Protocol overview

A frame-locked, peer-to-peer exchange over a plain 8N1 UART.

Line layer

Parameter	Value
Wiring	null-modem crossover (GND straight, TXD↔RXD, /CTS↔/RTS)
Baud	19200
Framing	8N1, LSB first, idle-high, non-inverted
Levels	3.3 V CMOS
Flow control	hardware /CTS, software /RTS (I/O 0xB2)
Driver	TMP95C061 SIO channel 0

Games never touch SIO registers — all I/O goes through the BIOS COM services.

Byte classes

Range	Meaning
0xF0–0xFF	control tokens — handshake FC/FD, transport F0–F7, gameplay F8, bulk ack F9/FA, teardown FE, sentinel FF
other	payload — controller bitmaps, ring bytes, identity params

Roles

Endpoints are identical; roles are negotiated. Whoever gets FC onto the wire first becomes P1 (FC requester); the peer answers FD and becomes P2 (FD responder). In gameplay P2 sends the offer F8 00 <P2>, P1 the reply F8 <P1> <P2>.

State machine

Tick 0x206c8f dispatches on state byte 0x564e (a ×4 table offset).

stateDiagram-v2
    [*] --> DISABLED: FF
    DISABLED --> INIT: external write 00
    INIT --> ADVERTISE: 04
    ADVERTISE --> RESPOND_FD: 08 (got FC)
    ADVERTISE --> FINALIZE: 0C (got FD)
    RESPOND_FD --> FINALIZE: 0C
    FINALIZE --> ACTIVE: 14
    ACTIVE --> SHUTDOWN_FLUSH: external write 10
    SHUTDOWN_FLUSH --> DISABLED: FF

State	Value	Does
DISABLED	FF	no dispatch; external code writes 00 to start
INITIALIZE	00	COMINIT + COMRECIVESTART + COMONRTS → 04
ADVERTISE	04	send FC; got FC→08, got FD→0C
RESPOND_FD	08	send FD → 0C
FINALIZE	0C	finish handshake → 14
ACTIVE	14	transport / gameplay until reset
SHUTDOWN	1C	flush + /RTS cycle → FF

ACTIVE has two sub-modes (status bits): bit 7 set → normal F0–F7 transport; bit 7 clear + bit 3 set → F8 gameplay.

Timing

Lockstep at the frame cadence: 33.39 ms (~30 Hz) between exchanges — capture-measured.