ABB AC 900F Mounting And Installation Instructions Manual page 92

Serial interfaces of the CPU module
Redundant controller
The RS232/485 bus lines of a redundant controller should first be connected to
terminal strips and then be guided to the field. The terminal strips must be arranged
as close as possible to the controller. In this configuration, the pull up/down resistors
of the CPU module are not required.
RS232
RS 232 redundant PM.png
92
Section 6 Wiring
M&I – AC 900F Controller