RS485 4 Wire to 2 Wire for Micrologix 1100

69FIREBIRD

Member
Join Date
Oct 2004
Location
New Zealand
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102
Hi all, I have a project where I am going to be talking to a power revenue meter via Modbus with a Micrologix 1100 and 1763-NC01 on channel 1.
I have received the meter specs and it advises that it is a 4 wire RS485 comms configuration. The Micrologix 1100 is a 2 wire RS485 configuration.

How do I hook this up? -
- Can I hook up just one pair on the 4 wire RS485?
- Can I parallel up pairs of the 4 wire connection and connect the paralleled pair to the 2 wire terminations on the 1763-NC01?
- Do I need a signal converter between the to RS485 if so what works?
- Any other options?

Appreciate your feedback.

PS the modbus registers come in as floating point and cover two registers per variable. How can I tie these together in the PLC to get the actual valve?

Modbus FP Info.png
 
Hi Stu, Thanks. Goggled around a bit myself. I'll had seen comments that the signals are reflected back and can cause comms issues - will have to test and see if it will work.
 
As to the Data Format, once you get Communications going and have Data to work with. You can use a Copy Instruction to move the Data into a Float, and see if it is correct. Otherwise, you will have to do some Data manipulation to get it in the right order.

Stu....
 
Stu's diagram shows the wiring, plus jumpered to plus, minus to minus.

2 wire is inherently half duplex. One side talks, the other listens.

4 wire RS-485 could be full duplex, not necessarily, but it could be. It's protocol dependent.

If it's Modbus, it's not an issue, because of the master/slave query/response format.

But if it's some other protocol, and the power meter decides to talk while it is receiving some data, because it expects full duplex functionality, then your comm doesn't work. Collisions. Bad data. Retry. Time out. Etc.
 

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