Rockwell Automation 1771-IS 5V DC INPUT MOD - Manual (Page 29)

Programming and Operation
Chapter 3
3-10
Important: When momentary switch contacts are used, the momentary
(latched) contact position (switch #4, ON) should be selected. Otherwise,
bits will be reset when the momentary contact switches open and data
representing OFF-to-ON transitions can be lost.
The maintained contact position (switch #4 OFF) should be selected if the
application calls for the use of up to 72 maintained contact switches.
3-digit Thumbwheel Switch Data (12-bit storage)
Data is loaded into memory on the basis of whether you have selected the
maintained or momentary switch contact mode (switch #4).
Maintained Contact Mode
When 3-digit BCD thumbwheel switches are wired according to the block
diagram in Figure 3.6a, the switch settings are stored automatically in the
data table after a block transfer as shown in Figure 3.6b. The first word is
reserved for diagnostic data. Each remaining word stores the three digits
of the thumbwheel setting. The digits MSD, MD and LSD have been
loaded, respectively, from left to right, in bits 13-00 in the same order that
they are normally read. For example, observe how the 3-digit switch
setting 642 of switch #3 is loaded into the data table (Figure 3.6b).