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Technical Supplement
9.6.13.2
Printer Flex Circuit and User Controls
9-30
Inside the print head are seven resistors that heat up when power is applied, and
in turn create dark dots on the thermal paper. One lead of the print-head
resistors is connected to the printer supply voltage VPRN; the other lead is
connected to the driver chip (see "Optional Printer Flex Circuit with User
Controls"). One of the print dot resistor leads (DOT4) is also fed back to the
printer interface circuitry. The DOT4 signal is a print dot resistor with a range
of 11-16 ohms, which is connected to VPRN.
The print head resistance is measured by U36. R143, R144, R145, R146, and
head resistor DOT4 form a two-level resistor bridge. The resistor bridge is
switched on when PR_MEAS is pulled high, pulling TP77 low and biasing the
resistor bridge. The logic outputs of PR_HEAD1 and PR_HEAD2 are read in by
the CPU to determine the head-resistance category of this particular head. R156
ensures that Q20 does not switch on when the batteries are installed backward.
Due to the large current draw of the resistor bridge and the fact that the head
resistance does not change significantly over time, the head resistance is only
measured once at every power-on.
The CPU starts the printer motor running by setting PR_MOTOR high. A single
motor drives both the print head and paper advance mechanisms. The printer
provides a printer timing generator (TG) signal, which is an AC waveform of
about 4 Vpp. Q19, R106, R142, and CR29 convert the AC waveform to a
CMOS level square wave; this signal (PR_TACH) is then used as a CPU
interrupt line. An interrupt routine services the printer, thereby producing the
required dot patterns to create the characters. C127 is used to filter noise.
The position of the print head is sensed by the signal PR_HOME. Whenever the
print head is not in the home position, a switch in the printer closes, shorting
PR_HOME to switched ground. Whenever the print head is in the home
position, the switch opens, allowing R118 to pull PR_HOME high.
The print head dot pattern and pulse width are controlled by the CPU. The
proper printer dot values are loaded into the output port, then the proper pulse
width is loaded into the CPU CAM for PR_STROBE. The signal PR_STROBE
enables the outputs for the specified pulse width. When the PR_DOTX lines are
high, a dot will be printed.
The printer flex circuit is illustrated in Figure 9-33, at the end of section 9.
The thermal printer is plugged in via connectors JP10 and JP11. The
PR_PRESENT signal is connected to switched ground to tell the CPU that a
printer is installed. U1 is a Darlington pair driver chip that is used to drive the
printer dots and motor. When an input is high, the output is shorted to ground,
driving the output load.
The auxiliary PCB provides constant power (VPRN) and a power control line
(PWR_ON). Q1 is used as a power control FET. Both halves are used in
parallel to reduce the on resistance. When PWR_ON is high, the sources (S1,
S2) short to the drains (D1, D2), connecting ground to U1 and C3. PWR_ON
also controls the regulated power supplies; thus, Q1 and the power supplies are
both enabled and disabled at the same time.
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