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The high PRIME signal into pin 3 of the two shift register elements
(ME18 and ME19) disables all inputs to these elements and internally re-
sets all stages of the shift register. When PRIME goes low, recirculate
signal SRCL from LSI chip ME16-38 remains high and a single CLKTB2 pulse
is generated at ME16-36. The high SRCL (and low PRIME) connects shift
- -
register outputs TB1-TB7 back to inputs OSl-DS7. and enables a high (+5V)
input to stage 8. Note that since the shift register uses inverted sig-
nals for both inputs and outputs (e.g., DS1 and TBl), a high input to
stage 8 represents a ZERO into that stage.
The single CLKTB2 pulse then clocks a dummy character into the
register (i.e., ONES into stages 1-7 and a ZERO into stage 8).
4.3.2 SELECT
Before the printer can receive data, it must first be selected.
This can be done either by the SELECT switch on the operator panel or by
an octal 021 code on the input data lines.
Pressing the SELECT switch (S2) generates a low SELSW signal in-
to LSI chip ME25-31. The chip contains "anti-bounce" protection which
requires the SELSW line to be noise-free for approximately 2-8 msec
before the level is recognized by the chip.
After this delay, the low
SELSW input sets a Select latch in the chip. The output of this latch
appears as signal SLCT ' on LSI chip ME25-40.
Similarly, a decoded octal 021 code on data inputs DS1-DS7 ANOed
with ungated data strobe pulse OSTA also sets this Select latch.
The printer is deselected (i.e., the internal Select latch is
reset) either by again pressing the SELECT switch or by an octal 023
on data lines OSl-DS7 and a DSTA pulse. Alternately pressing the SELECT
switch alternately selects and deselects the printer. Note that when
power is turned on, PWRPRM resets the internal Select latch so that the
printer initially appears in a deselect state.
A high SLCT ' signal at ME2S-40 indicates that the printer is
selected. This signal, inverted by ME22-6 (SLCT) turns on the SELECT
lamp on the operator panel. SLCT inverted by ME20-4 generates a SLCT
signal to the interface connector.
4-S
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