Converter RS232 to analogue 4-20mA

[MaxPlanck]

Dear all,

In our lab we want have an analytical balance which has a RS232 output.
Now we want to connect this balance to a PLC, that way we can also connect a pressure-measurement-signal en a temperature-signal (both 4-20mA) and measure all these parameters at once.

Is it possible to convert the RS232 signal to an analogue signal 4-20mA?
Can you recommend me some companies who produce these converters?

Thanks in advance for all the comments.

Best regards,
MaxPlanck

 

[mikek10]

Not directly, the RS232 connection is using some protocol to communicate numerical data, you would need an intermediate device e.g. PC to interpret this data and an addon card which allows Analogue output.

I would be suprised if the balance doesn't have an analogue out or an optional extra which gives you this ability

 

[GTstartup]

Why convert to mA? Will the PLC take ethernet? If yes, there are RS232 to ethernet converters out there.

 

[sreid]

Or purchase an RS-232 I/O "Block" for the PLC.

 

[Standingback]

Max,
All of these guys have valid points, but it won't matter which way you decide to go, ie RS232 to 4-20mA, RS232 to Ethernet, RS232 to RS232 PLC input even RS232 to Or anything else.

There are manufactures that will come up with some black box to convert from one to the other. But they won't be able to help you unless you have a driver or communication protocol for your analytical balance. This will define what information the balance is prepared to release via the RS232 port and how any requesting device will have to ask for it.

My suggestion would be to firstly check the analytical balance technical manual for the RS232 communications protocol. Even ask the manufacturer if there is an an adapter that they would reccomend. Secondly you can investigate which is the lowest cost option. 4-20ma, rs232 or something else, given what I/O you plc has now and what converters support your protocol.

I trust this helps,
Mlv

 

[MikeHalloran]

As an example, I once worked with an analytical balance that accepted commands and returned data over an RS232 line. I think you sent it a letter 'T' to make it tare, and it would acknowledge with something like 'ok' when it had done so. Then you would send it a letter 'M' to make it measure, and it would respond with a printable ASCII string including the measurement and the units. I think there was a mode where you could command it to transmit continuously, so it would send measurements as fast as it could make them, and you could watch it settle.

A friend who insisted he was not actually a programmer built a nice graphical front end for calibrating precision pumps, all built in HyperCard stacks and running on a Mac. Because he didn't know any better, and because the Mac only had one serial port, he ran the balance _and_ an intelligent stepping motor controller over the same wire at a different baud rate at the same time. Some of the two machines' commands collided, but luckily not the ones he needed.

If your balance is anything like ours was, you need bidirectional communication, and I don't think 4..20mA can do that.

On the other hand, it's not rocket science. If you have time, you could probably make your own black box using nothing smarter than a Basic Stamp.



Mike Halloran
Pembroke Pines, FL, USA

 

[controlsdude]

If it is an AB plc, I know that the AB website has standard code to bring RS232 through your serial port on the front of the processor. If your using that same port to get online then that would not be a good way.

otherwise go to this website

Converters - bb-elec.com has a good line of converters.

 

[controlsdude]

forgot this product from phoenix contact

RS232 to TTY Converter
PSM-ME-RS232/TTY-P
2744458

http://www.phoenixcon.com/

 

[felixc]

Your balance probably has an RS232 output because it is a precision instrument. Using 4-20mA to deliver measurements adds two error sources, the 4-20mA transmitter, and the 4-20mA receiver. This may destroy the precision of your balance.

 

[123MB]

plug RS232 into PLC. Get an RS232 comms module.