Hammer Driver Board; Hammer Driver Logic And Control - Printronix

P9212 Maintenance Manual

Hammer Driver Board

The hammer driver board consists of three functional elements: hammer
driver logic and control circuits, blower drive and monitoring circuits, and
filter capacitors for the +48 V and +12.5 V power supplies.

Hammer Driver Logic and Control

Each hammer spring is controlled by two electromagnetic coils, a driver, and
a logic circuit. The hammer logic circuits perform the following functions:
♦
Convert serial data bits on the COM line into parallel data bits.
♦
Control the energizing of hammer coils to print dots in accordance with
the parallel data.
♦
Provide safety features to prevent coils from energizing under conditions
that could damage the coils and hammer drivers.
The buffered hammer shift clock (BHSC) pulses load data from the COM
DATA line into the hammer driver shift register. Every bit on the COM line
is clocked into the shift register by the rising edge of BHSC, containing dot
information for the characters to be printed by each hammer.
After the last bit is clocked into the shift register, the FIRE signal causes the
contents of the shift register to be loaded into the data latches. These latches
drive the gates of each lower drive MOSFET (Metal Oxide Semi–conductor
Field Effect Transistor).
The FIRE signal also turns the upper drive transistors on. When FIRE is high
and a lower driver is on, 48 volts are applied across the hammer coil. This
causes the coil current to rise rapidly, cancelling the magnetic field holding
the hammer retracted. With the magnetic field cancelled, the hammer starts
to fly forward. The FIRE signal drops, disabling the upper drivers. The coil
current is sustained by the upper driver diodes and the lower driver
MOSFETs. This combination applies 12.5 volts across the coil, keeping the
magnetic field cancelled until hammer impact.
After the dot is printed, the NLD_RST signal resets the lower driver
MOSFETs. The remaining coil current returns to the 48 volt supply through
flyback diodes. The magnetic field is restored and the permanent magnet
captures the hammer. (See Figure 2–19.)
Principles of Operation 2–35