The Zilog Z80 PIO chip (aka Z84C20) is a Dual 8bit I/O Port (not to be confused with the Triple 8bit 8255 PPI chip, which is sometimes referred to as PIO, too).
The chip has B/A (Port B/A Select) and C/D (Control/Data Select) pins, which may be connected to the CPUs A0 and A1 pins (or vice-versa, A1 and A0) (or other pins like A3 and A4). Anyways, no matter how the addressing is done:
There are 4 registers - 2 Data registers, and 2 Control registers.
Data Registers A and B
Data direction can be configured in the corresponding control register. Depending on the direction, the data bits are inputs or outputs with following meaning:
7-0 Data (0=Low, 1=High)
Aside from the 8 data lines, each port additionaly supports automatic /STROBE and READY handshaking signals (whereas, unlike as in printer ports, STROBE can act as an input, not an output), the handshaking stuff isn't used in Mode3.
Control Registers A and B
7-1 Interrupt Vectors Bit7-1 (Vector Bit0 is fixed = zero) 0 Must be 00h
7 Interrupt Enable (1=Enable) (same as when D0..D3=0Fh) 6-4 Not used (should be zero) 0-3 Must be 03h
7 Interrupt Enable (1=Enable) 6 Interrupt AND/OR (0=OR, 1=AND) ;\ 5 Interrupt High Low (1=High) ; used in Mode3 only 4 Interrupt Mask Follows ;/ ;<--- also ACK interrupt? 0-3 Must be 07h
7-6 Mode (0=out, 1=in, 2=bidirectional,3=control) (mode2 on PortA only) 5-4 Not used (should be zero) 0-3 Must be 0Fh
After Mode3 (control mode) was set, then next control write is:
0-7 Direction bits, separately for each pin (0=out, 1=in)
After InterruptMaskFollows was set (in Mode3 only?), then next write is:
0-7 Select bits to be monitored (in OR mode: 0=Select)
Usage in CPC
A Z80 PIO is used by the following CPC hardware expansions:
- Media:Z80pio z84c20.pdf - Zilog Z80 PIO Datasheet