Serial connection to RS/6000

[Jonathan Eunice]

I'd like to make an RS-232 connection with an RS/6000 320 planar serial
port.  Two devices are of interest: a 2400b Hayes-compatible modem
(DB-25 connector) and a Macintosh Plus (DIN-8 connector).  I have
naively tried some existing cables, and they do not work in a variety
of software configurations.  This much is to be expected.  Time to
RTFM.  But, I don't have the FM for the hardware.  So, does anyone know
the appropriate serial connections?

Many thanks in advance,

Jonathan Eunice