Hi!

I need to have multiple devices with the RS485 interface communicating with an S7-1200 CPU 1215C PLC (using a communication module CM1241). Hardware-wise, it is possible to have those devices sharing the same bus, and both the device and the communication module have RS485 interfaces. However, I realize that some of the devices (for example, this distance sensor https://docs-apac.rs-online.com/webdocs/1534/0900766b8153436e.pdf) don't support Modbus RTU protocol (they use another protocol, specified in the datasheet).

I would like to know if it is possible to configure the communication module and/or the PLC to accept other communication protocols.

Thank you in advance!

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Split from RS-485 2-wire communication from S7-1200 PLC (CM 1241).

Hello...

this distance sensor has RS232 serial...so You have to use CM 1241 RS232 card....

and of course You can use different protocols on RS485......

Thank you for your reply!

Yes, I'm sorry, I forgot to mention that I'm also planning on using RS232 to RS485 converters. 

So, regarding the programming, if I'm using other communication protocol, I need to write my own library (similar to the LMRTU for Modbus RTU protocol)?

Hello Maria;

The suggested way to "create" your own protocol on a CM 1241 would be to use the Freeport mode. 

Freeôrt is a freely programmable, frame-based protocol, very close to the ASCII protocol. You simply define the frames you need to send and receive,m the start-of-text and end-of-text characters (or the number of characters to be transmitted) and use the SEnd/Receive functions for the CM to communicate with your serial partner.

CM PtP - Configurations for point-to-point connections

https://support.industry.siemens.com/cs/ww/en/view/59057093

Study this document's information on Freeport before asking more questions, please.

Hope this helps,

Daniel Chartier

Thank you for your replies!

All sensors on the bus have the same protocol (different from Modbus RTU).

The success of multidrop bus communications over RS-485 is highly unlikely and improbable for these devices.

This device broadcasts, on a schedule, its process variable via RS-232, which is a point-to-point serial connection.

RS-232 devices almost never have the ability to be "polled' or identified with a node ID or address, because it is expected that in point-to-point communications that the device on the other end is always available and knows whom it is talking to.

I doubt that there is any means of

- identifying which value comes from which device

- synchronizing whatever number of devices on an RS-485 comm net so that only one device speaks at time.

When there is no synchronization, there are communications collisions.

Even if data comes through because other devices happen to be silent at that moment, how does one identify the source of the data?

Just because RS-485 is capable of multidrop communications does not mean that converting RS-232 point-to-point devices to RS-485 with a converter grants the devices and the network multidrop functionality.  The communications protocol needs to provide multidrop functionality as well as the hardware layer.   The broadcast protocol used by these devices (over RS-232) does not provide multidrop functionality.

I did not read the spec in detail, but the timing diagrams for serial output are revealing.  I'd suggest proving me wrong before proceeding to avoid disappointment.

Thank you very much for your reply!

I had not thought about those problems, but you're right. Thank you!

I found a similar sensor (http://image.dfrobot.com/image/data/SEN0002/URM04V2.0Mannual1.1.pdf) that has an RS485 interface, which allows having up to 32 devices connected (according to the datasheet).

In the communication protocols section, they include the slaves' addresses in the data frames. So, I guess the problem of identifying from which slave comes the data on the bus is solved (?).

Also, the synchronization is ensured by having the master initiating communication with one device at a time. Each sensor only places its data on the bus when requested by the master. The master waits for the data and, only after receiving it, initiates communication with a second device (?).

Please correct me if I'm wrong.

Thank you in advance!