Dear all,

I try to use PLC S7-1200 CPU (1211C DC/DC/DC, firmware 4.1) and module CM 1241 (RS422/485, firmware 2.1) for communicate with frequency inverter TECO L510 by Modbus RTU protocol.

After days of trials have no idea what is wrong.

From beginning.

I tryed connect these devices by 1m plain Ethernet cable, without shield, terminated on both sides by resistors 120 Ohm. Inverter has Ethernet plug, so terminating resistor I connected by welding it before plug. Some times I had communications, but always with error (frame error, no response error etc.). Many times I changes program, try with examples from forum etc. After hours of attempts communication does not work properly. I decided to change cable to Industrial Ethernet cat 5e and plugs to Siemens Profibus and Siemens Profinet.

At this time I have 2m Industial Ethernet cable, on PLC side connected to Profibus plug and terminated by 120 Ohm resistor (build-in resistor on plug is off, this is different resistanse?), and Profinet plug with resistor at inverter side.

From few hours I don’t have any response from slave. I bought Modbus receiver connected to USB to test the devices. When I try connect to inverter, it work properly. Using “IO Ninja” or “APBModbus Test” programs I can send and receive data from inverter. But when I connect to CM 1241 I don’t have any communication. In PLC I send data, and “IO Ninja” software should receive and show it, but this is not happening, as if no data is sending. What is important, one times I received response for loop test, but I don’t have any idea why this not work all times. I am beginning to suspect damage to the CM 1241 module, but why…

I have already read all threads on the forum, trying solutions, but no success.

Normaly I try with this parameters:

• Address “2”,
• 9600 Bits/sec,
• 1 Stop Bit,
• No Parity

After read some posts I change to 2 Stop Bit, in inverter and PLC.

Tx diode on CM 1241 module blinks, Rx diode not. Some time ago I had at least error on slave, now nothing. All time I have error 808C (“The slave does not respond within the set time /Check the data transmission rate, parity and wiring of the slave.”).

I'm worried that despite the flashing diode I do not get any data on the USB receiver, it's possible that this data is not sent at all?

Here Siemens TIA V13 project and PDF with screenshots and program:

[snip]

Someone has an idea?

EDIT 08.07.2018:

Here attached project on older library (in TIA communication is new Modbus RTU and older Modbus):

[snip]

Communication still does not work, the same symptoms. I downgrade firmware on module CM1241 to v2.0, it also did not help.

Hello,

lot of different tests over each other. I recommend:

1. set speed 9600,8,N,1 to both devices. In other next tests you can set also parity.

2. proper termination on both ends.

3. test of CM1241 functionality, you can start send some message and attach this to some PC app that reads in hex format, for correct see all bytes incoming. If message from CM is correct, CM is ok.

4. test again with PC MB Master application for data from inverter. If all working, initial test is ok.

Steps for running MB instructions/blocks in S7-1200 is strictly defined with success end of one block, then you can call Master block. Check your Step7 TIA version System manual for correct steps, help. I don't see reason to manipulate with firmware of CPU, modules. Come here agin after tests.

Hello,

next known problem can be not correctly set MODE in instance block of Comm_Load. Due some bug, in case of configured 2 wire RS-485 connection (your situation), check that value, it need to be 4. In other case, set this to 4 prior start call of Comm_Load block. Next recommendation is check you have swapped Data+ and Data- wires.

In previous ZIP file you have four .html files. It's too long for screenshot, and html retains formatting.

Unfortunately I do not have another converter betters than Moxa. I have tried one "no name" and Moxa.

In attachment program and DB config.

[snip]