Yes, we had a similar problem in a machine shop with one computer talking
to the NC machines.  Eventually this was solved by using a RS232/Ethernet
bridge that was installed close to the machines.  But I would personally
recommend optically isolated RS422 for point to point. (That would be 4
wires).

When I design with RS485 I use 3 wires (1 gnd, TDA, TDB) and each device
connects to the ground with a serial 100ohm resistor, this limits the
ground currents.  I put a 100 ohms across TDA and TDB on each end of the
cable to match impedance and a 560ohm pull-up on TDB, 560ohm pull down on
TDA for failsafe biasing.


On Sat, 29 May 1999, John wrote:

> A recent post mentioned running RS422 full-duplex with a 5-wire cable.
> Is the 5th line for ground?  I've assumed (uh oh!) one of the benefits of
> RS422 and RS485 was the absence of a common ground running a couple
> thousand feet (ground loops, noise, etc).
>
> A friend working as a network consultant has a client who recently moved
> into a new and much larger facility.  A PC communicates via RS232 to some
> uP-based I/O boxes out in the warehouse (cable run was <50ft. now is
> ~150ft.).  The PC's serial port has blown several times since the move.
> The PC and the remote I/O boxes are on separate breakers, but
> the electrician says all is OK with AC service and grounding. Their plan
> is to convert to RS485.  My serial comm experience has been with RS232,
> so I can't offer anything beyond the MAXIM 485 data sheets, etc.
> Has anyone dealt sucessfully with this symptom by replacing RS232 with
> 485?
>
> John
> ______________________________________________________________________________
>
> John Leonard, instrument technician                        Electron Hacker
> College of Science & Engineering                           Bit Banger
> Saginaw Valley State University                            Bassist
> University Center, MI USA  48710                           Dad
> _____________________________________________________________________________
>