> What are the advantages / disadvantages of these two methods
> of null modem wiring ? Obviously the first uses cheaper 3-wire
> cable. This is the type I most commonly use after simple Tx/Rx/Gnd,
> but if an unspecified null modem is called for, would the second
> be advisable to cover any situation ?

The first type will not allow hardware flow control.  Worse, it will
false-pass all hardware flow control efforts, so devices at both ends
of the line will get "suprised" by extra data after they deassert
their "ready" lines.  This will lead to buffer overruns, and if one
device on the line thinks it has hardware flow control turned on,
that buffer overrun might not even be checked for.

The second cable-- with the modifications you realized you forgot--
is more correct for most situations.

However, having said all this, I must now admit that I have a lab
FULL of measuring instruments hooked up via 3-wire RS-232, mostly
because whoever wired their internal RS-232 jacks had some really
"interesting" ideas about flow control.

So start with what is most correct, then try the 3-wire.  The WORST
cable in the world is a serial cable that crosses RX and TX but does
NOT CROSS the flow-control lines!  Any device requiring such an
abomination should be reworked.  No one on the face of the earth
should ever release such an egregious mistake.

...so make up about four cables to deal with THAT situation, too. =3D]

Cheers,
   Matt

PS: Your 3-wire solution does not address possible chassis ground
differrential problems, unless shield-grounded, but we'll look over
that for now. =3D]

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