ePanorama.net - Data communications

Making RS232 line to work like a BUS

From: dan@dyndata.uucp (Dan Everhart)
Newsgroups: sci.electronics
Subject: Re: Low power multipoint RS-232 ideas?
Date: 24 Sep 91 19:16:27 GMT
Reply-To: dan@dyndata.celestial.com
Organization: Dynamic Data & Electronics
In-Reply-To: henry@zoo.toronto.edu's message of 16 Sep 91 19: 58:52 GMT
In article <1991Sep16.195852.21894@zoo.toronto.edu> henry@zoo.toronto.edu (Henry Spencer) writes:
> You forgot at least two important design criteria:  what data rate, and
> what distance?  If neither data rate nor distance is important, you can
> probably get away with the diode-resistor-capacitor solution recently
> posted.  When the specs start to get demanding, you start needing active
> components, and that's going to be hard because of item (2); you really
> can't get very much power from the RS232 lines.
The data rate and distance were the free variables. :-) The cost and power demands were what needed to be minimized. Although I should say that in a commercial device, the cost of components is a lot less than the case, connectors, packaging, assembly.... Let's say under $5 of components. You can get a lot of components for $5, including active ones, as Henry mentioned above. (Including active components makes the design a lot more interesting too.)

I forgot to mention another design criterion: The interface medium is RJ-11 connectors and grungy phone cable. (Not even-twisted pair!)

I missed the "diode-resistor-capacitor" solution. Could someone please forward me the article?

I've seen one commercial solution which used only one component. It was a reed relay with RTS and DTR driving the coil. RX and ground were connected to the network medium, and TX was connected to RX through the relay contacts.

Put RTS and DTS in opposite states when you want to transmit, closing the relay contacts, and connecting your TX line to everyone else's RX lines.

I don't like this solution because of the slowness of the relay. I suppose you could use FETs.

   _                                         dan@dyndata.celestial.com
  / \_   Dan Everhart                        uunet!{camco,fluke}!dyndata!dan
  \_/ \____________________________          206-743-6982
  / \_/                                      7107 179th St SW
  \_/    Dynamic Data & Electronics          Edmonds, WA 98026, USA

Is David Byrne really Carl Sagan's illegitimate son?

From: stevem@specialix.co.uk (Steven Murray)
Newsgroups: sci.electronics
Subject: Re: Low power multipoint RS-232 ideas?
Date: 16 Sep 91 13:14:58 GMT
Distribution: sci
Organization: Specialix International, London

dan@dyndata.uucp (Dan Everhart) writes:

>I'd like to hear if anyone has any circuits or ideas for hooking
>together several (4 to 6) RS-232 ports in a multipoint configuration.
>When any transmitter sends, all receivers should receive the data.

>Three design criteria:

>(1) Low-cost.  The circuit should use cheap and widely available

>(2) Low-power.  The circuit should be able to run off power stolen from
>the serial ports' DTR or RTS lines.

>(3) Modular.  I'd prefer a bus configuration so that each serial port
>connects to an adapter which connects to the bus.
Yes, I've been thinking about this too. Obviously RS485 has all the technical merit going for it. If you want to connect (say) little microprocessor-based units on a control-communication bus, then RS485 is the way to go. In practice however, there is just so much RS232 gear out there, with cables that aren't twisted pair already laid, modems in place (etc) that making such a bus using RS232 would get you into a lot of existing installations.

My thoughts had been moving toward using a circuit like:

              1N4148           1nf
             ____|/~|____________| |_____
             |   |\_|      |     | |    |
             |            <             |
             |             >            |
             |            <  15k        |
             |             >            |
             |             |            |
 1488 Out ___|____|~\|_____|____________|____  'Bus' serial line
                  |_/|     |
 1489 In __________________|
Okay, so its not exactly the most sophisticated circuit in the world - but with 5 devices, you would get bus mode operation. What ideas have other people got? Lets get some discussion going!

-Steven Murray

Steven Murray
uunet!slxsys!stevem  stevem@specialix.co.uk
I am speaking, but  | The art of efficient design is knowing and using
not for my employer.| good compromises