Re: Network

From: Andre Fachat (
Date: 1998-04-16 15:36:28

John West McKenna wrote:
> >> The physical layer could be derived from the RS485 specifications.
> >> The original RS485 specification uses a terminated bus, and allows
> >> one master to send at any time only. Otherwise drivers could be damaged.
> >Can you give me in a few lines some more specifications, please? How many
> >lines does it use? Why can the drivers be damaged? What ICs does it use?
> Unless you're doing really stupid things (like putting mains voltages on
> the data lines), you can't damage the drivers.  Part of the specification
> states that the drivers must be able to cope with certain conditions (from
> memory, it includes a shorted bus, and other active drivers) without
> damage.

Seems to be better than what I got from comp.arch.embedded (or was
it alt.comp.hardware.homebuilt....
I got the sheets from natsem (it's and will have
a look at them.
I still wonder if something like the schematics I described in an 
earlier mail works with multiple nodes. After all, there is some
(software) protection.

> Since there is only one pair of wires, only one device can be sending data
> at a time.  You need some software protocol to determine who gets to talk
> when.  Nothing gets damaged if two talk at once, but your data won't get
> through.

That is understood.

> Just remember that RS485 likes to see a simple cable with devices hanging
> off it, and a terminator at each end.  No branches, and definitely no
> loops.

uhm. Maybe a silly question, but neither the TI nor the Natsem docs
tell anything about the terminators at the end. Are they between the 
two data lines (I assume so), have they connection to the ground or what?
And what value does it have?

What type of cable are you using? unshielded twisted pair (telephone
wire i.e.) should be enough, probably with a third wire for ground,
right? Are there any standard connectors?

Thanks for the info. Now the network seems to be feasible at least.

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Andre Fachat, Institute of physics, Technische Universitšt Chemnitz, FRG

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