Siemens RS485

Hi

I am new to Siemens and I am looking into building my first system.

What I have is 10 x RS485 devices, each with their own ID/header. Which I would like to connect onto the same bus and then connect into a Siemens CM1241 module bolted onto a S7-1200.

Each device on the string will not contain much data, an example of a sting with 3 devices in is below. #109 is a serial number, 1.25 is the data required.

#109,1.25,#110,1.23,#111,3.36,

How would you go about configuring a the PLC to read the incoming RS485 string and pull out each serial number and data and put them into an individual tag?

Thanks,

Are these some kind of ASCII bar code readers or something like that ?

With RS485 it is theoretically possible to make daisychained bus system, but there must be a protocol that handles when each device may be allowed to talk on the "bus".
Problem 1, how do you implement the (homegrown) protocol in the ASCII devices ? How to stop them from all "talking" at the same time ?
Problem 2, it will be too big task to implement the protocol in the PLC side. Theoretically possible, in reality not.

The only real solution is if the RS485 devices support some standard protocol. For Siemens S7-1200 it should be Modbus RTU, Profibus DP or Profinet IO.

Start with this example from Siemens. Shows how to communicate with two S7-1200's via RS485. You can expand it from there.

keithkyll said:

Start with this example from Siemens. Shows how to communicate with two S7-1200's via RS485. You can expand it from there.

Click to expand...

Excellent - will have a read. Thanks.

keithkyll said:

Shows how to communicate with two S7-1200's via RS485.

Click to expand...

It is Modbus RTU.
So the "RS485 devices" that DLMUK talks about have to support Modbus RTU for the sample code to work.

JesperMP said:

It is Modbus RTU.
So the "RS485 devices" that DLMUK talks about have to support Modbus RTU for the sample code to work.

Click to expand...

They are Modbus RTU 4-20mA instrumentation amplifiers.

I just need to know how to interrogate the incoming RS485 and get the data into individual tags.

Cheers,

Thats good. Should have put that bit of information in the 1st post.