Difference between legacy and current MODBUS standard


Thread Starter



I am in the processing of evaluating options to interface to a MODBUS network running on the legacy MODBUS Over Serial Line standard. Is the new standard (Modbus Serial Line Protocol) backwards compatible?

If not, is there a hardware converter or similar available?

I can't find a definitive answer in the standards, modbus.org or these forums.


Lynn August Linse

You won't find a solid answer because it depends on the 2 products you compare. Few people have ever 'certified' their products to any of the standards - most companies test and adjust their products by working with other vendor's products. In other words, it was easy for company X to get in trouble with Modbus if they just create & test their own device with their own master. One really needs to test with other vendors products, adjust ones own design to make it interoperate.

The main differences are that the new standard is far more detailed and explicit, whereas the older PDF left a lot up to the imagination of the implementer. I would use the new standard, forget the older one (or use for reference only).
Thanks for the response. That was what I was feeling.. I take it that communication between a legacy device and a new modbus "compliant" device is probably not likely, let alone guaranteed?

Lynn August Linse

Actually, I would say it is VERY likely compliant. Modbus has thrived in a 'test against other common vendors' mode.

Yes, you will find small pockets of incestuous products where a company tweaks a master and slave product to be 'Modbus', yet not interoperable with our vendors.

But over all, Modbus vendors do NOT want such limited sales potential. Most Modbus vendors select Modbus explicitly because they want out of such binding contracts. They want to sell a slave-device (or master/OPC) and jettison all responsibility for 'the big-picture system'. They want to focus on their core competence, allowing Modbus interoperability to simplify their marketing, allowing other vendors to produce the products they do not wish to produce.