AnsweredAssumed Answered

RS485 half duplex communication problem

Question asked by roymessinger on Apr 25, 2017
Latest reply on May 1, 2017 by roymessinger

I've asked a similar question before, and the result Mark Butcher has given me (and proved to be correct) was to replace the 2 resistors shown in the picture (R189=0, R188 not assembled):


My board is installed on a big machine with lots of other electronics/mechanics going on. I have 2 boards on the machine. Each board is getting 24V and using DC/DC transfers it to 3.3. On this 24V I have other 'users' also.

What I am experiencing is that in one of the boards, whenever a different user (on the same 24V main power line) is turned on, I'm getting an endless loop of tx enable, shown here:


I did not see any voltage spikes on the 3.3 of the K64 nor on the 24V when the other user is turned on. I'm using a USB to RS485 dongle which is not shielded (ready made product) - so I'm guessing an outside noise is coupled on this cable. When it happens, even the slightest touch on this cable and it starts to show this tx enables, sometimes when I touch it it suddenly stops.


This goes forever, and obviously, the connection with the K64 is dead, until I power on it. Without turning On/Off this user, there is no problem with the connection.

The green line is the tx enable. The yellow line is the Rs485 transmission (A+)

a. Even if it is a noise problem, how come the K64 sends endless TX enable pulses ? I would have thought just a one time scenario, not all the time.

b. Any idea how to solve it? Maybe something with the RS485 termination resistors?