RS485 Modbus

teh

Supporting Member

T

Join Date

Nov 2002

Posts

140

• Mar 30, 2006

• #1

Hi Folks,

Is that any "2 wire" to "5 wire" converter for modbus ?

Cause i have 5 Terminal for modbus module and the field instruments

have only 2 wire ( modbus )..


Regards

 

Jim Dungar

Member

J

Join Date

Apr 2002

Location

WI

Posts

1,152

• Mar 30, 2006

• #2

First, Modbus doesn't care how many wires you have.
Second, do you have RS323, RS485, or RS422.

 

teh

Supporting Member

OP

T

Join Date

Nov 2002

Posts

140

• Mar 30, 2006

• #3

At Plc Module I Have 5 Terminal .. Tx+, Tx-, Rx+, Rx- And Ground..

But My Instrument Only Have 2 Wire ..

So How Should I Do The Terminaltion?

Regards

 

bernie_carlton

Lifetime Supporting Member + Moderator

Join Date

Apr 2002

Location

Yakima, Washington

Posts

6,413

• Mar 30, 2006

• #4

The "2 wire" is created by tying Tx+ to RX+ as 1 wire and Tx- to RX- as the other wire. Look up the PLC's requirement for termination resistors. Then try this hookup with the closest "2 wire" instrument. If it works, add on another always moving the other set of termination resistors to the far end of the line. Of course like any other network the Modbus devices will have to be assigned distinct addresses.

Be careful about major differences in the grounding. You may wish to use isolation between the PLC module and the line.

 

Jim Dungar

Member

J

Join Date

Apr 2002

Location

WI

Posts

1,152

• Mar 30, 2006

• #5

You probably have RS485. Just connecting the wires, as suggested by Bernie, may not always work. Some drivers are not universal.

My favorite RS485 technical literature comes from Robust Data Comm.

 

teh

Supporting Member

OP

T

Join Date

Nov 2002

Posts

140

• Mar 31, 2006

• #6

Both Side "2 Wire" And "4 Wire" Are Rs 485..

 

bernie_carlton

Lifetime Supporting Member + Moderator

Join Date

Apr 2002

Location

Yakima, Washington

Posts

6,413

• Mar 31, 2006

• #7

As noted by Jim Dungar the method behind the hardware is not always the same. Though the protocol, Modbus, is defined, its method of implementation could be taking advantage of the seperated transmit and recieve channels which the 4 wire allows. If so it may not like the 2 wire hookup. That is why I suggested trying it just on one unit. I assume you don't have any information on the PLC side requiring one or the other else you wouldn't have to ask.

 

jolio ST

Member

Join Date

Oct 2004

Location

central CDC

Posts

292

• Mar 31, 2006

• #8

The instrument you are using, reads from the PLC module or writes to the PLC module? maybe a manual written for the instrument might have stated what it needs and what it does. Sometimes you just need Tx+ and Tx-, or Rx+ and Rx-, or as Bern said, TRx+ and TRx- (jump the positive connections together and same goes for the negatives).

regards
Sherine T.

 

MikeGranby

Lifetime Supporting Member

Join Date

Aug 2005

Location

York, PA

Posts

399

• Mar 31, 2006

• #9

If you jumper the 'like' rx and tx lines on the 4-wire device, the issues you're going to need to watch for are (i) does the 4-wire device turn off its transmitter when not sending? and (ii) does it turn off its receiver when it is sending? If it's a 4-wire master device, it may not meet either of these conditions...

 

zova

Member

Join Date

Jan 2006

Location

Russia, Komi

Posts

118

• Apr 2, 2006

• #10

repeater

I had the same problem some time ago.

You need in RS485 (two wires) to RS422 (4 w) repeater.
I use I-7510a.

 

danw

Lifetime Supporting Member

Join Date

Oct 2004

Location

midwest, USA

Posts

3,427

• Apr 2, 2006

• #11

It's worth trying it as a 2 wire, half duplex link by jumpering as advised above.

Some companies, like Honeywell, provide all 4 terminals, but only implement half duplex (2 wire), and state as much.

Dan