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We have to send two 4-20ma signal to a distant control room (approximately 200 m away from instrument) Desktop computer. Is this ok or do we have to use some other technique? Also, if anyone has used any device to convert 4-20ma into RS232/485 or USB then please inform me.

It is perfectly ok to take your 4~20 mA signal to distant Control rooms. The reason for changing from voltage is to avoid voltage drops at different levels. Just make sure you don't overload your signal, I mean take it to more than two places.

The best device for you is ADAM 4017 or 4017+ module. Which acquires data from the field and gives output in form of RS-485. This protocol supports distance up to 900 meters. Then at your PC you need to convert this signal into RS-232 by ADAM-4520 module. For complete implementation of the system you can directly contact me on my mobile no: 09998823972.

Advantech Adam 4000 modules convert analog signals into Modbus RTU format on RS-485.

Your desktop needs HMI or data acquistion software to act as a Modbus master. And you'd need an RS-232/485 converter, because PCs never come with an RS-485 port.

4-20mA will easily travel 200m, but you still need some kind of I/O to convert an analog to something the PC can read.

There are various options (depends on your budget)

Wireless: use 2 / 4 / 8 Channel Transmitter which accepts 4-20mA signal and transmit signal (usually 2 miles in open ground), and on the receiver end use receiver that accepts wireless signal and gives output in ethernet protocol, which can be directly connected with your desktop. This is very advanced & cost effective way to go ahead. In the future if you want to expand your system you don't have to lay the wire again, just hook up with transmitter & there you go.

- Data Logger with RS485 output

Any more question, email instrumentation_control @ yahoo. ca
It should work fine as long as the voltage drop of the wire isn't too much. You can do a simple calculation that should indicate if you are going to have problems. The basics are to figure your total voltage of the system and divide it by the total resistance. The total must be greater than .020 or you may experience problems.

750 Ohm conversion resistor
400 Ohm wire resistance
24 volt power supply

Using Ohm's law
Amps = 24V/(400+750) = .021

I would probably actually subtract a volt from my supply volt total to account for any voltage drop at the transmitter.

23V/1100 ohms = .0209

Hope this helps,