🚀 go-pugleaf

jdc@isis.cs.wayne.edu (Jon Cardwell) writes:
+---------------
| We're looking for sources or references for a "multi-drop serial protocol"
| i.e. one that allows multiple devices to be attached to the same serial line.
| Our goal is to have data exchanged between a host cpu and any number of
| ( well ok, 8 :) attached slave devices. ... Any help (even remote references
| from 15 years ago) would be most welcome.
+---------------

If you can run sync instead of async and you're willing to do some
coding, Localtalk would be a good protocol for this sort of thing -
it's fast (in comparison to async), it only needs an 8530 and some
simple driver hardware, and you can take advantage of an existing base
of machines that talk it.

If you need to do async (e.g. your host only has async ports) then any
poll-response protocol would do - you just need to ensure that each
poll has an address in it, and require that slave devices only
transmit in response to a poll.

				Peter Desnoyers
-- 

>jdc@isis.cs.wayne.edu (Jon Cardwell) writes:
>+---------------
>| We're looking for sources or references for a "multi-drop serial protocol"
>| i.e. one that allows multiple devices to be attached to the same serial line.

RS-232 is not a multi-drop protocol *electrically* because it holds
the line at the mark (or space) state.  You need the RS-485 protocol
to multi-drop devices, because it lets the line float if a device
has nothing to say.  There are 232 to 485 converters available (or
there were 7 years ago).  Then you can implement a master-slave
software protocol for the universe of devices attached to the line.

In ISO terms, RS-232/485 is barely a physical layer specification.
You get to do anything you want with the rest of the layers.

Bill Hawkins

In <1993Jan24.181433.26225@rosevax.rosemount.com> bill@texan.rosemount.com (William Hawkins) writes:

| >jdc@isis.cs.wayne.edu (Jon Cardwell) writes:
| >+---------------
| >| We're looking for sources or references for a "multi-drop serial protocol"
| >| i.e. one that allows multiple devices to be attached to the same serial line.
| 
| RS-232 is not a multi-drop protocol *electrically* because it holds
| the line at the mark (or space) state.  You need the RS-485 protocol
| to multi-drop devices, because it lets the line float if a device
| has nothing to say.  There are 232 to 485 converters available (or
| there were 7 years ago).  Then you can implement a master-slave
| software protocol for the universe of devices attached to the line.
| 
| In ISO terms, RS-232/485 is barely a physical layer specification.
| You get to do anything you want with the rest of the layers.

Wyse makes (made?) both 485 terminals, and 485 drops to rs232 protocol
boxes (2 and 8 232 ports per box).  I don't remember if they ever made
the controller boards.  This was done about 5 years back, originally as
a joint project with Altos.  I did a lot of the driver work, and it
worked pretty well (we ran 64 terminals at full speed both directions
off a 286 box).  As I recall, the datarate was about 1.4 Mbits/sec on
the multidrop wire. 
--
Let no one tell me that silence gives consent,  |   Dave Olson
because whoever is silent dissents.             |   Silicon Graphics, Inc.
    Maria Isabel Barreno                        |   olson@sgi.com

In article <1993Jan24.181433.26225@rosevax.rosemount.com> bill@texan.rosemount.com (William Hawkins) writes:
>>jdc@isis.cs.wayne.edu (Jon Cardwell) writes:
>>+---------------
>>| We're looking for sources or references for a "multi-drop serial protocol"
>>| i.e. one that allows multiple devices to be attached to the same serial line.
>
>RS-232 is not a multi-drop protocol *electrically* because it holds
>the line at the mark (or space) state.  

Hogwash...

RS232 has been used for years with BSC 3270 multi-drop circuits. Generally,
the trick is to use the fastest modem turn around time possible to get best
results. The "poll-ack" protocols are above this physical standard.

Remember, multi-drop does not mean a "bus architecture" in the strictest
sense. One orders a multi-drop circuit from the Telco. I have not been
involved with BSC 3270 circuits for many years (about 15) so I have no idea
how one would go about building an in-house multi-drop circuit....


-- 
---
Michael Burke

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