I am implementing a Modbus converter from a RS-485 serial bus to a TCP network. I didn't see an equivalent for the serial broadcast messages in the TCP specifications.

What is the proposed handling of such messages?

Is there a real usage of broadcast messages in the real-life serial applications?

 

TCP is a a connected state. In order to communicate to a Device that is using TCP you have to establish a socket connection to the target before being able to send and recieve data. TCP is used to ensure that the data you are getting to a request is from the device it was sent to and all TCP messages have an embedded checksum to make sure the data is not corrupted. What this means is that you cannot do broadcast transmissions to multiple device in TCP like you could with a serial connection.

However if you use UDP, you can broadcast messages. Typically a UDP brodcast message is an IP of 255.255.255.255. The following link explains in more details how broadcasting works.

http://www.cisco.com/en/US/docs/internetworking/case/studies/cs006.html

Of course the other piece to this equation is the ability of the serial to Ethernet converter you are using to accept UDP prodcast.

Fred Loveless
Applications Manager
Kepware Technologies
http://www.kepware.com

 

Modbus/TCP uses TCP/IP to deliver the messages. Therefore, broadcast is a TCP/IP feature, not a Modbus feature. The following is a link to Wikipedia which explains IP broadcasting.

"http://en.wikipedia.org/wiki/Broadcast_address"

You can find all kinds of other information about TCP/IP broadcasting by using Google.

 

In Modbus RTU broadcast is achieved using 0 as the slave address. TCP only encapsulate RTU frames in transport protocol data units (TPDU). Once you have an active TCP connection (socket) between Master and each slave you don't need a "TCP broadcast". Each slave will receive the Modbus RTU broadcast frame regardless of the underlying TCP connection. By the way TCP is and end-to-end protocol and broadcast are not its concern as in the UDP protocol. May be that in a Modbus/UDP impementation (i wonder if there exist any) you need to set the broadcast in the UDP packets as you don't have an end-to-end reliable connection between Master and slaves.