RS485 Multidrop Issues

Question

I have an application where we use RS485 to communicate messages to a series of LED Displays on the bus. Our first version of the board seemed to work fine with no problems, however we have made a new revision that is using a different RS485 Transceiver and now some displays do not receive their messages reliably. However not all displays on the bus are being affected. Only one or two are being affected. We never had to use any biasing resistors or terminating resistors with the first version so I can't see that as the problem for this case. The only thing I can see that might be causing the problem is the change in RS485 Transceiver that was used.

The first 485 Transceiver we used was a ST485C, however, the new board is using a SN65HVD485E 485 transceiver. Is it possible that the change in chip has caused a difference in performance? I know there is only one way to find out, but I just wanted to know if this was a reasonable causation?

We also use Cat5 UTP cable and use the following pin configuration. This is T568a standard.

1. RS485 B
2. RS485 B
3. RS485 A
4. GND
5. GND
6. RS485 A
7. VCC
8. VCC

I know that the above wiring doesn't follow RS485 standard wiring, but it worked with the last Revision and I can't see how this could be the reason for the current problems.

Answer 1

From looking at both datasheets the SN65HVD485E draws 50 times the current on its data input DI, or about 100 uA. The ST485C draws about 2 uA on its DI pin with a 24 K load, so there is a major difference.

In addition the changes in input loading can cause reflections of the signal from end to end, such that at some places it is strong while in others it is weak due to 'running into itself' out of phase.

Since both IC's are supposed to work as advertised I would make sure they are all the same. The mix of 485 types maybe more of a problem then having just one type for all points of use.

The load resistor is still needed at each end of the line.

Answer 2

You would need terminating and biasing resistors at each end of the transmission line. Tha cable has to have a characteristics impedance of 120 ohms, twisted pair. Ethernet cable has twisted pairs, but 100 ohm impedance, a little mismatch but not critical. It depends on cable length if the cross section of Cat5 is enough.

I usually use shielded twisted pair with 0.5mm^2 cross section for approx. 100m, like LiYCY TP 4x2x0.5.

The RS485 transceivers are all compliant and compatible, I guess you were just lucky the first time, but you would need to improve it anyway if you want a stable and robust RS485 network.

^{simulate this circuit – Schematic created using CircuitLab}

$$R_{T_{eq}}=\dfrac{R_{term}\cdot (R_{bias_{up}} + R_{bias_{dw}}) }{R_{term}+R_{bias_{up}}+R_{bias_{dw}}}$$

For given example the equivalent Rt is 112 ohms. You can tune resistors to match used cable. R_bias shall not be less than 560 ohm.