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Re: RS-485 termination



Hi

Could you please explain the difference in the two GND pins on the RS485
connector. Which is Ground?

Regards
Cedric
----- Original Message -----
From: "Johan Adolfsson" <johan.adolfsson@xxxxxxx.com>
To: <cedric.stevens@xxxxxxx.com>
Sent: Tuesday, October 05, 2004 9:43 AM
Subject: 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
>
>
>