Printronix p300 Maintenance Manual page 43

2–5. Print State
–continued
f. Hammer Driver Circuits
The hammer logic circuits perform the following functions:
–Convert serial data bits (44 for P300; 66 for P600) on the COM line into parallel data bits (44 for
P300; 66 for P600).
–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 coils
and hammer drivers (P600 only).
–Provide an isolated and regulated +5 VDC supply, 5HL, for TTL logic on the hammer driver PCBA
Following a SYNC pulse, hammer shift clock pulses, NHSC, load 66 bits from the COM line into the ham-
mer driver shift register. Every bit (of 132 bits) on the COM line is clocked into the shift register, selecting
dot information for the numbered characters to be printed by each hammer.
After 132 microseconds, the last bit is clocked into the shift register. The next SYNC pulse causes hammer
clock, NHCK, to load the contents of the shift register into the data latches.
When a dot is to be printed, the data will set the latch for that hammer and the corresponding Darlington
driver transistor is forward biased. This completes the current path for the hammer supply through the
hammer coil to its return. The coil is energized and the hammer is released to print the dot.
Hammer reset signal, HRS, resets the OE level of the tri–state latch. The output disables all tri–state buffer
outputs and deenergizes the coils. The rebounding hammers are recaptured by the permanent magnet.
g. Safety Circuits, P600 Only (Figure 2–14)
Excessive current and overheating can damage coils and hammer drivers in either of the following situ-
ations:
–Current turned on when not needed to release hammer springs.
–Current left on after hammer spring release.
Three safety circuits on the hammer driver board prevent excessive current by disabling the tri–state buff-
ers under control of the OE signal. For each of the following three conditions, OE is held low:
– Initial power on – the OE flip–flop is held reset until +9.5 V rises to a value within tolerance. This
prevents spurious release of hammer springs.
– +5HL supply out of tolerance – when the 5HL voltage output exceeds Zener diode avalanche levels,
the OE flip–flop is reset and held low.
– +36V supply ground missing – If the +36 V return is not connected to the hammer driver board, the
OE line is held reset.
P300/P600 Principles of Operation
–continued
2–21