====== 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.

^Bit^Description                   ^Cleared by   ^Hardware reset value^Software reset value^
|0|Parity error                    |Self-clearing|0|-|
|1|Framing error                   |Self-clearing|0|-|
|2|Overrun                         |Self-clearing|0|0|
|3|Receive data register full      |Reading data register|0|-|
|4|Transmit data register empty    |Writing data register|1|-|
|5|/DCD                            |Reflects /DCD pin state|-|-|
|6|/DSR                            |Reflects /DSR pin state|-|-|
|7|IRQ                             |Reading status register|0|-|

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

===== A7EA/E7EA: Command register =====
<columns>
==== Bits 7-5: parity check configuration ====
^Value^^^Operation^
^7^6^5| |
|-|-|0|Parity disabled|
|0|0|1|Odd parity|
|0|1|1|Even parity|
|1|0|1|Mark parity bit transmitted, no parity check|
|1|1|1|Space parity bit transmitted, no parity check|
<newcolumn>
==== Bits 4-2: transmitter control ====
^Value^^^TX interrupt^/RTS level^Transmitter^
^4^3^2| |
|1|0|0|Disabled|High|Off (local echo)|
|0|0|0|Disabled|High|Off|
|0|0|1|Enabled|Low|On|
|0|1|0|Disabled|Low|On|
|0|1|1|Disabled|Low|Transmit BRK|
</columns>

==== 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:
  * If word length is 8 bit and parity enabled: 1 stop bit (9-bit words mode)
  * If word length is 5 bit and parity disabled: 1.5 stop bits
  * In all other cases: 2 stop bits

==== Bits 6-5: word length ====
  * 0: 8 bits
  * 1: 7 bits
  * 2: 6 bits
  * 3: 5 bits

==== Bit 4: receiver clock source ====
  * 0: use external clock on pin 17
  * 1: use baud rate generator

==== Bits 3-0: baud rate divider ====
  * 0: Divider disabled - 115200bps
  * 1: 50 baud
  * 2: 75 baud
  * 3: 109.92 baud
  * 4: 134.58 baud
  * 5: 150 baud
  * 6: 300 baud
  * 7: 600 baud
  * 8: 1200 baud
  * 9: 1800 baud
  * A: 2400 baud
  * B: 3600 baud
  * C: 4800 baud
  * D: 7200 baud
  * E: 9600 baud
  * F: 19200 baud