Modbus and cyclic transmission


Automation Linse

Maybe you're not asking exactly what you want. Cyclic is no problem - a device can act as Master (especially easy on TCP/IP medias) and write data cyclically. Or a remote master can poll cyclically.

What Modbus cannot do is the IEC870/DNP3 style of Master-initiated "report-by-exception" where the Master just asks/polls the Slave "Anything to report?" and the Slave uses it's own condition to return data as appropriate. This allows a natural priority in data returned.

With Modbus, the Master MUST ask for some specific data range, even if the values are not important and something else much more important has happened. Some Vendors simulate this by packing some status bits in a common place to poll, so when the remote Masters sees certain bits go high, it knows to poll some important data that has changed. But this is a Vendor and Master-specific method an likely your average OPC-style master could not be so flexible.

best regards
- LynnL,
Plain serial MODBUS is a strictly Query-Reply protocol. In other words, if the Master does not send a Query, the Slave is not supposed to "talk back".

What exactly do you mean by cyclic transmission? Is it continous polling -- sending a request continously after an interval?
I already replied via the Moderator - but I'll expand.
If you mean by cyclic that a station sends data at regular intervals without being asked (queried) each time to do so, then this is called in MODBUS terms "unsolicited". MODBUS is defined as precisely one reply for each correctly addressed query.

On the other hand, there is nothing to prevent a device to accept and process an unsolicited message in MODBUS format - if it is programmed properly to do so.
The difference is between MODBUS (a defintion of message formats and procedure) and a device being programmed to process them in a special way.