On this diagram [1] for a modification to a GameCube cable, the author
is disconnecting a pin from ground and attaching it to Vcc through a
pull-up resistor. The chip uses this pin to determine whether it
should output component video (YPbPr) or VGA video (RGBHV). Since this
pin appears to be an input pin, is the 10k pull-up resistor really
necessary? Shouldn't the input pin have a natural input impedence of
mega-ohms? A well explained guess is all that's necessary. A
datasheet for the chip in the photo (MX B012355 CMPV-DOL T21643K1)
would be great, but not necessary.

Thanks,
Shaun

[1] http://www.geocities.co.jp/Technopolis-Mars/9984/dac_e.htm

Declaiming on a subject about which I know very little:
It depends on the chip technology and the design.  It "might" float
high, but you should use a resistor if for no other reason than to
prevent a stray static charge coming along and frying the chip with
50kV +.

Regardless of claiming no authority, xarqi is dead right. One never
leaves inputs open circuit, especially high input impedance MOS. If
you do, the status is indeterminate and causes unpredictable results.
Can also be damaged by static.

Connecting to the rail via a resistor is to some extent a throwback to
earlier chips which in certain conditions could cause problems. Rarely
now however is it necessary to incorporate a resistor -- just connect
straight to rail.

Couldn?t find refs to your device I?m afraid. Not clear which is the
part number either because some of the numbers refer to batch or date
etc. Make it difficult for us why don?t you manufacturers! An all too
common problem.

However, since I cannot say categorically that a link rather than a
resistor would do, on the presumption the modder has actually access
to the data sheet, go with a resistor. Value not at all critical.

Best