Powering isolated RS485 transceivers over cable

Hi All, I have an application that requires about 12 sensor devices over a RS485 cable, evenly spaced over about 20m with a controoler at the end.

I'd like to use galvanically isolated RS485 buffers such as IL3185 on each sensor, but would also like to not have to put isolated DC-DC converters to supply the power to the cable side of the buffers.

So can I supply +5V up the cable to all the devices to power their cable side buffers? Since it is RS485 only one sensor will be transmitting at a time so as long as the input buffers don't take much current it would seem practicable (with some capacitance and possible ferrite filtering where the power comes in to each sensor).

First, is this a pratical solution? If so, assuming CAT5 cable with a pair dedicated to the 2 wire RS485 signals, a pair to +5V and a pair to ground, how long a cable would be possible, or is it best to use a higher voltage and use a cheap LDO on the receivers to generate +5V?

Thanks! Mark.

Reply to
Loading thread data ...





Sure. Typically this is done with a pair of wires (one each for voltage and ground) Transformer couple the RS485 data and apply the power to the center taps. In a similar way power is pulled off the center taps at the other end. Power over Ethernet works the same way.


We power our remote widget over 1500' of CAT-5. We start out with 24V and use a buck regulator at the far end. We could go further but the drop-out on the regulator is pretty bad. 1500' is far enough for our application so we haven't bothered fixing it.

Reply to

I guess I'm concernred about the voltage drop across a 20m cable due to the resistance of the cable when the worst case of all 12 sensor receivers are powered and one sensor's transmitter is on. The caps would smooth things locally so it's down to DC losses.

As a matter of interest, what drop-out voltages were you seeing (I'm assuming that's down to cable losses), and how much current were you supplying? At the moment I've got a 5V regulated supply available, but the chips require 4.5V.

Also I guess since power has return currents I should use a twisted pair for ground and power to reduce noise, maybe tripling up as I have 4 wire pairs available and one is taken by the signal pair (?).


Reply to

While not exactly RS-485, the CAN bus based DeviceNet standard transfers the actual data in one pair and the power for the transceivers and small loads in an other pair. You might find some interesting information about power arrangements etc.

If you have full control of both the master and slave side, have you considered the good old current loop system, which is quite easy to optoisolate (no local floating power needed at slaves) ?

Connect all devices, both master as well as slaves, both receivers (photo transistors) and transmitters (optoisolator LEDs) into a single

20 mA current loop. When idle, all transmitter transistors are conducting and the 20 mA current flows through the loop and when the active station (master or selected slave) wants to send the "0" bit, it will cut the loop current. Since the active station will hear its own transmission on the receiver side, it must contain some echo cancellation in software.

The problem with this arrangement, especially with optoisolators with bipolar transistors, is the large voltage drop at each station, nearly

2 V for the receiver LED and 1 V for the Vce of the transmitter. With 12+1 stations the loop current loop would have to provide nearly 40 V, which might be too much for some optoisolator transistors.

One way around this problem is to use optoisolators with FET output stage and hence a low voltage drop across the transmitter, reducing the loop voltage requirement.

An other approach is to use two loops, one containing the master transmitter and all the slave receiver LEDs in series, the other containing the master receiver LED and the slave transmitter transmitters (bipolar or FET) in series. 24 V should be sufficient for both loops. In this configuration, echo cancellation is not needed.

Reply to
Paul Keinanen

I had a simple three station design, but there were 120 feet between stations. I used one pair for power, and individual bucks at the stations. I had an old programmable laptop power supply on hand, so just used it (I think I had it set to around 12 volts.) Depending on the current draw for your individual stations, you shouldn't need too high a voltage. Remember, voltage loss is a function of both voltage and current!


Reply to
Charlie E.




t a





20M isn't very much cable. The I2R drop is pretty easy to calculate at .08ohms/meter. The transformer's DCR will swamp that. Don't forget that there are two wires and the two transformer windings are in parallel (though there are two transformers in series).

Something like 70mA. 1500' of cat-5 is 15ohms (two wires) and another

3 ohms or so for the transformer, so the drop is about 1.25V.



Ideally, but if they're used for data, not so much. The POE connection I described works quite well. Cat-5 pairs are twisted together, so it's not a disaster to have a differential current between pairs. The thing to watch with all UTP connections is common mode EMI coupling. Switchers tend to make this a real problem. Passing the cable through a ferrite core once or twice tends to knock a lot of that down.

Reply to

Funny you would talk about this, we have some Red Lion devices that use the current loop and if you don't have more than one slave unit on the loop, in hot operating conditions, the crappie current source circuit used will bias itself more than 20 ma's and over time burn out the optical input on the slave device!. So, what I have done to correct the issue for now is to use a R in series. This really should be documented in the Red Lion manual but I haven't see it..

The current source that is being used is your first year 101 basic PNP transistor being biased to 20 ma. there is a diode on the base to the (+) rail to help out but its not enough, it still gets out of spec.. I've see it reach up to 35 ma when thing get warm..


Reply to

markp schrieb:


I would look for the cable data, especially the resistance per length of one wire and calculate the voltage loss over a 20 m long cable with the necessary current for the tranceivers.


Reply to
Uwe Hercksen

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.