RS-422/485 Communication

  • Thread starter Waddell, Reginald E
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Thread Starter

Waddell, Reginald E

I am setting up a communications link between a Honeywell TDC3000 DCS system and a Bently Nevada vibration monitoring system. The Honeywell system
communicates via MODBUS RTU protocol using RS422/485. The Honeywell configuration of the RS422/485 link uses a two wire connection (TX+ and RX+ are tied together; TX- and Rx- are tied together), in a master-slave, half-duplex configuration. The Honeywell system is already communicating with two other devices (non-Bently), in a daisy chain configuration,
successfully. My problem is that when we connect the Bently system (in a two-wire configuration) to the communication link, all communication stops. When the Bently is disconnected, communication resumes. Anyone experienced this problem before? Any suggestions as to how to solve this problem?

Reggie Waddell
BP Amoco Chemicals
[email protected]

(TECO) David Bergeron

Is it possible that the Bentley Nevada system is also acting as a master? If it is that would certainly kill the communications because you can only have one master on the link.

David Bergeron, P.E.

Jerry Herman

Make sure that the Bently Nevada vibration monitoring system disables its driver when it is not transmitting. Also, test whether the Bently system responds only to request at its address.

Arnold Dillon

MODBUS communication can be supported on either a 2 wire hook-up, or 4 wire connection, and on either an RTU protocol or another serial protocol. The fact that the Honeywell system and the other existing devices work when connected
indicates that their wiring and protocol are matched. If the Bently system shuts everything down, then you need to verify that the protocol is properly matched; that the Bentley system is not trying to act as the MODBUS Master; that the Bently system is not duplicating an existing address, and the the wiring required by the Bently system is 2 wire and not 4 wire.

Hope that helps.

Arnold Dillon
I.C. Thomasson Associates
Just some experienced guesses:

1) First make sure their "+" is the same as Honeywell's - many times I've had to connect company A's "+" to Company B's "-". If the Bently includes a line bias, reverse wiring could stop the data comm.

2) If the Bently "expects" a 4-wire connection, then it may not like the "echo" shorting the TX pair to RX pair creates. Devices designed for 2-wire mode either disable their receiver or flush the echo received. This shouldn't "stop" all data comm, but will cause the Bently to be confused.

3) Take a look at this app note - about
trouble-shooting RS-485 circuits. It may be that the Bently interface is drawing too much power - loading the line to death.


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

Darold Woodward

As you may already have surmised, transmitter control is critical on a two-wire EIA485 system. The same pair of wires is used to transmit and receive data. If you are using a converter on the new node, it is important to determine what
transmitter control mode it is operating in and that the slected mode will work properly with the serial port you are using.

If there is an EIA-485 port on the Bently equipment, you should verify that it is configured to operate in a 2-wire handshaking mode. If there are 4 wires available on the Bently equipment, there may be a handshaking selection and a selection for a termination resistor.

The other consideration is that as your network grows, so does the capacitance. This means that you definitely need to install the required termination resistors to bleed down residual voltages otherwise some devices may not turn
around properly and transmit. There is a good application note on proper construction of the network at the B&B Electronics web site at

Darold Woodward
SEL Inc.
[email protected]

Sanji Ekanayake


Which model Bentley-Nevada TSI are you using?
We have implemented B-N TSI to Intellution FIX SCADA system interfaces at multiple Power Plant Steam Turbine vibration monitoring applications and experienced a few unexpected hurdles.
Note that on the B-N 3300/03 there are jumpers to configure the B-N Serial data interface on both the Serial interface module as well as the Power
Input module. You may also want to test the B-N connected directly to the TDC3000 to check for cable configuration .

Sanji Ekanayake
SE-ACE Innovations, Inc.
[email protected]
(954) 584-9525

David Diaz Burgos

Have you checked the RS485 addresses?
What about termination resistors?
Have you tried using a data spy to see the frames?

David Diaz
General Manager
Xcontrol Limitada
Calle 74 #15-80 Int.2 Of.310
Santafe de Bogota, D.C.
Colombia, S.A.

Phone: +571 3216315
Fax: +571 3216739
e-mail: [email protected]

Jeff P. LeBlanc

Also check the for terminating resistors in the middle of the network. If the device your adding has terminating resistors make sure it is on the end of the wire.
If you are using BNC 3300 series monitors as per the catalogue it supports only RS-422.It doesnot support RS-485.

Hi Abhishek,

I am not sure on what exactly you are looking for on smart transmitters and your kind of project?
As I know,Vendor catalogues gives you different types of smart xmtrs, their construction and principle of working, etc.You can refer some
standard vendor catalogues of Rosemount,Yokogawa,etc. or log on to their websites.

I am currently working on a project with the same interface.

After looking at the jumper requirements for the BN SDI board I find BN documentation to be good but not what I consider 100%.

Particularly confusing is the way they list jumpers to be installed versus ones to be removed. If a jumper is not installed, then is it not removed!?

Anyway. . .

I have two electronic govenors up and talking on one HPM SI FTA. On the other FTA I am only going to be talking to a single BN rack.

I have pinned everything on the SDI & pwr board according to my first review of the BN instructions, and I have the 4 wire BN com jumpered down to 2 for the SI connection.

Unfortunately I am still getting a MSGTIMOUT
error on the single SI Array point I've built for the BN rack. This tells me that my SI requests are not being answered.

[email protected]