Casio PB-700 - hardware

Random Access Memory

See the HD61914 chip description. The PB-700 uses 4 of them.

The Expansion Port

expansion port

Pin functions

Expansion Port Pin	Symbol	Function
1	CE1	chip enable of the printer interface, active low
2	GND	ground (positive supply voltage rail)
3	CE2	chip enable of the cassette interface, active low
4	CK1	bi-phase clock
5	INIT	output for the printer control signal INIT
6	CK2	bi-phase clock
7	VDD3	-5V when operational, 0V when stand-by
8	D1	data 1
9	BUSY	input for the printer status signal BUSY
10	D2	data 2
11	FAULT	input for the printer status signal FAULT
12	D3	data 3
13	OP	register select
14	D4	data 4
15	GND	ground (positive supply voltage rail)

The Cassette Interface

The cassette subsystem in the FA-4 interface is almost identical with the FA-3 one, so the tape interface protocol used by the FX-700P applies to the PB-700 as well.

However, there's an interesting quirk in the PB-700 protocol. The calculator writes to the tape more data than reads back. For example the last 3 bytes of the file header segment remain unread. To skip these data the calculator inserts a close/open sequence between the segments which causes loss of a few bytes read from the tape. This effectively places the tape head to the lead-in of the next file segment. Seems like a kludge to me...

The Printer Interface

The printer communication protocol can be easily figured out from the circuit diagram of the FA-4 parallel port interface.

The printer port is selected by the CE1 signal latched by the rightmost D flip/flop.
Printed data (inverted) are sent through the 4-bit port in two bus cycles, the least significant nibble first. Data transfer is controlled by a binary counter formed of both remaining D flip/flops. Counter states:
11 - idle
00 - latch the least significant nibble in the TC4042 register
01 - idle (each data word is accompanied by two CK1 pulses, this state skips the second clock pulse)
10 - the most significant nibble and the Strobe pulse
The printer status signals (Busy, Fault) are fed directly to the calculator port.
The calculator uses the positive supply voltage rail as ground, while the parallel printer uses negative supply voltage rail as ground. Thus the circuit uses a TC5067 level translator chip in the data path, and two level translators built from discrete parts in the printer status path.