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USB Out to RS 485 2 Terminal Input

10/14/2016 3:56 PM

I thought about posting this earlier, but until I got the customer to agree to bring in the manufacturer to re-program their standard 4-20ma + Hart coriolis flow meter, I didn't feel comfortable end running the manufacturer for a $100 cable. So here is the question after the solution has been determined.

We have a new flow meter on a customer's process skid that is locked so we can't enter to reprogram the factory settings. (yes - this has been confirmed as the problem) Manufacturer of same provided a program for a PC to output from the USB port to the instruments RS 485 port to allow re-programming. We happily down loaded the program to my PC then proceeded to the instrument, only to find the RS 485 input was not a 9 pin D-sub, but two terminals. Said manufacturer is more than happy to sell us a $100 cable to allow this. We chose the other option - a free field visit by a trained distributor tech, as timing was essentially the same, and the cost far less. (warranty work)

Now, if we had wanted to go ahead and try it ourselves to save a couple of days, what is the method for connecting a 4 wire signal (RS 485) to a 2 terminal device. (i.e. - what is in that adapter cable in that bulge that we can't see?)

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#1

Re: USB Out to RS 485 2 Terminal Input

10/14/2016 4:13 PM

2 terminal input is a little vague....but I suppose you could work it out...

http://electronicdesign.com/what-s-difference-between/what-s-difference-between-rs-232-and-rs-485-serial-interfaces

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#2

Re: USB Out to RS 485 2 Terminal Input

10/14/2016 6:32 PM

https://en.wikipedia.org/wiki/RS-485

What the flow meter uses is half-duplex RS-485. You would need a USB to serial converter (many are RS-232) and then another converter to go from the single ended RS-232 to the differential RS-485 and all the drivers from the flow meter to each of the dongles you have to make this mess work.

It's a pain. I know. We have to it all the time.

You chose the best solution.

P.S. There's nothing in that cable but two unterminated wires. Some RS-485 transceivers can be operated either as four-wire full duplex or two-wire half-duplex.

Half-duplex transceiver;

Full-duplex transceiver:

Cheers!

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#3

Re: USB Out to RS 485 2 Terminal Input

10/14/2016 8:22 PM

If I understand your situation properly, the PC has a USB/RS-485 converter with 4 pin (presumably plus signal ground) connections, but the flow meter is 2 wire RS-485.

To convert from 4 wire 485 to 2 wire 485, jumper (+) to (+), (-) to (-):

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#4

Re: USB Out to RS 485 2 Terminal Input

10/15/2016 12:58 AM

I would go with the free visit by the company technician. Then I would talk to the technician about the idea that your posting here, you never know - the tech might actually like your idea and help you put a cable together, and if that happens, you could use the fabricated cable for the next job or if you have another issue, the good rapport you have with the tech will allow you to call him directly, and chit - chat.

Or, if you piece together a cable and it Fry's the flow meter, you might have to pull a chunk of change out your pocket book to buy your boss another set up.

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#5

Re: USB Out to RS 485 2 Terminal Input

10/15/2016 11:09 AM

I use the US Converters adapter regularly at work. The directional communication lights are very helpful in troubleshooting when communication breaks down. This is but one of many ways to solve this problem. Moxa also makes several adapters.

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Engineering Fields - Instrumentation Engineering - EE from the the Wilds of Pa.

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#6

Re: USB Out to RS 485 2 Terminal Input

10/17/2016 8:14 AM

Sorry I didn't reply to all who answered so quickly, but something in my title has been interpreted by the wall at work as "spam". I should have checked my post over the weekend, but this one was special - I got out and away ALL weekend from any connectivity. Just me, a bike, and lots of wooded trails.

Just a few hours now and the manufacturers rep will come and electronically open up that little box on the flow meter so we can set it right.

Thanks to all.

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#7
In reply to #6

Re: USB Out to RS 485 2 Terminal Input

10/17/2016 3:00 PM

How come the client locks you out of the place where the flow meter is? Sounds kind of silly.

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Engineering Fields - Instrumentation Engineering - EE from the the Wilds of Pa.

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#8
In reply to #7

Re: USB Out to RS 485 2 Terminal Input

10/17/2016 3:24 PM

Cause this client likes problems. Every skid takes about twice as long to FAT as it should.

Does sound that way when you read my post, but it is strictly an electronic issue. Funny thing is, the manufacturer's tech can't unlock it either. He got in with the RS 485 cable and re-programmed to what we need, but he can't unlock the display to let us change it. If we need to change in the field (on site) we'll have to use the PC and a free cable he gave us, plus get driver software from the cable manufacturer. Pays to bring them in - that cable is $98 if you have to buy it from them. Probably good we can't get in through the display, as the actual end user won't want anyone messing with the programming, which, when working, should be easily accessible from the display with a couple of those little lights you put your finger over to trigger. Maybe a bit too simple for a plant floor and an instrument at about 4' off the ground.

The post which suggested the doubling of the + and - cables and included a new driver is the correct one.

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Engineering Fields - Instrumentation Engineering - EE from the the Wilds of Pa.

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#9

Re: USB Out to RS 485 2 Terminal Input

10/18/2016 1:28 PM

Additional up date- yes the flow meter is correctly set now, with the same program as they allowed me to down load to my PC and the $98 RS485 cable adapter to 2 wire, and yes, the tech finally figured a way to unlock the write protection via the instrument face, not the PC. No explanation of how that was set in the first place. The way to over ride it is still a mystery, as he didn't really explain it - just said it was a very convoluted route to remove it. (guess he wants to return someday) Gave us the $98 cable to use for a while, in case we find it locked again. (???) Only took him about 2 minutes after I removed the cover to the instrument case. It would have taken me far longer, as I would have had to figure out where to start and what to do in the programming. As most CR4 Gurus always say - call the manufacturer.

Just for fun, I checked the price of the cable from the actual manufacturer and on Amazon, and the price quoted by the flow meter manufacturer is the going price for it.

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#10
In reply to #9

Re: USB Out to RS 485 2 Terminal Input

10/18/2016 3:37 PM

Good to hear your problem has been solved. For now . . .

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Engineering Fields - Instrumentation Engineering - EE from the the Wilds of Pa.

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#11
In reply to #10

Re: USB Out to RS 485 2 Terminal Input

10/18/2016 3:58 PM

I'm done with it now. Once it leaves here, it is the immediate customer's problem, as in this particular case, we sell to another company that sells to the end user. The wiring and plumbing issues are still ours, but not instrumentation warranties, as the intermediate bought them, nor the programming of them. Actually, the instrument programming here is technically not my issue, since the instruments belong to them, but we have historically done it for them.

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