Z80-DART/Z80-SIO chip

Used in standard RS232 interfaces for the CPC (Amstrad_Serial_Interface), mapped to following Ports:

 FADCh Amstrad RS323 Z8470 (Z80 DART) Channel A Data             (R/W)
 FADDh Amstrad RS323 Z8470 (Z80 DART) Channel A Control/Status   (R/W)
 FADEh Amstrad RS323 Z8470 (Z80 DART) Channel B Data             (R/W)
 FADFh Amstrad RS323 Z8470 (Z80 DART) Channel B Control/Status   (R/W)

Chip Variants (DART, SIO/0, SIO/1, SIO/2, SIO/3, SIO/4) Most CPC interfaces should contain the DART chips. Eventually some might use the SIO chips (which are including some additional features).

 Zilog Z8470 - Z80 DART (Dual-channel Asynchronous Receiver Transmitter)
 Zilog Z8440 - Z80 SIO (Dual-channel Serial Input/Output Controller)

Note: Five different variants of the SIO chips exist: types 0/1/2 are 40pin DIP chips with slightly different features/pinouts, types 3 (QFP package) and 4 (PLCC package) are 44pin chips, both combining all features of the three 40pin chips.

Control/Status Registers In the default state, reads/writes on the Control/Status port are accessing the RR0/WR0 registers. After writing a non-zero index value "n" to Bit0-2 of WR0, the next read/write operation on the Control/Status port will access the corresponding RRn/WRn register.

Control Registers

WR0 Write register 0

 0-2  Register Index (0-5 for write, 0-2 for read) (SIO: 0-7 for write)
 3-5  Command (0..7)
       0=No action
       1=DART: Reserved / SIO: Send Abort (SDLC)
       2=Reset Ext/Status Interrupts
       3=Reset Channel
       4=Enable Interrupt on next Rx character
       5=Reset Tx Int pending
       6=Reset Error
       7=Return from Int (Ch-A only)
 6-7  CRC Reset Code (DART: Reserved, should be 0) (SIO: 0..3)
       0=No action
       1=DART: Reserved, SIO: Reset Receive CRC Checker
       2=DART: Reserved, SIO: Reset Transmit CRC Generator
       3=DART: Reserved, SIO: Reset Tx Underrun/End of Message latch

WR1 Write register 1

 0   External Interrupts Enable
 1   Tx Interrupt Enable
 2   Status Affects Vector (Channel B only) (see WR2/RR2)
 3-4 Rx Interrupt Mode (0..3)
      0=None
      1=On first Rx char
      2=On all Rx chars (Parity affects vector)
      3=On all Rx chars (Parity does not affect vector)
 5   Wait/Ready on Receive/Transmit
 6   Wait/Ready Function
 7   Wait/Ready Enable

WR2 Write register 2 - Interrupt Vector (Channel B only)

 0-7 Interrupt Vector (Bit1-3=no effect when WR1.Bit2=1, see RR2 for details)

WR3 Write register 3 - Rx Control

 0   Rx Enable
 1   DART: Reserved (must be 0), SIO: Sync Char Load Inhibit
 2   DART: Reserved (must be 0), SIO: Address Search Mode
 3   DART: Reserved (must be 0), SIO: Receiver CRC Enable
 4   DART: Reserved (must be 0), SIO: Enter Hunt Phase
 5   Enable automatic hardware handshaking using RTS/CTS
 6-7 Rx data bits          (0..3 = 5bits, 7bits, 6bits, 8bits)

WR4 Write register 4 - Rx/Tx Control

 0   Rx/Tx Parity bit      (0=None, 1=Enable)
 1   Rx/Tx Parity type     (0=Odd, 1=Even)
 2-3 Rx/Tx Stop bits (0..3=Reserved, 1bit, 1.5bits, 2bits) (SIO: 0=Sync Modes)
 4-5 DART: Reserved (must be 0), SIO: Sync Mode (enabled when Bit2-3=zero)
      0=8bit SYNC Character        (SIO only, not DART)
      1=16bit SYNC Character       (SIO only, not DART)
      2=SDLC Mode (0111 1110 Flag) (SIO only, not DART)
      3=External SYNC Mode         (SIO only, not DART)
 6-7 Rx/Tx DART clock mode (0..3=X1, X16, X32, X64)

WR5 Write register 5 - Tx Control

 0   DART: Reserved (must be 0), SIO: Tx CRC Enable
 1   RTS enabled/disabled
 2   DART: Reserved (must be 0), SIO: Rx/Tx CRC Type (0=SDLC, 1=CRC-16)
 3   Tx Enable
 4   Tx Send break
 6-5 Tx data bits          (0..3 = 5bits, 7bits, 6bits, 8bits)
 7   DTR enabled/disabled

WR6 Write register 6 - Sync Character or SDLC address (Z80 SIO only)

 0-7 DART: N/A, SIO: LSBs of 8bit/16bit sync char, or SDLC address

WR7 Write register 7 - Sync Character or SDLC flag (Z80 SIO only)

 0-7 DART: N/A, SIO: MSBs of 16bit sync char, or SDLC flag (should be 7Eh)

Status Registers

RR0 Read register 0 (General Status Bits)

 0   Rx character available (3-stage RXFIFO not emtpy)
 1   interrupt pending      (Channel A only) (always 0 on Channel B)
 2   Tx buffer empty        (1-stage RXFIFO emtpy)
 3   DCD (carrier detect)
 4   DART: RI (ring indicate), SIO: Sync/Hunt
 5   CTS
 6   DART: Not used,           SIO: Transmit Underrun/End of Message
 7   break received,           SIO: Break/Abort

RR1 Read register 1 (Used with Special Receive Condition Mode)

 0   Tx all sent (1-stage TXFIFO and TX-shift register empty)
 1-3 DART: Not used,           SIO: Residue Codes
 4   Rx parity error
 5   Rx overrun error
 6   framing error             SIO: CRC or Framing Error
 7   DART: Not used,           SIO: End of Frame

RR2 Read register 2 (Interrupt Vector) (Channel B only)

 0-7 Interrupt Vector (as set via WR2) (Bit1-3=variable when WR1.Bit2=1)
      0 Channel B Transmit Buffer Empty       (1-stage TXFIFO empty)
      1 Channel B External/Status Change
      2 Channel B Receive Character Available (3-stage RXFIFO not empty)
      3 Channel B Parity/Rx Overrun/Framing Error, or End-of-Frame (SDLC)
      4-7 Same as above, but for Channel A

RR3-7 Read registers 3..7 - Reserved

 -   Reserved / don't use

Data Registers

Rx Data Register

 has a 3-stage FIFO, plus 1 rx shift register (4-stages in total)

Tx Data Register

 has a 1-stage FIFO, plus 1 tx shift register (2-stages in total)

Interrupt Notes

 Interrupts can occur on both channel A and channel B, certain bits (like
 interrupt enable flags) can be configured separately for each channel,
 other bits are shared for both channels:
 the interrupt vector can be accessed via RR2/WR2 of channel B only
 the interrupt pending flag can be read via RR0.Bit1 of channel A only

No info if the interrupt signal is connected to the CPC hardware, probably... it isn't?