Converting 4-20mA signals into fibre optic media


Thread Starter



There are two separate DCS systems (1 & 2) at our site. Due to the legacy design of both systems, it is not possible to implement OPC connectivity between the two. The distance between the two DCS systems is also quite huge.

I want to transfer some 4-20mA analog signals from DCS System 1 to DCS system 2. It will be quite simple to lay down a standard instrument multi-pair cable between the two but there are limitations like EMI, cable length, high costs, etc.

Due to these factors, I have another idea. I want to convert these 4-20mA signals from DCS system 1 into fibre optic media, lay a fibre cable between the two DCS and finally, convert it back to 4-20mA at the other DCS.

I want to know if this is really possible. Has somebody implemented this type of topology at his/her site? Please guide me in this.



Honestly, we have not done this type of conversion. But we can suggest alternatives to the solution you are looking for based on the assumption that both DCSes support Modbus RTU [Master protocol], this is very common for even legacy DCS systems. I suggest the following steps:

1] For signal input, use signal splitter from
either MTL, P & F, TURCK. One input & 2 output
2] Provide DAS system/IO modules from Acromag
or Moore industries [NCS].
3] One output from signal splitter would go as
input to either of the DAS/IO modules.
4] These I/O modules would have to be built in Modbus RTU slave protocol with RS485.
5] Provide Modbus antenna for transmission of
the signal over Radio Modem.
6] DCS2 location would also have Antenna radio
signal with converter to Modbus RTU [RS 485]
7] DCS2 should have Modbus Master card, which
will access the data from IO/DAS system.
If DCS1 has a Modbus slave card, then the above splitter & I/O modules are not required, only radio modem would serve the purpose.

We hope to have explained this to you logically, if you send me your email id, we can give you details with architecture diagram.

Even if you go through web site of both companies
you would be able to work the solution.

We have not suggested DCS1 with Modbus slave card
because this might load your control processor, for which you are the best judge.

[email protected]
If you want to use modbus you should configure one DCS as master and the other system as slave. Try to link them with fibre optic. The same connection betweem DCS and RTU

I hope it works.

I am assuming you require 4-20ma output at one site transmitted to a 4-20ma input at another site. This is what the post requires.

Remember this. Fiber optic cannot flow current. Because of this restriction, Most everything you use will require the 4-20 signal to be converted from analog to some sort of digital representation of itself and then be converted back from digital to analog. Loss of accuracy is a major decision in this process.

Almost all ethernet enabled PLCs are capable of this function. Another problem with ethernet and TCP/IP is it is usually a connection-less protocol that can be subject to lost communication packets. If this is not addressed from the beginning, communication loss and the loss of an accurate and timely signal could cause problems.

Regarding topology. If you accept the above listed problems, Why spend funds properly installing fiber optic? Wireless routers have reached amazing speeds. The cost of a high end wireless network is inexpensive in comparsion to a properly installed fiber network. The wireless network will never be subjected to a cut cable, although they can be subjected to solar storms and hackers with a knowledge of wireless networks.

Just throwing things out there. Food for thought!

Michael Griffin

In reply to CTTech: I have a minor point to make about one of your statements where you said: "TCP/IP is it is usually a connection-less protocol that can be subject to lost communication packets". UPD is connectionless, but TCP/IP uses a logical connection. TCP/IP will try to make sure that all packets get through. This is why UDP has less overhead than TCP/IP and why most automation systems have limits on the number of TCP/IP connections that are allowed.

With UDP you just send off a packet of data addressed to a particular IP number. With TCP/IP you first establish a "connection" and then transmit the packet(s). It's like the difference between sending a letter and making a telephone call. With the telephone call you know that the person at the other end of the line is there and can hear you. On the other hand, you can send off large numbers of letters at once but can only carry on one (or a very few) telephone calls at a time (you only have two ears and one mouth). Each method has it's advantages and disadvantages.

Even though UDP is connectionless, it can still be quite reliable as part of a complete system if errors are handled at the protocol level. For example, if you are polling I/O using UDP in a master/slave configuration, the master knows that it asked for a reply and didn't get one. The application software can then do whatever is necessary to handle the error.

Any form of communications though is ultimately subject to some degree of failure or error.
You have mentioned two issues in installing these signal converting equipments.
1. Loss of accuracy: You have commented that due to A/D conversion inside this signal converting equipment, accuracy of actual 4-20mA signal may be lost.

My question is that in such case, while selecting such equipment, should A/D conversion rate (like most vendor say 16-bit resolution or 20-bit resolution) match the A/D conversion rate of DCS I/O module?

In our case, there are two separate DCS systems. Hence, A/D conversion of respective I/Os of these two systems must be different. Which A/D conversion rate should we take as reference?

2. Time lag during signal transmission, do you think that using RS-485 Modbus RTU protocol may introduce such time lag?

If you think otherwise, do you still suggest to use conventional wiring system? Remember that we just not only want to transfer monitoring signals but also control signals (4-20mA output).

Thanks alot for your proposals. Actually I dont intent to convert 4-20mA to Rs-485 and send to RS-485 card on DCS. Instead, I want to convert 4-20mA signal from DCS system-1output card into RS-485/Fiber optic and then re-convert into 4-20mA and send to DCS system-2 Input card.
Is their any equipment which provides redundancy in all electronics and communication network?
Even if you have 16 bit conversion or more, it is likely the calibration is slightly out. Probably less than 1 mA, but a little bit in each of the two inputs and two outputs. This adds up. That is why I think that once you have converted it to digital in the remote-end, don't convert it back to analog again only to digitize it one more time.

Once digital, keep it digital. This is true regardless of you using fiber or wireless. That is, use Modbus or some other legacy protocol your old system supports.

Fiber optics is OK if you are able to run a fiber. But sometimes they may be roads, other people's property, river/brook or other obstacles making fiber impractical. In this case wireless can be used. Fiber works well if only two or a few nodes are involved. Once there are many nodes I'd go for radio.