Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 43)

Installation
Chapter 3
3-19
The ACTV light remains on as long as the cable between the
COMPUTER ASYNCHRONOUS socket and the interfacing
asynchronous device is properly connected. This light will appear to
flicker whenever characters are being transmitted across the asynchronous
link. If this light goes off, check the cable and connectors for possible
problems.
The CPU light comes on for about half a second every time the module
transmits or receives a DLE NAK protocol sequence (Chapter 4). If this
light flickers frequently or stays on, the asynchronous link might need
better isolation or noise immunity.
If the KF2 is on a PCL, the CPU light also flashes at about 1 Hz when a
duplicate node (two stations with the same station number) is detected and
stays on when an internal hardware fault is detected. For more on the
KF2 self-diagnostics, see Chapter 1 under “Module Description.”