Each channel address for the board responds to only 7 of the address bits. When this address is matched, then the 8th address bit controls the tracking/latching mode for all channels on the board. So, for example, if the board is addressed starting at signal channel S0, the board actually responds to channel S0 and channel S256. For signal channel S1, the board actually responds to channel S0 and channel S257. And so forth.

When the low 7 address bits are in range, and the eighth bit is "on" (address 256 or above) the entire board operates in track mode and the signals propagate through every channel immediately. If you operated this way all of the time, there would be no point in having the simultaneous sampling feature. The board would act like an ordinary termination board.

When the low 7 address bits are in range, and the eighth bit is "off", the entire board operates in hold mode and signal values will be latched and held for all channels. Basically, nothing new will happen until the eighth bit is again set "on" again to allow new signal values through.

The difficulty is that when you previously were reading hold values and then start reading track values, this begins electrical transients from the previously latched levels to some new levels. This happens at the same time that a sample value is captured by the DAP. As a result, you cannot trust the captured value at a latched-to-hold change for anything. While you "hold" consistently or change from "tracking" to hold, there is no transient and the readings are valid.

For example:

  IDEFINE  A
  CHANNELS  5
  SET IPIPE0 S256   // Enter tracking mode, value unpredictable
  SET IPIPE1 S256   // Same as signal S0 but current value
  SET IPIPE2 S257   // Same as signal S1 but current value
  SET IPIPE3 S0       // Signal S0 but signals latched 
  SET IPIPE4 S1       // Signal S1 but signals latched
  TIME 50
  END