Rockwell Automation 1771-DB Basic Module User Manual - Series A - Manual (Page 24)

Installing the BASIC Module
Chapter 3
3–10
The BAT LOW indicator should go out.
You can monitor the battery low condition in revision A and revision B
modules using a XBY(77B4H) statement. Bit 2 high indicates the battery
low condition.
With revision C modules use CALL 80 to monitor battery status.
3.5
Battery (continued)