Parallel transfer schemes and 8255A block diagram

Schemes of parallel data transfer

1. Simple (unconditional / synchronous) I/O: device is always ready (switches, LEDs); the CPU reads or writes directly with IN/OUT, no handshake.
2. Strobed I/O (single handshake / polled): the device gives a strobe when data is valid; the CPU checks a status flag then reads.
3. Handshake (double handshake) I/O: both sides exchange signals (STB' / IBF for input, OBF' / ACK' for output). Used for slow peripherals like printers; supports polled or interrupt (INTR) transfer.
4. Interrupt driven and DMA for further efficiency.

Timing: strobed input (8255 mode 1)

 Data     ----< valid data >------------
 STB'     ------\______/---------------
 IBF      ________/-----------\________   (cleared by RD' rising)
 INTR     ____________/--------\_______
 RD'      -----------------\____/------

STB' low loads the data into the port latch; IBF goes high to tell the device "buffer full"; INTR goes high (if INTE set) to interrupt the CPU; RD' from the CPU resets IBF and INTR.

Timing: output with handshake (mode 1)

 WR'   ---\__/---------------------
 OBF'  -----\___________/---------
 ACK'  ----------------\___/------
 INTR  ______________________/----

Functional block diagram of 8255A

 D0-D7 <-> [Data bus buffer]   +--> [Group A control] -> PA0-PA7
 RD',WR' ->[Read/Write control]|        [Port A]
 A0,A1  -> [   logic         ] |        [Port C upper] PC4-PC7
 CS',RESET                     |
                               +--> [Group B control] -> PB0-PB7
                                        [Port B]
                                        [Port C lower] PC0-PC3

• Data bus buffer: 8-bit bidirectional buffer to the system bus.
• Read/write control logic: uses CS', A0, A1, RD', WR', RESET to select ports/control register and direction.
• Group A: port A and upper port C (PC7-PC4). Group B: port B and lower port C (PC3-PC0).
• Operating modes: mode 0 (basic I/O), mode 1 (strobed I/O), mode 2 (bidirectional, port A only), and BSR (bit set/reset of port C).