Good question, and a good opportunity to highlight the differences between an I-PAC and KeyWiz shift function.
Here is how I understand how the I-PAC shift function works and then I will get to your specific example.
When you press the shift button on the I-PAC, no code is sent until you either
1) Press another button (Sends Shifted code for that button immediately)
2) Release the Shift button (sends the Shift button's normal code).
When I press in the IPAC Shift button and hold it (activating the SHIFT) then I press P1 (to make the TAB menu come up), does the TAB menu come up right away or does it wait until I 'release' the P1 button? Or does it wait until both buttons are released?
Also when I do this, will Mame pause (since my shift key is the same as pause)? It seems logical to make my Pause button the IPAC Shift button also, but I don't want Mame pausing and unpausing back and forth as I'm doing things in the TAB menu everytime I have to press the IPAC Shift button...
Again, as I understand it your pause button will work normally as long as you don't press any other buttons. However, MAME will pause and unpause on the Button RELEASE (not press).
In the example above, MAME will NOT pause when the Shift button is first pressed. When you then Press P1 Start, the TAB menu will come up immediately, but MAME will still remain unpaused unless you release both buttons and press and release the PAUSE key again.
If this is acceptable, that's fine.
I know I sound like I use every post to recommend the KeyWiz lately, but if you want MAME to pause when the Shift key is pressed, you might want to consider it.
The KeyWiz (
www.groovygamegear.com), gives you four more inputs, so you could have dedicated Pause and Tab buttons.
Also see
http://www.fraggersxtreme.com/arcadepanels/encoder/shiftkeys.htm .
If you wanted extra admin buttons, you could wire them up as shown in the "Stealth Mode" section of the page above.
If you wanted less buttons on your panel and wanted the Shift (Shazaaam!) button to function like the I-PAC but actually Pause MAME before activating, you could wire the KeyWiz as shown in "I-PAC Compatibility Mode" and it will do this.
Basically, this is a good example of a case where the differences in shift methodology determines which encoder you want:
Pause and then activate - KeyWiz
Activate without Pausing - I-PAC
Let me know if you didn't follow all that :-)