Fault: The analyzer is connected in the segment mode (bus routed through the analyzer) and the communication is interrupted or data can be sent only in one direction. The communication without analyzer works.
Reason: There can be two reasons for this behavior.
1) One or both bus segment(s) is not correctly terminated and biased as recommended for RS485 buses. In the inactive bus state the difference voltage is higher than about 600mV. In this case the analyzer interprets the bus segment as active and sets the direction of the routing switch from the active segment to the other. As no inactivity is ever detected the switch will never change and the bus is never released.
2) Just the opposite behavior. The bus is overloaded or one device defective so that the bus swing is not more than ±500mV. In this case the analyzer will never or seldom detect an active bus, the routing switch is closed and the transceivers disabled.
Check: Remove the analyzer and recombine the segments. Connect the analyzer in tap mode with channel 1 parallel to the bus. Now start the communication and do a short logging. Open the signal view and check the data signal.
Now examine the signal before and after a transmission byte sequence. In case 1 you will note that the bus is not released down to a difference voltage of <600mV, the signal line will be high between the packets of different sender instead of being displayed in the middle between high and low, which is the inactive state.
In case 2 the signal is always or most time in the middle, even if bytes are transmitted.
Remedy: At first check if the resistors are correct as described in the part Connecting the analyzer in segment mode. Use a multimeter to measure the voltage difference A+ - B- in the inactive state (transmission not started). It must be between 200mV and 300mV as a recommendation.
Furthermore use a digital scope to verify that the bus and its voltages (swing at least 1V) are correct over the complete time. Use repeaters if the bus is overloaded.
The error behavior of the segment mode can be a hint for a bus working at its limits. You should follow this hint and optimize the bus, otherwise sporadic faults can happen. That is what the analyzer is made for, giving indications of error sources.
More entries concerning the segment mode:
How does the segment mode work