Monitoring A Modbus RTU Network

J

Thread Starter

J

I would like to monitor a modbus rtu network that has about 10 slaves (multilins) and one master (DCS) to find out if one slave is actually working properly. I don't want to take down the network so i figured i would ask before i try to connect. I downloaded CAS Modbus scanner but am unsure how to change the master ID. Can both masters be set to address 0? Also, can there be 2 masters on the network...say a DCS serial controller and a PC running the scanner software?
 
Hello,

> Can both masters be set to address 0?

No. Address zero (0) is reserved for broadcast messages. The legal address range is 1-247. Most software allows 1-255.

> Also, can there be 2 masters on the network

No, not a serial (485/422) network.

Regards,

Mark
http://peakhmi.com/
 
The PC based monitor (sniffer) is NOT a ModBus Master, it just captures the Master and Slave communications and displays in a window and logs to a file.

There is always a 3 byte (or so) delay between the polls from the master and the responses from the slave. The sniffer uses these delays to detect the end of a poll/response and inserts a carriage return/line feed on the displayed activity and in the file. The one I use also posts a Time Stamp before each line feed.
 
I saw a industrial sniffer on a tank farm several weeks ago that showed the tank level at the bottom of the stairs. Kinda cool, trying to remember who it was...
 
P

Patrick Lansdorf

How are you planning to figure out if one slave is actually working correctly? Why do you suspect it?

If there are any problems with the Modbus RTU communication, you should be getting Modbus exception responses from the slave, can you read out any?
 
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