Printronix P9012, Maintenance Manual

Page: 55 / 256

2–37 Principles of Operation
Safety Circuits
(Figure 2–28)
Excessive current and overheating can damage coils and hammer drivers in
either of the following situations:
•
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 latch output drivers under control of the OE signal. For each
of the following three conditions, OE is held low:
•
+5VHL supply out of tolerance — when the +5VHL voltage output
exceeds Zener diode avalanche levels, OE is held low.
•
–12V, +36V supply ground missing — if the –12V, +36V return is not
connected to the hammer driver board, the OE line is held low.
•
+5V, –12V voltages levels are out of tolerance.
Two other circuits monitor the leakage currents of both the overdrive
transistor and the lower hammer driver transistors and if the currents are too
large, the fault circuit signals the MCU which in turn turns off the printer via
BRKRTRIP.
A fourth safety circuit halts hammer firing in the event of a hammer master
clear, HMC, or the loss of the hammer reset pulse, HRS. When HMC is low,
the output of the one millisecond one–shot, U16, is normally kept high while
being triggered by HRS pulses. Whenever HRS pulses stop or HMC goes
high, the one–shot resets within one millisecond and hammer firing stops.
1 MILLISECOND
U16
ONE – SHOT
HRS
HMC
4
3
2
1
Q
OE
8
SD
NORMALLY HIGH
GOES LOW FOR HMC
OR LOSS OF HRS PULSES
1
2
3
4
+5VHL
HRS
Q
Figure 2–28. Safety Circuits