MODBUS connectivity

Hi , I have a limited knowledge of MODBUS. I have been asked about connecting a device that needs to be able to listen to 2 way traffic between a master and one slave but for reasons can not be set up as a slave and canot be a master as there is already one on the MODBUS. Also is there a site you could recomend where I could get a practical tutorial?


I may be wrong but the issue is that the master PLC and slaves are already set up but we now want to extract data from the master back to another system. Ethernet would be an answer but would require difficult new cable runs.

Here are devices that sit on the serial line which allow monitoring.

Here is a tutorial

For Modbus RTU on a serial line, only one master is allowed, meaning that if you want data from the master, the master has to write that data to a slave.

One work-around might be a serial sniffer like Bernie mentions.

Another might be a bus controller for Modbus that allows more than one Modbus master

Miille Applied Research does that kind of thing:
http://www.miille.com/stdprod.htm

He pops up on control.com with suggestions for dealing with Modbus where multiple are needed.

Dan

Thankyou

Hi
Thankyou Bernie and Danw, this was very helpful. I shall spend some time with the tutorials and try to get up to speed.