Multiple devices in a JTAG chain can reduce the signal quality of the TMS and TCK signals which are connected in parallel. Is there a rule of thumb for when to add buffers to these signals?
The answer is dependent on the environment and the cable. The experimental rule of thumb is that after five devices, the signals should run through a buffer. However, under noisy circumstances, buffers may be needed when using less than 3 devices.
The LS244 is an example of a buffer that has been used successfully with our devices.