Rockwell Automation 1785-LTx,D17856.2.1 Classic PLC-5 Programmable Controllers Users Manual - Manual (Page 22)

Choosing Hardware
Chapter 2
2-3
Master/Expander I/O Modules
Some I/O modules (called “masters”) communicate with their expanders
over the backplane. These master/expander combinations either:
can time-share the backplane, or
cannot time-share the backplane
For masters that can time-share the backplane, you can use two masters in
the same chassis. For a master/expander combination that cannot
time-share the backplane, you cannot put another master/expander
combination in the same I/O chassis.
Example: The stepper-controller module (cat. no. 1771-M1, part of a
1771-QA assembly) and the servo-controller module (cat. no. 1771-M3,
part of a 1771-QC assembly) always act as masters and cannot time-share
the backplane. Therefore, you cannot put a second master module in the
same chassis with either of these modules.
Table 2.C summarizes the compatibility of master modules within a single
I/O chassis.
Table 2.C
Compatibility of Master Modules within a Single I/O Chassis
1st Master
Module
2nd Master Module
1771ĆIX 1 1771ĆIF 1 1771ĆOF 1 1771ĆM1 1771ĆM3
1771ĆIX 1 Valid 2 Valid 2
1771ĆIF 1 Valid 2 Valid 2 Valid 2
1771ĆOF 1 Valid 2 Valid 2 Valid 2
1771ĆM1
1771ĆM3
1
These modules have been superseded by 1771ĆIXE, ĆIFE, and OFE master modules that
do not exhibit the master/expander conflict in a chassis as 1771ĆIX, ĆIF, and OF master
modules shown in this table.
2
These are the only master combinations that you can use in a single I/O chassis. These
combinations are valid with or without the module's associated expanders (1771ĆM1 and
ĆM3 have expander modules). You can use a maximum of two masters in the same
chassis; you can use any other intelligent I/O modules not shown here with these masters.
Important: Density is not relevant to an expander module because it
communicates only with its master; an expander module does not
communicate directly with an adapter.