6850 ACIA chip

MC6850 Asynchronous Communications Interface Adapter (ACIA) (24pin)

Usage in CPC

Used by CPCI_RS232_Interface (DIY german magazine CPC Schneider International):

 F8DCh CPCI Serial Interface MC6850 Control/Status Register (R/W)
 F8DDh CPCI Serial Interface MC6850 Data Register (R/W)
 Input CLK is 19.2kHz or 38.4kHz (manually selectable via switch)
 Status.Bit2 is always 0 in this circuit
 IRQ pin is not connected (can be read only by software, in Status.Bit7)

Used by KDS Electronics Serial Interface:

 FBECh KDS RS232 6850 (reversed-bit-order) Control (W)
 FBEDh KDS RS232 6850 (reversed-bit-order) TX Data (W)
 FBEEh KDS RS232 6850 (reversed-bit-order) Status  (R)
 FBEFh KDS RS232 6850 (reversed-bit-order) RX Data (R)
 Caution: KDS has wired the 6850 chip's data bus incorrectly: D7 is D0,
 D6 is D1, etc. (so the bit order must be reversed by software).
 Input CLK is 2MHz with variable 16bit divider (programmable 8253 chip)

Used by AMSSIO (DIY by Martin Zacho):

 FBF0h AMSSIO Serial Interface MC6850 Control/Status Register (R/W)
 FBF1h AMSSIO Serial Interface MC6850 Data Register (R/W)
 Input CLK varies from version to version (v1,v2,v3), in some versions,
 the input CLK is jumper-selectable
 IRQ pin is not connected (can be read only by software, in Status.Bit7)

Used by Music Machine (in this case, as MIDI interface, not as RS232 interface):

 F8ECh Music Machine ACIA Control (W)
 F8EDh Music Machine ACIA Data write (W)
 F8EEh Music Machine ACIA Status (R)
 F8EFh Music Machine ACIA Data read (R)

Registers

MC6850 Control Register (W)

 0-1  Baudrate (0=CLK/64, 1=CLK/16, 2=CLK/1, 3=RESET)
 2-4  Mode (0..7 = 7e2,7o2,7e1,7o1,8n2,8n1,8e1,8o1) (data/stop bits, parity)
 5-6  Transmit Interrupt/RTS/Break control (0..3)
       0 = Output /RTS=low,  and disable Tx Interrupt
       1 = Output /RTS=low,  and enable Tx Interrupt
       2 = Output /RTS=high, and disable Tx Interrupt
       3 = Output /RTS=low,  and disable Tx Interrupt, and send a Break
 7    Receive Interrupt (1=Enable on buffer full/buffer overrun)

MC6850 Status Register (R)

 0    Receive Data  (0=No data, 1=Data can be read)
 1    Transmit Data (0=Busy, 1=Ready/Empty, Data can be written)
 2    /DCD level
 3    /CTS level
 4    Receive Framing Error (1=Error)
 5    Receive Overrun Error (1=Error)
 6    Receive Parity Error  (1=Error)
 7    Interrupt Flag (see Control Bits 5-7) (IRQ pin is not connected)

MC6850 Data Register (R/W) Data can be read when Status.Bit0=1, and written when Status.Bit1=1.

 0-7  Data

Rx has a 1-stage fifo, plus 1 shift register (2 stages in total) Tx has a 1-stage fifo, plus 1 shift register (2 stages in total)

Datasheet

• Motorola MC6850 datasheet