Serial I/O (RS232 / RS423)

Serial I/O — MOS Interface

Reference for MOS-level serial port use. For the chip-level details, see 6850-acia and serial-ula.

The 5-pin DIN socket

Pin	Signal	Direction
1	Signal ground	—
2	CTS — clear to send	input
3	TD — transmit data	output
4	RD — receive data	input
5	RTS — request to send	output

Notch on metal shield → left side of computer (correct orientation). The plug inserts physically rotated 180° too — won’t damage but won’t work.

BBC-to-BBC null modem: cross-wire TD↔RD and RTS↔CTS on each end, plus ground.

RTS/CTS semantics on the BBC

Not standard EIA RS232. The BBC’s RTS is asserted unless the receive buffer is full, and the BBC stops transmitting when its CTS input is unasserted. This is closer to standard DTR/DSR than to RS232 RTS/CTS — useful caveat when connecting a BBC to a PC.

For PC interop, you typically need a hardware cable that:

• Cross-wires the data lines (TD↔RD).
• Cross-wires RTS↔CTS and asserts the PC’s DTR/DSR/DCD permanently.

A “null modem with hardware handshake” cable is what works.

OSBYTE summary

OSBYTE	Function	Notes
&02 (2)	Select input stream	X=0 kbd, X=1 RS423, X=2 both
&03 (3)	Select output stream	Bit-packed (see below)
&07 (7)	Set rx baud	X = code 0-8
&08 (8)	Set tx baud	X = code 0-8
&89 (137)	Cassette motor	X=0 off, X=1 on
&9C (156)	R/W 6850 control register + OS shadow	Tube-safe path
&B0 (176)	R/W CFS timeout counter	Decremented per vsync (50 Hz)
&B1 (177)	R/W input source flag	Used by &02
&B5 (181)	R/W RS423 mode	See below
&BF (191)	R/W RS423 use flag (bit 7)	Bit 7 set = free
&C0 (192)	Read shadow of 6850 control reg	Don’t write — would desync
&CB (203)	R/W RS423 handshake threshold	Default 9 bytes
&CC (204)	R/W RS423 input suppression	Non-zero = ignore input
&CD (205)	R/W RS423/cassette select	&00 serial, &40 cassette
&E8 (232)	R/W 6850 IRQ bit mask	See interrupts
&EC (236)	R/W output stream destination flag	Used by &03
&F2 (242)	Read shadow of serial ULA register	Read-only access

Baud rates

OSBYTE &07 (rx) and OSBYTE &08 (tx) take X = code:

X	Baud
0	9600
1	75
2	150
3	300
4	1200
5	2400
6	4800
7	9600
8	19200

X=0 and X=7 both give 9600 baud (legacy code aliasing). Use X=7 for clarity.

Input stream selection (OSBYTE &02)

LDA #&02 : LDX #x_value : JSR &FFF4

X	Effect
0	keyboard only (default)
1	RS423 only (keyboard disabled)
2	both keyboard + RS423

Returns previous source in X.

Output stream selection (OSBYTE &03)

X is a bit field:

Bit	Set means
0	Enable RS232/423 output
1	Disable VDU
2	Disable printer driver
3	Enable printer independent of VDU 2/VDU 3
4	Disable spooled output
6	Disable printer unless preceded by VDU 1

*FX 3,0 = defaults: RS423 disabled, VDU enabled, printer follows VDU 2/VDU 3, spool follows *SPOOL.

*FX 3,1 adds RS423 output (so OSWRCH bytes go to both screen and serial).

RS423 mode (OSBYTE &B5)

Controls how received RS423 bytes are processed:

Flag	Behaviour
0	ESCAPE bytes generate ESCAPE events; soft-key expansion applies; input-buffer event fires per byte; cursor editing keys work
1 (default)	Bytes go straight into buffer; no events; no escape recognition

Mode 1 is correct for terminal emulators and file transfer (don’t want stray &1B to abort). Mode 0 makes RS423 input behave exactly like keyboard input — useful for remote console.

Handshake threshold (OSBYTE &CB)

When the RS423 input buffer has fewer than X bytes free, the OS deasserts RTS to halt the sender. Default 9. Increase for slow-responding senders, decrease if you really need every byte of buffer (and trust the sender to stop on time).

6850 control word — basic configuration

For standard RS423 8-N-1 at the selected baud:

; 6850 control: clk÷64, 8 bits no parity 1 stop, RTS low, TX IRQ disabled, RX IRQ enabled
LDA #&95         ; 1001 0101 — bit 7=1 (RX IRQ), bits 6-5=00 (RTS low TX off), bits 4-2=101 (8-N-1), bits 1-0=10 (÷64)
LDX #&95
LDA #&9C : JSR &FFF4

For raw 7-E-1 (common for ASCII terminals):

LDA #&91         ; bits 4-2=010 (7 bits, even, 1 stop)
LDX #&91
LDA #&9C : JSR &FFF4

Bulk transfer pattern

For sending a buffer of bytes:

.send_loop
    LDA &FE08       ; status
    AND #&02        ; TDRE
    BEQ send_loop   ; wait until TX register empty
    LDA (src),Y
    STA &FE09       ; transmit
    INY
    BNE send_loop
    ; ... next page handling ...

For 9600 baud this is fine; ~960 bytes/sec means the CPU spends most of its time waiting. At 19200 it’s ~1.9 KB/sec — still loosely bounded by the serial line, not the CPU.

For higher throughput consider:

• IRQ-driven transmit — let the TDRE IRQ wake up your buffer-drain code; foreground does other work.
• Direct ULA write for baud-rate switches (skips the OSBYTE overhead).

CFS (cassette) tangent

The same 6850 + Serial ULA drive the cassette interface. Switching:

LDA #&CD : LDX #&40 : LDY #0 : JSR &FFF4   ; cassette mode flag
LDA #&8C : LDX #12 : JSR &FFF4             ; *TAPE 12 (1200 baud)

Back to RS423:

LDA #&CD : LDX #0 : LDY #0 : JSR &FFF4     ; serial mode flag
LDA #&07 : LDX #7 : JSR &FFF4              ; rx 9600
LDA #&08 : LDX #7 : JSR &FFF4              ; tx 9600

CFS timeout via OSBYTE &B0 is a vsync-counted (50 Hz) timer for tape block gaps.

Bypass-MOS strategies

• Direct STA &FE09 for transmit — saves the OSWRCH dispatch but you must check status yourself.
• Direct LDA &FE09 for receive — pulls the byte and clears RX flag in one read.
• Custom IRQ2V handler — claim 6850 IRQs via OSBYTE &E8 mask, handle yourself for custom protocols (e.g. packet framing).
• Bit-bang via User VIA — for arbitrary baud rates or non-standard framing (SPI etc.). Costs CPU cycles but unconstrained by the 6850’s framing rules.

See also

• 6850-acia — Chip details.
• serial-ula — Baud rate + cassette switch.
• interrupts — 6850 IRQ in MOS dispatch chain.
• buffers — RS423 in/out buffers (IDs 1, 2).

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