RS232-RS485

Tron9000 · Aug 24, 2011

First post, so here goes.

Apologies if this is not in the right category.

I've been asked to design a RS485, multidrop network to interface with data terminals (DCE) that spit their information out in RS232. The problem is I haven't touched anything to do with RS485 since uni - 5 years ago!

The reason behind using RS485 is that we may need to connect more DCE's at a later date.

I've read up a little about it here:http://www.lammertbies.nl/comm/info/RS-485.html, but am still a little fuzzy on the subject and wikipedia hasn't helped at all (goes mostly into the history if anything!)

I've been looking at RS232 - RS485 conversion by searching the web and have found very little.

My question is:
Am I right in thinking that you can't just plug in an RS232 DCE on a RS485 network? (from what I understand slew rate would be a problem of one)

And is there a simple solution - IC, circuit with as few components as possible to convert RS232 to RS485?

I did find this circuit: http://www.rmv.com/232-485.pdf - would this be a valid solution?

Has anyone done something like this, and if so, what pearls of wisdom can you offer?

Many thanks!

(*steve*) · Aug 25, 2011

It's a long time since I was doing any work with serial networks (thinks back to running terminals over lines several km in length).

I'm almost 100% certain that you need a converter fro RS485 vs RS232. I'm not sure that RS232 would cope very well in a multidrop situation because it's simply not designed for that.

Sorry I'm not really helpful, but I wanted to make sure you knew you weren't being ignored

Laplace · Aug 25, 2011

Check out the technical library at B&B Electronics.
http://bb-elec.com/technical_library.asp

They also have all the 232/485 interface products that you might need. Interfacing a 232 device to a 485 device can be fairly simple; however, interfacing a 232 device to a 485 network can be tricky. RS485 uses tri-state line drivers whereas RS232 does not, so the logic in the RS232 device needs to account for that. Otherwise you will get signal collisions like on a shared Ethernet bus (does anyone still use those?)

Tron9000 · Aug 25, 2011

Thanks, that's helped a bit more to understand.

JoeyAVR · Aug 25, 2011

I wouldn't be surprised if Maxim do a chip that would help.

Well whoopy-doo, this should answer your prayers:

http://www.maxim-ic.com/app-notes/index.mvp/id/723

Tron9000 · Aug 25, 2011

Excellent!

I knew an IC solution was around

Tron9000 · Aug 26, 2011

Just hada a look on http://www.findchips.com

its quite a rare IC.....not to mention a bit expensive.

Reckon it could be done by a mirco controller?

JoeyAVR · Aug 26, 2011

A micro can do protocol translations, but won't handle voltage level shifting.

RS232-RS485

Tron9000

(steve) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

Laplace

Tron9000

JoeyAVR

Tron9000

Tron9000

JoeyAVR

propblem with RS232 to RS485 Pleased help!

RS232

rs485 communication

Rs485 to Ethernet & ethe to rs485

Compitable RS485 Transceviers

RS232

cheap RS485 device

Help with RS485

RS232-RS485

(*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

(steve) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator