Hello,

I am trying to change a RS232 network to an IP network to support modbus. In the applications, broacast is used and I was wondering how modbus TCP is handling this. By an IP broadcast or by parsing a table of RS232/IP gateway table sending to each a modbus broadcast encapsulated in an IP frame?

Does the implementation depends on the manufacturer of the gateway? or worse, is there any manufacturer supporting broadcast in their RS232/IP modbus gateway?

Thanks for your help

 

Hi Martin,

First you need to run IP over the RS232 link. For this you would use PPP protocol or possibly SLIP (older and not much used). However RS232 is a point to point protocol so I'm not sure why you would use broadcast.

Maybe you need to convert each device from RS232 to RS485 (many devices available to do this) which is a multidrop protocol but then you can't support PPP since PPP is a point to point protocol.

So if you are saying you want to use Modbus TCP over an RS485 network then the question is why. Just run Modbus RTU over the RS485 and provide a single gateway to convert Modbus RTU RS485 to Modbus TCP (over Ethernet maybe).

Regards

Peter Whalley

 

I don't know of any Bridge that will map a serial broadcast to an IP broadcast, nor any that map an IP broadcast to a serial Broadcast. I cannot speak for Modicon, but I doubt they accept/handle a Modbus/TCP packet addressed to 255.255.255.255 and likely would just discard it (?? any SE guys know ??).

So on the IP side, the bridge receives a unicast MB/TCP message with a Unit Id of zero (result is vendor & user-config specific) 1) auto-assumed to means serial Slave Address #1 2) or sent out as a serial Modbus broadcast 3) or silently discarded (or exception 0x0A returned) 4) or a few bad implementors may send to serial Slave #0 and expect a response!

Generally sending IP broadcasts are seriously frowned upon by network people - plus that broadcast won't go through routers or will be limited by VLAN or other filters, so results will be unpredictable and very site-specific.

I'd suggest your application should be modified to (if the effect of the broadcast is required) to include a user-defined list of IP who desire the "broadcast" and unicast to them all with a Unit Id of zero. Some of these slaves will respond (some may not) so the app must now deal with responses (or lack of them).

Best regards

Lynn August Linse IA Firmware Specialist, Digi Int'l (www.digi.com)