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Re: RS-485 termination




----- Original Message ----- 
From: <cedric.stevens@xxxxxxx.za>
To: "dev-etrax" <dev-etrax@xxxxxxx.com>
Sent: Tuesday, October 05, 2004 8:29 AM
Subject: RS-485 termination


> Hi
>
> We are testing RS422/RS485 communication using the Axis 83 devboard. We
seem
> to be able to communicate using RS485 4-wire without terminating any of
the
> devices (which should not be possible, even if we are testing over 1m of
> cable ??). The datasheets (if I am using the correct one) shows that the
> Axis83 uses a Max3483 driver for the ttyS3 synchronous port.
>
> My questions are:
> 1) Does the Axis83 use the Max3483 tranceiver? Which are the correct
> datasheets for the latest version?

The devboards 82/83 uses either LTC1480CS8 or LTC1480IS8 from Linear
Technology or
ADM3485EAR from Analog Devices.
To be sure you need to open the box and check. One of them is specified to
support 10 Mbit/s, the other 2.5 Mbit/s if I recall correctly.

> 2) What about termination. Do we have to terminate our tx/rx pairs?

I guess that depends on your wiring (cable length), and your baudrate,
I don't think it's an absolute requirement.

See e.g. :
http://www.bb-elec.com/tech_articles/rs422_485_app_note/system_configuration.asp#termination

Assume propagation delay = 1/(0.66* c) = 50 ns/m
baudrate: 6250000 gives the bittime  160 ns
baudrate:  115200 gives the bittime 8680 ns

If you were to use 2-wire, you might need biasing resistors on the wires
though
(either weak pullup and pulldown and no termination, or strong pullup and
pulldown
and a termination resistor)


> If not,
> how do we remove termination as this will limit the capabilities of the
> RS485 device.

As far as the devboard 82/83 there is nothing to remove.
If you have other devices connected, check their datasheet/manuals.

> 3) Or are we simply experiencing fantastic operation of the
> Max3483 chip under bad (incorrect) termination conditions?

No, I dont think there is anything fantastic about it working with
1 m of cable, unless you mean 1 mile or something with 1m.

> Regards
>
> Cedric

/Johan