General Description: Can a board supply of 3V or 2.5V be used as the power supply for the JTAG cable?
Yes, the Parallel Cable III (JTAG Cable) will accept 5V, 3V, or even 2.5V as the power supply.
To select a voltage standard to use with the JTAG cable, connect the VCC pin of the parallel cable to the desired level. Typically, the VCC is supplied from the target board. If an external supply is used, ensure that the power supply and board are connected to a common ground pin.
NOTE: The Parallel Cable III can be powered from 2.5V to 5V; however, it can only interface with 3.3V and 5V logic levels reliably. The buffers (74HC125) do not perform any logic level translation. The buffers in the Parallel Cable III were designed to be powered with a wide VCC range, but were not designed to perform level translation.
Do not use the Parallel Cable III to configure a part that interfaces at 2.5V levels. If this is required, you must provide your own level-shifter.