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Serial Port (SIO)

1F801050h SIO_TX_DATA (W)

  0-7   Data to be sent
  8-31  Not used

Writing to this register starts transmit (if, or as soon as, TXEN=1 and CTS=on and SIO_STAT.2=Ready). Writing to this register while SIO_STAT.0=Busy causes the old value to be overwritten.
The "TXEN=1" condition is a bit more complex: Writing to SIO_TX_DATA latches the current TXEN value, and the transfer DOES start if the current TXEN value OR the latched TXEN value is set (ie. if TXEN gets cleared after writing to SIO_TX_DATA, then the transfer may STILL start if the old latched TXEN value was set; this appears for SIO transfers in Wipeout 2097).

1F801050h SIO_RX_DATA (R)

  0-7   Received Data      (1st RX FIFO entry) (oldest entry)
  8-15  Preview            (2nd RX FIFO entry)
  16-23 Preview            (3rd RX FIFO entry)
  24-31 Preview            (4th RX FIFO entry) (5th..8th cannot be previewed)

A data byte can be read when SIO_STAT.1=1. Data should be read only via 8bit memory access (the 16bit/32bit "preview" feature is rather unusable).

1F801054h SIO_STAT (R)

  0     TX Ready Flag 1   (1=Ready/Started)  (depends on CTS) (TX requires CTS)
  1     RX FIFO Not Empty (0=Empty, 1=Not Empty)
  2     TX Ready Flag 2   (1=Ready/Finished) (depends on TXEN and on CTS)
  3     RX Parity Error   (0=No, 1=Error; Wrong Parity, when enabled) (sticky)
  4     RX FIFO Overrun   (0=No, 1=Error; Received more than 8 bytes) (sticky)
  5     RX Bad Stop Bit   (0=No, 1=Error; Bad Stop Bit) (when RXEN)   (sticky)
  6     RX Input Level    (0=Normal, 1=Inverted) ;only AFTER receiving Stop Bit
  7     DSR Input Level   (0=Off, 1=On) (remote DTR) ;DSR not required to be on
  8     CTS Input Level   (0=Off, 1=On) (remote RTS) ;CTS required for TX
  9     Interrupt Request (0=None, 1=IRQ)                             (sticky)
  10    Unknown           (always zero)
  11-25 Baudrate Timer    (15bit timer, decrementing at 33MHz)
  26-31 Unknown (usually zero, sometimes all bits set)

Note: Bit0 gets cleared after sending the Startbit, Bit2 gets cleared after sending all bits up to including the Stopbit.

1F801058h SIO_MODE (R/W) (eg. 004Eh --> 8N1 with Factor=MUL16)

  0-1   Baudrate Reload Factor (1=MUL1, 2=MUL16, 3=MUL64) (or 0=STOP)
  2-3   Character Length       (0=5bits, 1=6bits, 2=7bits, 3=8bits)
  4     Parity Enable          (0=No, 1=Enable)
  5     Parity Type            (0=Even, 1=Odd) (seems to be vice-versa...?)
  6-7   Stop bit length        (0=Reserved/1bit, 1=1bit, 2=1.5bits, 3=2bits)
  8-15  Not used (always zero)

1F80105Ah SIO_CTRL (R/W)

  0     TX Enable (TXEN)  (0=Disable, 1=Enable, when CTS=On)
  1     DTR Output Level  (0=Off, 1=On)
  2     RX Enable (RXEN)  (0=Disable, 1=Enable)  ;Disable also clears RXFIFO
  3     TX Output Level   (0=Normal, 1=Inverted, during Inactivity & Stop bits)
  4     Acknowledge       (0=No change, 1=Reset SIO_STAT.Bits 3,4,5,9)      (W)
  5     RTS Output Level  (0=Off, 1=On)
  6     Reset             (0=No change, 1=Reset most SIO_registers to zero) (W)
  7     Unknown? (read/write-able when FACTOR non-zero) (otherwise always zero)
  8-9   RX Interrupt Mode    (0..3 = IRQ when RX FIFO contains 1,2,4,8 bytes)
  10    TX Interrupt Enable  (0=Disable, 1=Enable) ;when SIO_STAT.0-or-2 ;Ready
  11    RX Interrupt Enable  (0=Disable, 1=Enable) ;when N bytes in RX FIFO
  12    DSR Interrupt Enable (0=Disable, 1=Enable) ;when SIO_STAT.7  ;DSR=On
  13-15 Not used (always zero)

1F80105Ch SIO_MISC (R/W)

This is an internal register, which usually shouldn't be accessed by software. Messing with it has rather strange effects: After writing a value "X" to this register, reading returns "X ROR 8" eventually "ANDed with 1F1Fh and ORed with C0C0h or 8080h" (depending on the character length in SIO_MODE).

1F80105Eh SIO_BAUD (R/W) (eg. 00DCh --> 9600 bauds; when Factor=MUL16)

  0-15  Baudrate Reload value for decrementing Baudrate Timer

The Baudrate is calculated (based on SIO_BAUD, and on Factor in SIO_MODE):

  BitsPerSecond = (44100Hz*300h) / MIN(((Reload*Factor) AND NOT 1),Factor)

SIO_TX_DATA Notes

The hardware can hold (almost) 2 bytes in the TX direction (one being currently transferred, and, once when the start bit was sent, another byte can be stored in SIO_TX_DATA). When writing to SIO_TX_DATA, both SIO_STAT.0 and SIO_STAT.2 become zero. As soon as the transfer starts, SIO_STAT.0 becomes set (indicating that one can write a new byte to SIO_TX_DATA; although the transmission is still busy). As soon as the transfer of the most recently written byte ends, SIO_STAT.2 becomes set.

SIO_RX_DATA Notes

The hardware can hold 8 bytes in the RX direction (when receiving further byte(s) while the RX FIFO is full, then the last FIFO entry will by overwritten by the new byte, and SIO_STAT.4 gets set; the hardware does NOT automatically disable RTS when the FIFO becomes full).
Data can be read from SIO_RX_DATA when SIO_STAT.1 is set, that flag gets automatically cleared after reading from SIO_RX_DATA (unless there are still further bytes in the RX FIFO). Note: The hardware does always store incoming data in RX FIFO (even when Parity or Stop bits are invalid).
Note: A 16bit read allows to read two FIFO entries at once; nethertheless, it removes only ONE entry from the FIFO. On the contrary, a 32bit read DOES remove FOUR entries (although, there's nothing that'd indicate if the FIFO did actually contain four entries).
Reading from Empty RX FIFO returns either the most recently received byte or zero (the hardware stores incoming data in ALL unused FIFO entries; eg. if five entries are used, then the data gets stored thrice, after reading 6 bytes, the FIFO empty flag gets set, but nethertheless, the last byte can be read two more times, but doing further reads returns 00h).

Interrupt Acknowledge Notes

First reset I_STAT.8, then set SIO.CTRL.4 (when doing it vice-versa, the hardware may miss a new IRQ which may occur immediately after setting SIO.CTRL.4) (and I_STAT.8 is edge triggered, so that bit can be reset even while SIO_STAT.9 is still set).
When acknowledging via SIO_CTRL.4 with the enabled condition(s) in SIO_CTRL.10-12 still being true (eg. the RX FIFO is still not empty): the IRQ does trigger again (almost) immediately (it goes off only for a very short moment; barely enough to allow I_STAT.8 to sense a edge).

SIO_BAUD Notes

Timer reload occurs when writing to SIO_BAUD, and, automatically when the Baudrate Timer reaches zero. There should be two 16bit SIO timers (for TX and RX), the upper 15bit of one of that timers can be read from SIO_STAT (not sure which one, and no idea if there's a way to read the other timer, too).
Or... maybe there is only ONE timer, and RX/TX are separated only by separate "timer ellapsed" counters, in that case the MUL1 factor won't work properly, but, with the MUL16 or MUL64 factors, RX could start anytime (eg. when TX has already ellapsed a bunch of times)...?
The maximum baud rate may vary depending on the length and quality of the cable, whether and how many inverters and anti-inverters are used (on the mainboard and in external adaptor, and on whether signals are externally converted to +/-12V levels)... anyways, rates up to 9600 baud should be working in all cases.
However, running in no$psx, Wipeout 2097 seems to use about 2 million bauds... although, in older no$psx versions, I believe I did see it using some kind of baudrate detection, where it did try different rates in steps of 200 bauds or so...?

SIO Ports vs JOY Ports

SIO uses I/O Addresses 1F801050h..1F80105Fh, which seem to be organized similar to the Controller/Memory Card registers at 1F801040h..1F80104Fh, though not identical, and with an additional register at 1F80105Ch, which has no corresponding port at 1F80104Ch.
SIO_BAUD is \<effectively> same as for JOY_BAUD, but, \<internally> they are a bit different:

  JOY_BAUD is multiplied by Factor, and does then ellapse "2" times per bit.
  SIO_BAUD is NOT multiplied, and, instead, ellapses "2*Factor" times per bit.

Unlike for the Controller/Memory Card ports, the data is transferred without CLK signal, instead, it's using RS232 format, ie. the transfer starts with a start bit, and is then transferred at a specific baudrate (which must be configured identically at the receiver side). For RS232, data is usually 8bit, and may optionally end with a parity bit, and one or two stop bits.

Note

For SIO Pinouts, PSone SIO upgrading, and for building RS232 adaptors, see:
Pinouts - SIO Pinouts
Aside from the internal SIO port, the PSX BIOS supports two additional external serial ports, connected to the expansion port,
EXP2 Dual Serial Port (for TTY Debug Terminal)

SIO Games

The SIO ports on two consoles can be connected with an SCPH-1040 Link Cable (known as Taisen Cable, or "Fight Cable" in Japan) for multiplayer functionality on games that support this method. This was used by a small number of games in the console's lifecycle, but inconveniently required a second console and copy of the game.

Two-Console Link Cable Games (Incomplete List):

Andretti Racing
Armored Core (and Armored Core "Link Versus Demo" disc)
Armored Core Project Phantasma
Armored Core Master of Arena
Assault Rigs
Ayrton Senna Kart Duel
Blast Radius
Bogey Dead 6
Burning Road
Bushido Blade
Bushido Blade 2
C1 -Circuit-
CART World Series
Command & Conquer Red Alert
Command & Conquer Red Alert Retaliation
Cool Boarders 2
Dead in the Water
Descent
Descent Maximum
Destruction Derby
Duke Nukem Total Meltdown
Dodgem Arena
Doom
Dune 2000
Explosive Racing (X Racing in NTSC-J)
Final Doom
Formula 1
Formula 1 98
Grand Tour Racing '98 (Gekisou!! Grand Racing -Total Driving'- in NTSC-J, Total Drivin in PAL)
Independence Day
Krazy Ivan
Leading Jockey Highbred
Metal Jacket
Mobile Suit Z-Gundam
Monaco Grand Prix Racing Simulation 2 (Monaco Grand Prix in NTSC-U/C)
Motor Toon Grand Prix (reportedly NTSC-U/C version only)
Motor Toon Grand Prix 2
Motor Toon Grand Prix USA Edition
The Need for Speed (Over Drivin' DX in NTSC-J)
PrePre Vol. 2
Pro Pinball Big Race USA
RacinGroovy
Real Robots Final Attack
Red Asphalt (Rock & Roll Racing 2 Red Asphalt in PAL)
Ridge Racer Revolution
R4 Ridge Racer Type 4
Robo Pit
Rogue Trip Vacation 2012
San Francisco Rush Extreme Racing (reportedly PAL version only)
Shutokou Battle R
Sidewinder
Sidewinder USA
Soukou Kihei Votoms Gaiden: Ao no Kishi Berserga Monogatari
Streak Hoverboard Racing
Test Drive 4
Test Drive Off-Road (reportedly NTSC-U/C only)
TOCA 2 Touring Car Challenge (TOCA 2 Touring Cars in PAL)
Trick'N Snowboarder (Tricky Sliders Freestyle Snowboard in NTSC-J)
Twisted Metal III
Wing Over
Wipeout
Wipeout 3 Special Edition
Wipeout XL (Wipeout 2097 in PAL)
Zero Pilot Ginyoku no Senshi

The serial port is used (for 2-player link) by Wipeout 2097 (that game accidently assumes BAUDs based on 64*1024*1025 Hz rather than on 600h*44100 Hz).
Ridge Racer Revolution is also said to support 2P link.
Keitai Eddy seems to allow to connect a mobile phone to the SIO port (the games CD cover suggests so; this seems to be something different than the "normal" I-Mode adaptor, which would connect to controller port, not to SIO port).

8251A Note

The Playstation Serial Port is apparently based/inspired on the Intel 8251A USART chip; which has very similar 8bit Mode/Command/Status registers.