RS-422 to RS-485 communication

  • Thread starter Suresh Kumar Devapiriyam
  • Start date

Thread Starter

Suresh Kumar Devapiriyam

Dear list,

We have an application where a RS-422 communication port in a Gas
Chromatograph has to communicate to RS-485 modbus port in a DCS.

Does this conversion require a RS-422 to RS-485 converter or can it be
achieved through some wiring changes. RS-422 port has got 5 pins
(Receive-2 nos, Transmit-2nos and Ground) and the RS-485 has got 3 pins.
(Receive, Transmit and Ground)

The supplier who is responsible for this conversion is not able to give
a clear answer. Any help from you will be highly appreciated.

Thanks in advance

With Warm Regards
D.Suresh Kumar
Chennai, India

Darold Woodward

In actual practice, there is very little difference between EIA-422 and
EIA-485 (same as old RS designation). The signal levels and specs are
identical making it possible for them to talk together. The difference is
that 422 was designed to be point-to-point and 485 is designed for
multidrop. To that end, a 485 device can put its transmitter in a
tri-state mode to allow other transmitters to talk on the network and allow
the ultimate network size of 32 nodes. If you have a 485, 4-wire port, you
can connect to 485 4-wire, 2-wire or 422.

Chances are quite good that depending on how your 422 end works that you
can simply short the Tx+ to Rx+ and Tx- to Rx- and connect the 422 and 485
devices together. You will need to talk to the Gas Chromatograph people to
find out when their transmitter is on and when it is off. Also, recheck
your information from the DCS hardware, the PLC Modbus adapter cards that
I'm familiar with like Prosoft and CTI which support 485 allow you to go
either 4-wire or 2-wire which solves your problem.

I also suggest that you or anybody else doing a 422 or 485 application
check out the excellent B&B Electronics RS-422/485 Application note at: B&B Electronics
is also a good source for any converters you may need.

Darold Woodward PE
SEL Inc.
[email protected]
I believe the 422 is a point to point protocol and the 485 is a multidrop.
The conversion requires a protocol converter. Also check the baud rates.

David Lee
SiteScan Project Coordinator
Liebert Global Services
(614) 841-6310 phone
(614) 841-6315 fax
(800) LIEBERT pager

Maybe the following information will help you finding the solution for
your problem.

RS485 is a standard which describes the signal levels on a bus system.
So which voltage equals logic "0", or logic "1".

The main difference between RS485 and RS422 is that RS485 can handle 3
states instead of 2 states, knowing:
* Logic "0"
* Logic "1"
* Tri state (not "0", not "1", the line is available for other
communication partners)

>From hardware point of view, it shouldn't be a big problem to wire one
device to antoher.

BUT: Just the hardware is not enough to do the job.
With a bus system, all communication partners can send or receive to =
partners, using the same two wires.
Of course, if all more than one station send different information at
the same time, this will result in chaos.
That's why you need a protocoll which defines when a station is allowed
to send.
One of those protocolls is Profibus. In this protocoll there are
definitions when a station is allowed to send, and what to do if for
what ever reason two stations had a conflict anyway (e.g. after

You first had one problem "What about my hardware?".
Now you got two problems, "What about my hardware? and Which protocolls
do I need?".

By a better definition of your problems, the suppliers must be able to
give more accurate answers.

Good luck.

With kind regards,
Mit freundlichem Gru=DF,
Met vriendelijke groeten,
Bonne Salutions,

Wim Vrinds,
Software & commissioning engineer,
System Controls Division,
Stork BP&L B.V.
At the Gas Chromatograph side, RS485 port, you should connect a 100 ohms
resistor between line Tx- and Rx-, and another 100 ohms resistor between Tx+
and Rx+.

Then you should connect the DCS line RS422 Rx with point Tx- and the Tx
with Rx+.

good luck
I assume your RS-485 3 pins are (+, -, G) or (A, B, G) and not Recv, Tran, Ground.

The problem with just shorting the RS-422's T+ & R+ and T- and R- together is that this creates an "echo" in the RS-422 device. Everything
it transmits it will receive back. If the Gas Chromatograph manual shows multi-drop solutions, then it MAY handle this Ok and ignore the echo
(suppress receive while transmitting). If all the drawings show a point-to-point link, then it will not likely work.

It is easy to test. Just do it, then watch the response. If the Gas Chromatograph only sends 1 or 2 chars in reply & then stops, then it is
seeing the "echo" and assuming the master has started a new poll & aborts it's reply. If this is the case, then you'll need a physical
converter to convert the full-duplex 4-wire into a half-duplex 2-wire and suppresses the echo back on the 2nd wire pair of the RS-422.

I had to do this with a set of Philip weigh bridges once - they had only 4-wire RS-485 and the slave shut up the instant it saw the echo. You
could see exactly 1 and only 1 char of the response each time.

Best Regards

Lynn August Linse, Senior Product Application Engineer
15353 Barranca Parkway, Lantronix Inc, Irvine CA 92618
[email protected]
Tel: (949)450-7272 Fax: (949)453-7132

List Management Account

---------- Forwarded message ----------
From: "Mecene Desormice" <[email protected]>
To: [email protected]
Subject: RE: COMM: Serial communication


My name is Mecene. I am junior progrmmer. I am working on project involving
serial communication programming (console). I would like to know what
compiler (c/c++) should I use?

Thank in advance for your conditions et your response is important for me.
Dear D.Suresh Kumar,

You could use a Prosoft 3150MCM module which operates modbus on both RS-422 and RS-485 communication. It is jumper selectable.

The card has two ports, you can connect one to your DCS and second to your Gas Chromotograph.

You can contact Prosoft for more details or your local distributor.