Rockwell Automation 1771-DA,D17716.5.13 U MNL 1771-DA ASCII I/O MODULE - Manual (Page 162)

Troubleshooting
Chapter 7
7Ć3
When the input buffer is full and when you use control lines, the module
signals the ASCII device to stop sending data. If you do not use control
lines and the ASCII device continues to send data when the input buffer is
full, the data spills over and is lost.
Typical examples of fault conditions displayed by status indicators, and
corrective action that you can take, are shown in the troubleshooting chart
(Table 6.B). If the FAULT indicator should remain on, return the module
to Allen-Bradley for service.