> Can anyone fill me in on the technical details of AB DH485 ?
Depends how "technical" you need. It's one of two serial protocols AB use on their PLCs, the other being DF1. As AB normally use it, DH485 is electrically based on the RS485 2-wire system i.e. differential pair with bidirectional transmission. The data side of the protocol uses standard NRZ data such as you'd get out of a normal PC serial port or UART. It is a token passing network with each machine on the network being allocated it's own node address, usually from 0 to 31.
The more modern AB processors tend to now just have an RS232 port which is configured by default to run their DF1 protocol, which is more of a point to point idea. What many people fail to realise is that with the modern AB software, this port on say a SLC5/04 can be configured to run DH485, even though electrically the port remains RS232. Why bother ? Well if you add an AIC+ module for around $100 you can convert RS232 back to RS485 (whoops, I mean DH485) and regain a multidrop port.
For our HMI range we have both DF1 and DH485 protocols, but I personally prefer the latter, as it gives much more flexibility.
Sorry, can't help you with the data structure. Although we have this protocol for our HMIs, I believe it's under license from AB, and therefore probably not in the public domain. If you do come across it I'd be interested though, as from time to time we need "Specials" for which we program some microcontroller based black boxes of our own design, doing things like protocol bridges i.e. Modbus back to back slaves etc.
Similarly if I do come across it, I'll let you know, but beware copyright !!!
Correct me if I'm wrong, but it has been my understanding that DH485 is just the DF1 protocol used on RS485. If that's the case, DF1 specifics are available on the AB web site as publication 1770.6.5.16
> Correct me if I'm wrong, but it has been my understanding that DH485 is just the DF1 protocol used on RS485. If that's the case, DF1 specifics are available on the AB web site as publication 1770.6.5.16
DH485 is definitely a different protocol. We've used it over both RS232 and RS485, but it's definitely not the same as DF1. For expample DF1 is point to point, but DH485 supports multiple nodes. I'll check the AB web-site, but as far as I know only DF1 is published.
