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4-20ma Signals collection onto a Network
How to get 4-20ma Signals from different locations in a facility to a common point and transmit it on a Network

I am interested to get all the 4-20ma signals from different locations (100-600m apart) to a common point. Can Anybody suggest me a solution?

I have done previously such project by means of some wireless I/O modules. But now I am not interested to use wireless technology here because radio frequency cannot travel undisturbed in this environment.

Waiting for your comments :)

Thanks & Regards

1) copper wire home runs.

2) copper wire to remote I/O rack(s). Connect remote I/O racks to central point with copper or fiber optic

First, it may not be necessary to convert the 4-20ma signals to digital form in the field. The 4-20ma signals can be aggregated into multipair cable where the total wire length can be in excess of 600 meters. Each wire pair can then be terminated in a conventional analog input multiplexer card for input to a DCS, SCADA, or PLC system.

Please do not assume that wireless cannot operate in an electrically noisy environment. While this is true of WiFi (IEEE 802.11 a/b/n), using a channel hopping standard such as ISA100.11a or WirelessHART can operate over the data collection area allowing you to collect data at their Gateways to transmit digitally via Ethernet networks where fiber optics can provide long distance connections. However, I expect that the cost of the converters for each point from 4-20ma to either wireless network would be more costly than wiring the original 4-20ma in multipair cable.

Dick Caro
Richard H. Caro, CEO, CMC Associates
Certified Automation Professional (ISA)
Buy my books at the ISA Bookstore:
Wireless Networks for Industrial Automation
Automation Network Selection
Consumers Guide to Fieldbus Network Equipment for Process Control

If you are looking to send 4-20ma from various locations to one common point across a network, you might want to look into a vendor named MOXA.

By James Ingraham on 4 October, 2011 - 10:30 am

Most of my comments from this thread also apply:

The up to 600m length is an issue. That's a LONG way for most industrial fieldbuses, and Ethernet is limited to 100m. Fiber is an option, however. I've used fiber with both Ethernet and Profibus with good results. Unfortunately, that's going to get expensive. Your 4-20mA would plug in to an I/O module (e.g. Wago), the module would plug Ethernet (or whatever) in to a media converter, the media converter would be connected via fiber either directly to a switch with fiber ports or another media converter and then a conventional copper switch. Not a cheap solution.

-James Ingraham
Sage Automation, Inc.

All of you nice people.

Thank you very much for your useful suggestions.

Best Regards,
Jawwad :)

By automationtechie on 30 April, 2012 - 7:11 pm

You could use Modbus RTU modules over RS-485 twisted pair. Each AI (4-20mA) signal could be connected to one input of M-7017C

This one handles up to 8 AI / module. Using Modbus protocol, you can read the AI values from a central point.