SIO Normal Mode

This mode is used to communicate between two units.
Transfer rates of 256KBit/s or 2MBit/s can be selected, however, the fast 2MBit/s is intended ONLY for special hardware expansions that are DIRECTLY connected to the GBA link port (ie. without a cable being located between the GBA and expansion hardware). In normal cases, always use 256KBit/s transfer rate which provides stable results.
Transfer lengths of 8bit or 32bit may be used, the 8bit mode is the same as for older DMG/CGB gameboys, however, the voltages for "GBA cartridges in GBAs" are different as for "GMG/CGB cartridges in DMG/CGB/GBAs", ie. it is not possible to communicate between DMG/CGB games and GBA games.

134h - RCNT (R) - Mode Selction, in Normal/Multiplayer/UART modes (R/W)

Bit Expl.

0-14 Not used

15 Must be zero (0) for Normal/Multiplayer/UART modes

128h - SIOCNT (SCCNT_L) - SIO Control, usage in NORMAL Mode (R/W)

Bit Expl.

0 Shift Clock (SC) (0=External, 1=Internal)

1 Internal Shift Clock (0=256KHz, 1=2MHz)

2 SI State (opponents SO) (0=Low, 1=High/None) --- (Read Only)

3 SO during inactivity (0=Low, 1=High)

4-6 Not used

7 Start Bit (0=Inactive/Ready, 1=Start/Active)

8-11 Not used

12 Transfer Length (0=8bit, 1=32bit)

13 Must be "0" for Normal Mode

14 IRQ Enable (0=Disable, 1=Want IRQ upon completion)

15 Not used

The Start bit is automatically reset when the transfer completes, ie. when all 8 or 32 bits are transferred, at that time an IRQ may be generated.

12Ah - SIODATA8 (SCCNT_H) - SIO Normal Communication 8bit Data (R/W)
For 8bit normal mode. Contains 8bit data (only lower 8bit are used). Outgoing data should be written to this register before starting the transfer. During transfer, transmitted bits are shifted-out (MSB first), and received bits are shifted-in simultaneously. Upon transfer completion, the register contains the received 8bit value.

120h - SIODATA32_L (SCD0) - SIO Normal Communication lower 16bit data (R/W)
122h - SIODATA32_H (SCD1) - SIO Normal Communication upper 16bit data (R/W)
Same as above SIODATA8, for 32bit normal transfer mode respectively.

Initialization
First, initialze RCNT register. Second, set mode/clock bits in SIOCNT with startbit cleared. For master: select internal clock, and (in most cases) specify 256KHz as transfer rate. For slave: select external clock, the local transfer rate selection is then ignored, as the transfer rate is supplied by the remote GBA (or other computer, which might supply custom transfer rates).
Third, set the startbit in SIOCNT with mode/clock bits unchanged.

Synchronization
The SI and SO Bits in control register may be optionally used to determine whether the opponent is ready for starting a transmission (the actual transmission is then automatically synchronized by the shift clock signal).
Bit 2 (SI) always reflects the current SI state (ie. the opponents SO state). Obviously, Bit 3 (SO) is output to SO during transfer inactivity only.
Note that only GBA models support SI and SO synchronization bits - these bits cannot be used when communicating with CGBs or monochrome gameboys.

Recommended Communication Procedure for SLAVE unit (external clock)
- Initialize data which is to be sent to master.
- Set Start flag.
- Set SO to LOW to indicate that master may start now.
- Wait for IRQ (or for Start bit to become zero). (Check timeout here!)
- Set SO to HIGH to indicate that we are not ready.
- Process received data.
- Repeat procedure if more data is to be transferred.
(or is so=high done automatically ??? would be fine - more stable - otherwise master may still need delay)

Recommended Communication Procedure for MASTER unit (internal clock)
- Initialize data which is to be sent to slave.
- Wait for SI to become LOW (slave ready). (Check timeout here!)
- Set Start flag.
- Wait for IRQ (or for Start bit to become zero).
- Process received data.
- Repeat procedure if more data is to be transferred.

Cable Protocol
During inactive transfer, the shift clock (SC) is high. The transmit (SO) and receive (SI) data lines may be manually controlled as described above.
When master sends SC=low, each master and slave must output the next outgoing data bit to SO. When master sends SC=HIGH, each master and slave must read out the opponents data bit from SI. This is repeated for each of the 8 or 32 bits, and when completed SC will be kept high again.

Transfer Rates
Either 256KHz or 2MHz rates can be selected for SC, so max 32KBytes (256KBit) or 128KBytes (2MBit) can be transferred per second. However, the software must process each 8bit or 32bit of transmitted data separately, so the actual transfer rate will be reduced by the time spent on handling each data unit.
Only 256KHz provides stable results in most cases (such like when linking between two GBAs). The 2MHz rate is intended for special expansion hardware only.

Using Normal mode for One-Way Multiplayer communication
Whem more than two GBAs are connected, data isn't exchanged between first and second GBA as usually. Instead, data is rotated from first to last GBA (and then back to first ???).
This behaviour may be used for fast one-way data transfer from master (or childs ???) to all other GBAs. For example (3 GBAs linked):

Step Sender 1st Recepient 2nd Recipient

Transfer 1: DATA #0 --> UNDEF --> UNDEF -->

Transfer 2: DATA #1 --> DATA #0 --> UNDEF -->

Transfer 3: DATA #2 --> DATA #1 --> DATA #0 -->

Transfer 4: DATA #3 --> DATA #2 --> DATA #1 -->

The recepients should not output any own data, instead they should forward the previously received data to the next reciepint during next transfer (just keep the incoming data unmodified in the data register).
Due to the delayed forwarding, 2nd recepient should ignore the first incoming data. After the last transfer, the sender must send one (or more) dummy data unit(s), so that the last data is forwarded to the 2nd (or further) recepient(s).