Printronix L5000 Series Maintenance Manual page 183

The transfer of the next page of print data is initiated by the MPU asserting the
VSYNC signal. The 030 (or 040) controller responds to the VSYNC signal by
asserting the PRNT signal, indicating that the data for the next page is ready for
transfer.
The MPU begins to load the next page of print data only when it acknowledges the
030 (or 040) controller's PRNT signal.
Principles of Operation 5–23