Hi!

I need to ‘arise’ this old record, because I’m having some problem to connect CP441 to Modbus slave RTU devices from Schneider TRV00210. First I was thinking to attach my project, but I need to test some more days first. I have connected the CP441 via a Modbus repeater also from Schneider with choice for 10 or 11bits. I have all the needed HW CP441 with Modbus master driver and a ‘chip’ to connect back on the CP441. First I have some general questions. I see in the Schneider manuals Modbus terminating resistor 120ohm. Should I have this one on the cable between CP441 (in the plug) and the repeater connection side? I also measured 3VDC on pin 4 and 11 on my RS422/485 adapter, is that normal? I’m using half-duplex line – 2wires. I’m using BSEND and BREC functions, just BSEND now, because I still don’t manage to receive DONE flag. My BSEND function returns mostly error code 2 which is described in the manual as Negative acknowledgment, function cannot be executed, for example, link partner does not respond or sends negative acknowledgment. .

I have also one setup fail, which I don’t know if it is critical. When I defined my connection I have not placed a PTP communication device in my project, but I just defined ‘undefined’ PTP connection and as I see the parameters for the connection don’t differ at all. As the PLC is running production I can not change the configuration now, but I assume it is not the main problem.

As I told my repeater have 10 and 11 bits setup. In one position I think 11 bits, the CP is just sending the telegram TX light goes ON and no response from the Schneider slave, then the BSEND runs out in 2 sec and generates STATUS error 2. Then I switched to 10bits, it was obvious that both was ‘ talking’ RX and TX lights was running very quickly as my cycle sends all the time after error or done. But in this case I got Error all the time.

I understand my post is quite confusing, but I’m confused myself. I may be specify more after more tests.

I was thinking, can I use a program like Hyperterminal or other to check the serial communication, do you know, to see the bytes running and make some conclusions. Do I need a modem between the CP441 and the serial port of my computer?

Thank you!

OK, here is my simple code, I’m presently just trying to send with BSEND without reading. When I switch the Modbuis repeater (TRV00211) to 10bits communication and 19200baud rate I can see the Tx/Rx lights on the PLC and the Schneider TRR00210 module running very quickly. This shows me that the PLC sends and the Schneider answers but any way when I debug the program I receive fail each time PLC sends a message (the Rx/Tx lights are running all the time because as you see my code I have a cycle which sends again and again each time DONE or ERROR status is present). BSEND function returns ERROR after each send telegram and never return DONE. The error is number 02 which stands Negative acknowledgment, function cannot be executed, for example, link partner does not respond or sends negative acknowledgment in the BSEND description. As I see that Schneider is sending something I suppose may be the problem is Negative acknowledgment or some fail with the baudrates or something else.

As I read the manual once again, and I described earlier I haven’t declared any communicational partner if you see in the Connections in the Step7 project, I just made an Undefined ptp connection, may be that is also one part of the problem?

And one mote check point, I don’t have any Modbus terminators as I have just one communication partner now, I don’t know how critical the termination resistor is when I have the repeater and Modbus slave connected together on a backplane. And do I need a terminator resistors on the cable between PLC and Modbus repeater ?

If you have some more ideas, please write.

Kaulquappe

Hello Ico,
error 2 at BSEND is not caused by a missing termination resistor. Then you would get "transmission error" or "monitor time elapsed". In a surrounding without electrical noise you can work without terminating resistors.

Error 2 occurs when BRCV is not called unconditionally or when BRCV uses a wrong R_ID. Please note: R_ID is a initialisation parameter, it is only evaluated with the first execution after restart of the CPU.

Check also the diagnostic buffer of the CP or read the SYSTAT area.

Regards,
Kaulquappe