Modbus communication between CO sensor & S7-1200 PLC

Dear All,

I am facing problem in modbus communication between Siemens PLC S7-1200 and CO detector sensor (Make: Grey stone). I have ensured the PLC side is working fine by checking with slave PLC i can able to read/write datas. But when i am trying to communicate with CO sensor am getting error as " Timeout error" in PLC.

What could be the possible reason, i have tried with different possible settings in Co sensor and PLC as well but unable to read/write. Kindly advice to resolve at the earliest.

Thanks,
Dina.

Hi Dina,

Maybe this link will give you more information.....
http://www.plctalk.com/qanda/showthread.php?p=372422

In order for anybody here to be able to help you, you need to give more information.
How should the S7 communicate with the CO sensor, CP341? (Modbus TCP or Modbus RTU)
What FBs are you using?
A Timeout means that a command was sent, but no reply was recieved whitin the Timeout time.

In some cases it may be a better to select a gateway (Profibus to Modbus) instead of a CP341 card and let the gateway handle the Modbus communication.
From what I understand, setting up a S7 as Modbus RTU/TCP master can be somewhat of an challange, espcially after reading: http://www.plctalk.com/qanda/showthread.php?p=372422

Here is a link to the Anybus gateway Profibus slave to Modbus RTU master:
http://www.anybus.com/support/support.asp?PID=104&ProductType=Anybus Communicator

Our customers feedback regarding the Anybus gateways is that it is aesy to use and reliable.

Just my 2 cents.

//Pat L

Is the CP card initialised correctly in OB100 or first scan bit?

Are you using dedicated modbus instructions from the global library (eg FB1081)?

One thing that caught me out was communicating to Shinko temp controller's their data address structure was conveyed as (eg write set point) Function 06H, Register 0001H. So I figured for FB1081 I would use mode 1 (write) 40001 (First holding register). But alas, the Set Point register (and all others) needed a 1 added to them. So 40002 worked for set point, 40003 for PID 'P' etc etc.

Prehaps you may be experiencing the same thing?

Can you post your project and a pdf of the CO₂ sensor comms manual?

first off all, check your wire and connection settings, verify one more time if S7-1200 & CO sensors communication setting matches (baud rate, nr of bits, parity ...)

1) I assume this is half duplex, 2 wire Modbus RTU over RS-485.

Have you swapped the A and B lines?

There is no standard or convention of which side is A and which side is B.

Some large firms have a division that does it one way, another division that does it the other way.

Connection to wrong side does not damage the hardware drivers, but communications is impossible if A and B are reversed.

2) If there is a connection for signal ground, by all means, use it.

"2 wire" is a misnomer. It should be 2 wire with signal ground.

The lack of a signal ground can cause comm faults.

3) The Modbus standard specifically states "no parity shall be 2 stop bits".

Few slaves pay attention to the standard and do 'no parity' at one bit (the classic default 8-N-1). But I did run into a master once that would adhered to the standard and would not talk to a slave on a no parity setting when the slave only did one stop bit.

The work-around is to use even or odd parity, which is always 1 stop bit, no arguments.