27
In the DP6 the 64 KByte EPROM U22 is selected by U25 pin 10 whenever either A14 or A15 is high, allowing
48 Kbytes of addressable EPROM. (In the DP7 the 64 or 128 KByte EPROM U24 is selected by U13 pin 19
whenever either A15 is high or any bank select BNK1 to 3 is low, allowing 64 or 88 Kbytes of addressable
EPROM.) The EPROM contains the dot patterns for all printable characters and all programs that the 6809
executes to control the printer.
The 8251 serial communication controller U24 (U27) is selected by U11 pin 9 (U13 pin 14) and receives and
transmits asynchronous serial data. Baud rate, word length, parity and number of stop bits are programmed
into the 8251 from the keypad. The baud rate clock for the 8251 is generated by a 74LS161 binary counter
U16 (U19) that provides one clock pulse for every thirteen E pulses provided by the 6809. RS-232 interface
signals are level-shifted by receiver U13 (U15) and driver U8 (U9). Two OR gates U12 (U14) control dual port
mode transmission.
The 6522 I/O & Timer device U20 (U23) is selected by U11 pin 10 (U13 pin 18) and contains two 8 bit output
ports, 4 control lines and 2 programmable timers. Port A controls the document stepper motor and Port B
controls the carriage stepper motor. Stepper motor timing and printhead print pulse length are controlled by
the 2 timers.
4.2.3 Stepper Motor Drivers
Each stepper motor has 2 stator coils, each driven by a 3717 driver U1 to U4. Each 3717 contains 4
transistors arranged in a H-Bridge that can apply +30 VDC to either end of the coil, a 25KHz switching
oscillator and comparators that limit the coil current by sensing the voltage on an external resistor R4,7,13 or
18 (R2,6,15 or 18). Each 3717 is controlled by 3 signals that are latched in the 6522: a phase signal which
indicates which direction current is to flow through the coil and two signals which limit the current to 0,20,60 or
100% of a maximum value determined by the external resistor. 0% is used to turn off a winding, 20% is used
when the motor is holding a fixed position and 100% is used when the motor is rotating. See Figure 4-1 for
typical voltage waveforms on both ends of a motor coil and the external resistor. The ground plane under the
4 3717's is connected to a finned heat sink to reduce the temperature rise. Both stepper motor drivers are
protected by a single 0.75A fuse F1.