I have two devices, device 1 and Device 2. The devices are approximately 60 feet apart connected to each other through a long cable. They communicate with each other at 2400bps synchronous serial using the single clock source. Device A needs a clock for input and output clocking. The current design has an "In Cable" loopback of the clock as shown below to provide both needed clocks.
Sometimes it was noticed that Data In of device A would not receive data. When this situation occurred, device 1 could check the incoming RX clock (Which should be at 2400bps) and it would be at a value that was not 2400bps. It would usually be a multiple or more of the original 2400bps clock. Values such as 4800, 7200 and other values have been seen at Device 1. Th obvious reason that data was was not coming into device 1 is that the clock was invalid for the Data (which was clocked at 2400bps).
If I try to put an O-Scope up to the clock signal into deice 1 to see what it looks like, it immediately clears up and is seen as 2400bps.
We have 20+ data paths with this configuration. Some work and some do not. Swapping cables may make bad paths work, or might make them not work. Using cables known to work between two different device might cause the path not to work. All cables are professionally made and I do not think it is a cable issue. It is unpredictable what device/cable set will work or not.
I am not an EE but have been researching this issue and I think there could be some impedance issue at device 2. The clock signal could be reflected back and forth between device 1 and 2, ultimately the RX clock IN on Device A giving improper clock signaling, and thus no Data in at Device 1.
I have read that some resistor or capacitor added in the path can cut back this issue, but I am not an EE so I am not sure if a) this is an impedance issue and b)where would I put a circuit to fix it. Someone has mentioned an inline 22.5ohm resistor to start. Someone else has mentioned anything <100ohms. At this point, I can not tell you the output impedance of device A. it is an old component and is currently undocumented.