Re: Network

From: John (
Date: 1998-04-16 20:46:43

>> 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

Differential signals on a single twisted pair.  This gives excellent noise
immunity, since both wires will pick up roughly the same noise, and the
difference between them (which is your data) will still be the same.
Driver and receiver don't even need to use the same ground, as long as you
don't exceed the common-mode limits of the receiver (15V on the chips we

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

We use the 75176 tranceiver.  It has one receiver and one driver in an 8
pin chip.  Excellent little device.  There are lots of others (including
quad receivers and drivers).,, and should give you more numbers than you ever wanted.

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

>The only thing is I have no idea of is which hardware-driver to use to drive
>the bus. That's why I asked more info about the RS485. My idea is to use
>something like the 7406 but capable of driving a long bus (> 100m?) and at
>higher frequencies. I never had a good look at a networkcard but if someone
>can figure out what it uses to drive the bus, we have our solution.

For long cables and high data rates, you must use differential signals.  We
use RS485 on kilometres of cable with no trouble.  I've never done any tests
to see how fast it can go.  Maybe I'll find some time one day...

John West

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