I am looking for a way to install a remote IO using rs485. I need (8) 4-20ma input on one end, then, using a twisted shielded 2 wire cable (rs485?) connected to another end with (8) 4-20ma output from there. This exact application has come up before, I have 2 existing potential projects that could use this now, but I never found a good way to do it. I see many AI/AO to rs485 modules but I'm not sure what other components I would need or what other options there may be. It looks like if I use a Modbus module I would need a Modbus master which looks like another component but I really just want to link 2 modules. Any recommendations would be nice?

 

What sort of distances are we talking about here?

Would the 4-20mA signal itself simply be able to cover the distance? If not, what about using a signal repeater or signal conditioner?

Is there some other reason why you would need to use RS-485?

 

What about using Ethernet instead of RS-485, using two of these, for example?
https://www.fctele.com/serial-industries-fieldbus/8-channel-4-20ma-to-ethernet-converter/

 

Hi, Thanks for the response. Both applications are adding to existing installation. Both under 1000 feet and have 4-20ma running there now. I want to add IO to each location. I can use 1 of the twisted shielded cables to run communication and link the 2 locations. 1 location only has 1 twisted shielded pair, the other has a couple.

 

This is exactly the idea but I don't have ethernet in the area:

 

HART multiplexers have been used for remote I/O for many years. For example, the HiDMux2700 [ https://www.pepperl-fuchs.com/en/products-gp25581/42264 ] supports up to 32 4-20 mA channels, providing sensor data via RS-485 (sensors must support HART communication, although conceivably the multiplexer could have been designed to measure and return the loop current for each channel).
Typically, the control system (or asset management program) has built-in support for the multiplexer and uses the HART protocol [data format] over the RS-485 connection.

 

ICP-DAS has a so-called "pairing" solution. In wireless, I called it "wired-in/wired-out" with analog or digital inputs one on end mapped to corresponding analog or digital outputs on the other end. This is the wired version.

ICP DAS's 'pairing' is where one module 'pairs' its outputs with another module's inputs. ICP DAS has a Modbus/TCP client module with analog outputs that can poll/read a Modbus RTU server module with analog inputs with a Modbus TCP/RTU gateway providing the conversion from Ethernet to RS-485.



The RTU link on RS-485 should run on the shielded twisted pair.

Pages 80 through 84 in the ET-7028 Modbus/TCP-client/AO module user manual describes a pairing connection of analog inputs to analog outputs using the web configuration interface.
https://www.icpdas-usa.com/documents/et7000_et7200_user_manual_v112.pdf

Other documentation can be found by model number from the search on their home page:
https://www.icpdas-usa.com/