Electronic – Differential bus transmission

I think to properly diagnose this you will need to eavesdrop with something like a FTDI rs458 converter, along with an oscilloscope. With the scope you can see when a transceiver grabs and releases the line. You may have to manually poll each node and review each response. If you can pinpoint the module that is holding the line "down", see if its corresponding enable pin is actually held high. If the line is not being held down, it could mean that two nodes are attempting to reply at once(atmega firmware issue).

Be sure that you have terminated the system with two 120 ohm resistors per repeater instructions.

It looks to me like your DE lines on the slaves are being driven wrong. Your bus voltages are (qualitatively) what I'd expect if the "unselected" slaves are in fact being driven active with a high input to each "unselected" slave DI.

As the number of slaves increase, the total bus drive from the unselected units increases, and the selected output has less and less effect on the bus voltage.

The obvious check would be to get the effects you've shown, then pull the selected slave chip from the circuit, and look at bus voltage. If the other chips are inactive, the bus differential voltage should drop to more or less zero.

Heh. Or, of course, the reason that your chips cost 1/3 the Digikey rate might be that they don't work properly, and the DE line has no effect. Just sayin'.