RS422/485 is prety good, if done right, and with good quality shielded
cable. At least, in our works application, it survies OK in fields
genereated by a 15kW amp that it is used in (the antenna's are less than
50' away, and the building/rack's etc is/are unscreneed!) from kHz to a
couple hundred MHz.
That system has a "hearbeat" scheme. If a link does get corrupted, and
dosnt come back to life after a short period, the amp is shutdown by way
of contactors removing the 3-pase to the 50V PSU's. Brutal, but safe.
The ultimate serial control for "in high field" usage, is optical fibre
of course, plus you also get full galvanic isolation too.
Not as difficult as you might expect to do either. The parts are easy to
obtain (both in stepped index glass with FSMA or ST connectors, and 1mm
Plastic, FSMA or the HP push-n-click stuff) for the electrical/optical
interface, and not expensive. Also complete serial to fibre (and back)
interfaces too (some supporting handshake lines too!) if you don't mind
spending money.
We also use a lot of the 1mm plastic stuff within HV power supplies for
many kW pulse TWTA's, again, it's the technology for the job, both for
serial control data, and analogue signals (fast PPM coded etc.)
It all works very well.
73.
Dave G0WBX.
------- Original Message -------
Date: Wed, 21 Dec 2011 14:21:01 -0800
From: Jim Barber <audioguy@q.com>
Subject: [Amps] Making RS-4XX interfaces RF resistant
To: AMPS <amps@contesting.com>
Message-ID: <4EF25BCD.7090405@q.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Curious if anyone in the group had any strong feelings about how to go
about RF-proofing RS-4XX interfaces? (RS-232, 422, 485)
I have a couple of microprocessor projects in the works that will need to
operate in close proximity to strong HF fields. In normal operation,
loads
of Mix 31 ferrite on the twisted pair should keep the RF down to levels
that the RS485 differential can overcome, but in a few rare cases the
interfaces may just have to survive being saturated. The key word here is
survive. :-)
What I've done in the past is follow an old National (Semi, not Radio)
app
note that discusses using a small amount of series current-limiting
resistance and back-to-back zeners between each side of the differential
pair and ground. A newer TI app note states basically the same thing,
except they discuss using specific TVS (Transient Voltage Suppressor)
devices instead of discrete zeners. I've also seen designs that use
MOV's,
but the National note suggests they aren't as fast as zeners.
Any thoughts?
Thanks,
Jim, N7CXI
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