serial.device — UART and RS-232 Communication

Overview

serial.device provides access to the Amiga's built-in UART (Universal Asynchronous Receiver/Transmitter) for RS-232 serial communication. The hardware UART is implemented using CIA-B shift registers and custom chip DMA. Serial I/O is fundamental for debugging (serial console), modem communication, MIDI, and inter-machine networking.

Hardware

Serial Port Pins (DB-25)

Pin	Signal	Direction	Description
2	TxD	Output	Transmit data
3	RxD	Input	Receive data
4	RTS	Output	Request to send (active low)
5	CTS	Input	Clear to send (active low)
6	DSR	Input	Data set ready
7	GND	—	Signal ground
8	CD	Input	Carrier detect
20	DTR	Output	Data terminal ready

Custom Chip Registers

Register	Address	Description
`SERDAT`	`$DFF030`	Serial data write (transmit) — 9 bits: stop bit + 8 data
`SERDATR`	`$DFF018`	Serial data read (receive) — status + received byte
`SERPER`	`$DFF032`	Serial period (baud rate divider)
`ADKCON`	`$DFF09E`	UARTBRK bit for break signal

Baud Rate Calculation

/* SERPER value = (system_clock / baud_rate) - 1 */
/* PAL system clock for serial = 3,546,895 Hz */
/* NTSC = 3,579,545 Hz */

#define SERIAL_CLOCK_PAL   3546895
#define SERIAL_CLOCK_NTSC  3579545

UWORD serper_from_baud(ULONG baud, BOOL isPAL) {
    return (isPAL ? SERIAL_CLOCK_PAL : SERIAL_CLOCK_NTSC) / baud - 1;
}

Baud Rate	SERPER (PAL)	SERPER (NTSC)
1200	2955	2982
2400	1477	1491
9600	368	372
19200	184	185
31250 (MIDI)	112	113
57600	60	61
115200	29	30

Using serial.device

Open and Configure

struct MsgPort *serialPort = CreateMsgPort();
struct IOExtSer *serialReq = (struct IOExtSer *)
    CreateIORequest(serialPort, sizeof(struct IOExtSer));

/* Open serial.device unit 0: */
BYTE err = OpenDevice("serial.device", 0,
                       (struct IORequest *)serialReq, 0);

/* Configure: */
serialReq->io_SerFlags  = SERF_SHARED | SERF_XDISABLED;
serialReq->io_Baud      = 9600;
serialReq->io_ReadLen   = 8;     /* 8 data bits */
serialReq->io_WriteLen  = 8;
serialReq->io_StopBits  = 1;
serialReq->io_RBufLen   = 4096;  /* receive buffer size */
serialReq->IOSer.io_Command = SDCMD_SETPARAMS;
DoIO((struct IORequest *)serialReq);

Write Data

serialReq->IOSer.io_Command = CMD_WRITE;
serialReq->IOSer.io_Data    = "Hello Serial\r\n";
serialReq->IOSer.io_Length  = 14;
DoIO((struct IORequest *)serialReq);

Read Data

/* Check how many bytes are waiting: */
serialReq->IOSer.io_Command = SDCMD_QUERY;
DoIO((struct IORequest *)serialReq);
ULONG waiting = serialReq->IOSer.io_Actual;

if (waiting > 0) {
    serialReq->IOSer.io_Command = CMD_READ;
    serialReq->IOSer.io_Data    = buffer;
    serialReq->IOSer.io_Length  = waiting;
    DoIO((struct IORequest *)serialReq);
    /* buffer now contains received data */
}

Serial Debugging

Many developers use a null-modem cable to a terminal for kernel debugging:

/* Kprintf — ROM debug output via serial (ROMTags/exec): */
/* This bypasses serial.device entirely — writes directly to SERDAT */
void KPrintF(const char *fmt, ...);  /* available in debug ROMs */

/* Sushi/Sashimi — capture serial debug output on another Amiga */

References

NDK39: devices/serial.h
HRM: Amiga Hardware Reference Manual — Serial Port chapter
ADCD 2.1: serial.device autodocs