TM 5-6675-316-14
(3) Clock interface.
The clock interface on the board receives a 500 kHz
signal from the program bus. The clock signal is counted down by a four-stage
counter to obtain the various frequencies required to drive all the stepping motors.
The outputs are buffered and then bused via the
carriage escapement and font interface boards.
(4) Row shift stepping control . This circuit controls the complete sequence
required to change fonts by moving the font disc. The main program determines the
number of steps that the font disc stepping motor will be required to take. This
information is passed to the row select step/delay control counter. The direction
in which the move is to be made is decoded and fed to the row select motor sequenc-
er. When a START command is received by the START CONTROL, it removes a HOLD signal
from the row select step/delay control counter and, at the same time, the sync
control is enabled. The system is now in a stepping mode. The 250 Hz pulse starts
the row select motor sequencer, and the row select step/delay control counter begins
to decrement the value originally set by the program instruction. The output pulses
to the driver board will terminate only when the input pulses to the row select
motor sequencer end. Input pulses continue until the row select step/delay control
counter is empty. At this point, it resets the step/delay control counter, in turn
resetting the sync control,stopping the stepping sequence.
2-57
