Dead Channels on AK-4XRS485/422

We have purchased several AK-4XRS485/422. Some of them are 100% working, but 3 of them so far have dead channels — on 1 or 2 channels, no data passes through and the RX light is solid green instead of blinking. Are there any steps we can take to debug this issue?

Here is an example picture. The RX light stays solid green. It does not do this on channel C1, C3, or C4, just channel C2

After some testing I have found that these channels do still work, but they produce some invalid data (1 or 2 characters) at the beginning of received packets. To workaround, I simply filter out the bad data and everything seems to work OK, at least so far.

Hello @chrisvo,

Sorry for the late reply.

This is a problem we have investigated before. First try to remove any load (resistor) present in the bus and try again.

In short, in some cases a couple of resistors in the wires are required for reliable RS485 communication. The reason we didn’t include them in the board is because the values may vary accordingly to the bus topology, and in most cases, it may not be required.

This is the complete answer from our HW engineer:

ST485 has an internal failsafe circuit useful when the differential input is floating, to avoid undefined state of the receiver that stays in a stable logic level.
However, the internal failsafe resistors are influenced from the external termination loader so that an external failsafe circuit is required if the termination resistor is present.
A typical failsafe configuration is shown below. The two resistors Ra (pull-up) and Rc (pull-down) should be 500Ohm while Rb (load) is 130Ohm.

We have tested this circuitry in our laboratory and seems to work fine. We did the test using 470 Ohm resistors (we didn’t have 500 Ohm resistors at hand), and the error was fixed, the RX LED turns off and the RS485 channel works correctly. Although STM recommends using 130 Ohm load resistors, other manufacturers such as Texas and Maxim continue to use 120 Ohm. Please read the STM Application Note in attachment.
Texas and other brands recommend similar solutions. For example, Texas recommends:
Vs is 5V, Rfs (Texas says) must be 560 Ohm, Rt1 and Rt2 are the 120 Ohm load resistors (already included in the AK-4xRS485 board). Note that this pullup and pulldown resistors must be unique throughout the entire RS485 network, and can be put in any point of the network.

en.CD00004227.pdf (116.4 KB)

Thank you for your detailed response. That makes sense and we will try when we get a chance.