Equinox Systems MEGAPORT Hardware Reference Manual page 58

Modular adaptor wiring is shown in Figure 4-10. When making
modular adaptors, the 3K resistor shown may be eliminated for use
with most terminals. The resistor is used to loop control signals back
to the attached device when 4-wire cable is used and pin 6 is not
connected. When using 6-wire cable and there is voltage on pin 6 the
resistor has no effect and the voltage on pin 6 is passed through to
the attached device.
Figure 4-10. Modular adaptors
52 Equinox MEGAPORT Hardware Reference Manual