The ESP-12E operates on a voltage between 3.0 and 3.5 Volts. The devices that I want this programmer to service could have operational voltages as low at 1.8 Volts and as high as 5 Volts. Some routers operate on 12 Volts but their programming logic is probably in the 1.8 to 5.5 V range.
I searched for “level shifters” on the web and found a nice option from NXP. The 74LVC2T45 is a dual supply, two bit Bi-directional buffer. Maximum propagation delay is 10.1 nS which would be compatible with JTAG speeds up to about 50 Mbps. This would allow me to configure the I/O direction of each pair of bits at the connector.
Except for high voltage programming enable pins, These chips should be able to handle all the voltages for programming devices. I will still have to design the programming enable voltage pins separately.