As i understand it, it would be possible to have the master send a request to a device and then the device could postpond the respons until input signals changes (or a timeout)? This could reduce the network load on the network.

Are there any guidelines in the spec for implementing devices according to this? Are there any devices that operate as described?

 

If you have a plc on each end, you could use the XMIT block to send data from one plc to the other. The logic could be setup so that the XMIT is executed only if the source data has changed values from the previous scan.

 

Micke:

As far as I know, Modbus has no provision for this, therefore it can't be done within the protocol.

However, since you're on TCP, you can have two Modbus connections, one going each way; each device would be both a master and a slave, and the master part can send data to the other device at will.

Jiri
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Jiri Baum <[email protected]> http://www.csse.monash.edu.au/~jirib
MAT LinuxPLC project --- http://mat.sf.net --- Machine Automation Tools