On Sat, 11 Jul 2009 13:25:25 +0100, Clint Sharp
wrote:

IIRC the max length for RS-232 by the specs is about 15 metres.

The maximum specification cable length is 50 feet, or the cable length
equal to a capacitance of 2500 pF. It is the puffage rather than the
length which is critical so using a cable with low capacitance allows
you to span longer distances without going beyond the limitations of
the standard. Using CAT-5 cable with a typical capacitance of 17
pF/ft, the maximum allowed cable length is about 50m.

Somewhere in the garage I've some line drivers that will allow you to
go to about a km if you want.