RS485 bus and auto-addressing

While mechanical switches are OK in "friendly" environment (IP20), but still there are problems with unreliable DIP switches for address selection.

However, in some harsh environment such as IP65, using an external mechanical switch for address setting is problematic. Opening the device in the field for address selection might (let the inert gas out and) let moisture in, not to mention the risk of damaging the seals by enclosure opening.

So there are legitimate reasons for avoiding mechanical address selection switches.

That's fine, except that "a couple BCD knobs" can be _very_ expensive in many environments (e.g. explosion-proof or water-proof requirements).

Except the address could be programmed using RS485 in a standalone setup.

It's also possible to come up with a design for auto detect that doesn't demand any additional reliability compared to normal communication.

True, but now you've added a device commissioning step that has to be done on a bench somewhere.

I was merely thinking of a way to eliminate switches and knobs.

Any reliability will be down environmental noise and judicious use of CRCs. The CRC for setting the address could even be different to one used in normal communications.

It would be relatively easy to incorporate all the RS485 and programming utility in a small enclosure with a few AA batteries for field programming.

It all depends on whether you want DIP switches or other means of setting the address?

I was thinking of simplicity where peripherals are dumb units, where all each unit needs to do is match an address and accept the data, and possibly transmit a reply.

The alternative is to program each peripheral with details of its functionality, to tell the master what it is, and have the provision for reducing effects of clashes and cope with power-downs / brown-outs (forgetting address) etc.

I've always liked the simple approach for overall system reliability.

We do it after the devices are installed. It is a pain, but it's only done once for the life of the device. I put an "autodetect" design on paper, but it got deprioritized, and would be almost as much trouble as programming the devices one by one anyway. You have to connect them one at a time, walk to the master controller, do stuff off a sheet, repeat lather rinse. And if you make a mistake, it's no fun to debug.

Sure, reliable communications and setup require a bit of extra software. However, dip switches and other configuration require a bit of extra hardware, and manual configuration on the bench requires extra operator steps. Getting a human operator involved is likely to introduce more errors in the system than having an automatic procedure.

I've never known a RS485 system put together by the client himself, and whilst my knowledge is limited to security, an installer will generally set the addresses as per the customer's needs on site. He'd also ensure that termination was appropriate to the wiring style of the network.

The installer wouldn't leave the job unless it was all working.

To be honest, the idea of plug and play automatic process on a RS485 network fills me with horror.

It doesn't have to be any different. Installer comes to the site, plugs in RS485 unit, and verifies that it is automatically detected and that everything is working correctly.

How many devices are usually in a building network ?

How fast do those device enumerate with the master ?

Is this discussion about a trivial function that takes a few minutes at most during install, and can be re-done automatically ever few hours with out intervention ?

Seems like a problem that is not a problem.

hamilton

The answers depend entirely on the situation, of course. Some networks are pretty much fixed after initial installation. Others support hot plugging by the user, and require new devices to be recognized and running within seconds.

It's really dependent on your application. Things like turn around time and maximum inter-byte spacing are just the beginning. How much data are you sending and how often at what rate. You know your problem domain better than anyone here.

Sure, but is this one of those cases? And I've seen some really nice 'O' Ring sealed and internally lubed BCD switches that would be pretty hard to screw up.

OK, but *I* don't have to worry about that and the OP is pretty sketchy on just WHY this is needed, or maybe I missed that part of the thread. You can cherry pick specific reason why my suggestion won't work, but for a shit load of cases, it's just fine.

But have you paid for any of those switches? :)

Yes, I have, and we do. Small price to pay for a reliable system.