B lines of the CM module. I will try that next.

After connecting the terminal resistor (120 Ohms) I got no difference in response.

However, after changing the LINE_PRE static tag of the Modbus_Comm_Load block, from 0 to 2, then it finally worked.

Also, another thing to consider is that I needed to turn off the "optimized block access" on the DB where the register values are to be loaded into, at least when referencing the DATA_PTR on the form "BlockName.RegisterName". 

C1Ron

After connecting the terminal resistor (120 Ohms) I got no difference in response.

However, after changing the LINE_PRE static tag of the Modbus_Comm_Load block, from 0 to 2, then it finally worked.

Also, another thing to consider is that I needed to turn off the "optimized block access" on the DB where the register values are to be loaded into, at least when referencing the DATA_PTR on the form "BlockName.RegisterName". 

i faced the same issue i use the LINE_PRE then it work