DH485 protocol spec


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Paul Ventura

Does any know where I can get an Allen Bradley DH485 protocol specification. I've been unsuccessful in attempts to obtain one from Allen Bradley. Thanks.
DH485 is a proprietary protocol. You'd have to pay AB to license it. It's not an open protocol that somebody can just give you. DF1 is their free open protocol. Good luck, you'd pretty much have to pay an arm and a leg and be building alot of DH485 devices to make it worth the cost.

Unfortunately, a "minor" problem. This is just the DF1 serial spec - this is the MESSAGES which are transported by the DH485 protocol. This document does NOT explain how to do the actual DH485 transport. To my knowledge the token-passing scheme used in DH485 is not publicly available, so if you're trying to make your RS-485 device "play ball" with a string of Allen-Bradley DH485 nodes, this document will be of little help. I believe you'll either have to reverse engineer the DH485 transport - or if your need is small, AB does sell a real DF1 serial to DH485 converter - or see if your customer has the option of DF1 or DH485. For example, most MicroLogix PLC have a pure RS-232 port. If you enable the "DH485" mode, they are just streaming DH485 data by full-duplex RS-232 and you need that isolated AIC+ module to give you a safe RS-485 version of the data stream. Note that AIC+ does *NOT* convert DF1 to DH485 - the PLC *IS* sending native DH485 bits out as RS-232 and the AIC+ just is a media converter to get this to the DH485 RS-485 multidrop bus. regards - Lynn

Alex Pavloff

That's correct, but I believe its a little simplistic. DF1, DH-485, and DH+ are link protocols. According to the Allen-Bradley DF1, DH-485 and DH+ documentation, the command set is the same for each protocol.
That's true to a degree Alex -- one has to be very careful on what commands to use though between SLCs, PLC5s, MicroLogix and ControlLogix PLCs -- there are some that work across all AB PLCs but others that don't and one can quickly get themselves into a corner with a driver that only works with one family of PLC models if not careful of which of the myriad of commands to use. It gets even more twisted in that there are some commands that work with certain PLCs on one link protocol and not the other. John Weber