Table of Contents

General information

The 6551 ACIA is an RS232 serial controller. It is used in the RF57-232 expansion for Thomson computers, allowing use as a standard RS232 serial port, or an RS-232-like serial link at TTL levels, to communicate with a Minitel.

Programming

The ACIA has only 4 registers, making it relatively easy to work with.

A7E8/E7E8: Data register

This register is used to read received data bytes, and write outgoing ones.

A7E9/E7E9: Status register

this register is read-only.

BitDescription Cleared by Hardware reset valueSoftware reset value
0Parity error Self-clearing0-
1Framing error Self-clearing0-
2Overrun Self-clearing00
3Receive data register full Reading data register0-
4Transmit data register empty Writing data register1-
5/DCD Reflects /DCD pin state--
6/DSR Reflects /DSR pin state--
7IRQ Reading status register0-

The “self-clearing” status bits are cleared after reading the data register, and successfully receiving one byte.

A7EA/E7EA: Command register

Bits 7-5: parity check configuration

ValueOperation
765
--0Parity disabled
001Odd parity
011Even parity
101Mark parity bit transmitted, no parity check
111Space parity bit transmitted, no parity check

Bits 4-2: transmitter control

ValueTX interrupt/RTS levelTransmitter
432
100DisabledHighOff (local echo)
000DisabledHighOff
001EnabledLowOn
010DisabledLowOn
011DisabledLowTransmit BRK

Bit 1: receive interrupt enable

Bit 0: Data Terminal Ready

Controls the /DTR pin and enables the receiver.

A7EB/E7EB: Control register

Bit 7: stop bit

When set to 0, always use 1 stop bit. When set to 1:

Bits 6-5: word length

Bit 4: receiver clock source

Bits 3-0: baud rate divider