RS485 on a Scope

Zone A, E and B

Zone C: Unless you have a very high speed scope and the scope can takes a huge number of samples you are unlikely to see each bit in the message. In this capture we can see the bytes (roughly speaking) but not the bits. We were forced to accept this compromise because the number of samples we could capture at a sampling rate high enough to see the bits would mean that we could only capture the 1st couple of bytes of the message and we would have to set the trigger to ignore zone B. Except with a very expensive scope you will be unlikely to be able to see the whole message and all the bits.

Zone D: We have finished transmitting but the transmitter is still enabled. Normally the device should disable its transmitter as soon as possible after transmitting the last stop bit of the message but since that can be difficult to achieve in the hardware, many devices run a timer to make sure they don't disable the transmitter too soon. The problem with this approach is:
1) The longer the time the more potential bandwidth is lost.
2) The receiving device may have already processed the message and try to send a response by enabling its own transmitter causing collisions.

 

Probes on plus and minus conductors. Segment connected to slave device.

 

Probes on plus and minus conductors. This chart represent the capture from a master device with no slaves connected to the network leaving the cable ends to float.