RS485 Communication Error

Hello afifahasan,

Are you configuring the port as a master or a slave? Which way is the flowmeter configured?

How long is the drop? Are the connections properly terminated?

Is the flowmeter connected when you power up, or is it connected afterward?

How is the flowmeter powered? Is it possible you have a ground-loop problem?

hi europium,

the port at the flow computer is being configured as master and the flowmeter is used as a slave.

the connections are checked and are terminated properly and there's no issue of long distance as the wire is 1.5 m long at max.

the flowmeter is connected after first configuring the FComputer to communicate on rs-485.

the power to the flowmeter is through the flow computer and there is the least possibility of nay kind of temination or grounding errors.

Any solution ??

Is there any chance that configuring the computer end of the wire is tri-stating the driver?

Try this: Disconnect the flowmeter from the cable, connect a 100-ohm resistor across the cable-connector pins and watch the pins with a scope while you configure the computer-side port. Please let me know what happens.

RS485 works by sending +/- 3ma along the T+ and T- lines. There may be 5V out there referred to gnd, but you don't care about it. When you configure the port, it could be putting some resistance across the lines and changing the 5V, which you aren't interested in anyway. Usually when connecting a 485 link, the problems are:

1. T+ and T-, or R+ and R- reversed

2. Baud, stop bits, parity

Review these carefully and you will probably be working fine.

LG_DAVE

Hi Afifahasan,

You said "5V is available for 485 signalling", and then that, "the power to the flowmeter is through the flow computer"; also that "the signal is lost".

Is this synchronous or asynchronous comms.?

Does the actual power supply for the flow meter disappear, or, is it just the signalling pair going tri-state as Europium suggests.

I'm just shooting in the dark here, but "master/slave" sort of indicates half duplex, in which case maybe the computer has polled the meter and is waiting for a reply.

On the other hand LG_Dave is talking (and almost certainly knows more about this sort of application than me) about a transmit pair and a receive pair, which kind of indicates full duplex.

Do you have the pin-out of the connectors?

What do you mean by "the signal is lost"?

Have you added the 120 Ohm resistors? And check the wiring please.

Thanx alot for everyone's comments!

the succcessful communication has been established finally, and the main problem was related to the swapping of the pins, i.e. T+,T-and R+,R-.