Main PCB
Microprocessor
AUX PCB
Printer
Interface
N-20P only
Printer
Hex
Circuit
(N-20P only)
PCB
Printer
(N-20P only)
ADV
O n
D/D User Push Buttons
Figure 12.7 Printer Control Block Diagram
Printer Interface Circuit
The following is a description of the printer interface circuitry found on all N-20P auxiliary PCBs.
The CPU reads the PR PRESENT signal to determine if a printer is installed. With PR PRESENT left floating,
it is pulled high by the weak pull-up resistor inside the CPU. If a printer is installed, PR PRESENT is connected
to switched ground, which causes a low input to the CPU. The optional printer circuit is protected from
excessive battery currents by fuse F2. CR28 is used to block noise generated by the printer motor being injected
onto the batteries.
The N-20P printer is a 16-character-wide thermal dot matrix printer, which generates a CPU interrupt for every
dot column. The thermal energy given to the print head is controlled by the pulse width of the active high signals
PR DOTx. In order to provide consistent print quality, the ambient temperature, print drive voltage, and print
head resistance must be measured and accounted for.
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. A two-level resistor bridge is formed by R143, R144, R145,
R146, and head resistor DOT4. 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 which of the three head resistance categories this particular head is. 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.
12-23