Maple bus

The Maple bus is the communications bus the Dreamcast uses to connect controllers and other related peripherals. It is a symmetrical serial protocol.

Hardware Overview

Dreamcast Port

Four separate Maple Buses exist on a Dreamcast system - one for each player. Each Maple Bus consists of 2 signal/clock lines that are labeled SDCKA and SDCKB. Hardware on the Maple Bus consists of one host, zero or one main peripheral, and zero to five sub-peripherals. The only difference between a main peripheral and a sub-peripheral is that a main peripheral communicates to the host what sub-peripherals are attached during normal communication. The main peripheral is something like a Dreamcast controller, and the sub-peripherals are things like a VMU, jump pack, and microphone. The host and all connected peripheral devices communicate on the same 2-line Maple Bus. Devices are powered with 5V from the host, and signals on the Maple Bus operate at 3.3V TTL.

• Both lines on the Bus are pulled HIGH through weak pullup resistors
• Only one connected component on the bus may communicate at a time
• During communication, a device should not drive both lines HIGH for very long to prevent a downstream device from thinking the bus is free
• Before a component starts communicating, it must verify the bus is neutral for a sufficient amount of time
• A peripheral device will only communicate 1 packet of data in response to a request from the host

Sense

The Dreamcast does not rely on any sense line as some projects implement. Both ground wires from the controller are tied to ground at the controller port module on the Dreamcast. To detect the presence of a device, Dreamcast instead sends a device info request every 16 ms until a main peripheral responds on the bus.

Maple Bus Signals

There is no handshaking done to determine how fast each component may transmit on the Maple Bus. Transmission speeds are internally pre-determined by each hardware component. Generally speaking, for all signals, the minimum amount of time between an edge (transition between logic levels) of one line and an edge of the other is about 125 nanoseconds. The minimum amount of time between edges on the same line is about 225 nanoseconds. There is no maximum time limit as long as both lines aren't held HIGH during active transmission for any extended amount of time. These guidelines may be applied for any sequence below.

Start Sequence

Maple Bus Start Sequence

Every packet begins with a start sequence.

1. SDCKA is brought LOW
2. SDCKB is toggled 4 times
3. SDCKA is then brought back HIGH to complete the sequence

End Sequence

Maple Bus End Sequence

Every packet is completed with an end sequence to commit the data to the target component.

1. SDCKA is brought HIGH
2. SDCKB is toggled HIGH then LOW
3. SDCKA is toggled 2 times
4. SDCKB is brought back HIGH to complete the sequence

Data Bit Sequences

Maple Bus Clocking Phases

For each bit, one line of the maple bus acts as a clock while the other is the data to be sampled. A data bit is clocked when the designated clock line transitions from HIGH to LOW. The two lines trade their function after each bit. Line A acts as clock and B acts as data for the first bit. Line B acts as clock and A acts as data for the next bit. Line A acts as clock again for the bit after that. The pattern repeats until all data is transmitted.

Each bit transmission sequence can be broken down into 3 phases:

• Phase 1 - Clock Conditioning: Bring clock HIGH and keep data at the state it was previously
• Phase 2 - Data Conditioning: Transition the data bit to the target value
• Phase 3 - Clocking: Bring clock LOW in order to have the data bit sampled

There are a total of 6 types of state transitions, depending on what the previous phase was and the target bit value. A depiction of state transitions can be seen in the image below.

Notice that each line, A & B transitions states in a staggard pattern i.e., only one line may change its logic level within each phase. Because of the staggard pattern, the minimum time between an edge of one line and an edge of the other is 1 phase length, and the minimum time between edges on the same line is the sum of the time of 2 phases. The total bit transmission time is of course the sum of the time of all 3 phases.

• The Dreamcast host transmits each phase at about 160 nanoseconds
  • This works out to be about 2 mbps raw data transfer speed
• Dreamcast controllers and other peripheral devices usually transmit a little slower at about 250 nanoseconds per phase and with about 110 microsecond delays between each 3-word chunk after the first frame word
  • This works out to between about 0.5 and 1.3 mbps raw data transfer speed

SDCKB Occupancy

The SDCKB occupancy start sequence looks similar to a normal start sequence, but 8 pulses on the SDCKB line signify the start of this mode. This mode is used to gain precise timing for light gun triggering.

Reset Sequence

A reset sequence may be sent from the host in order to attempt to reset all devices on the bus. It is similar to a start sequence, but there are 14 pulses on SDCKB.

Maple Bus Packet

This section contains information about the packet structure inherent to the Maple Bus. Any data transmission follows this packet structure.

Word Format

Each word is 32 bits in length, transmitted in little-endian byte order. The most significant bit of each byte transmits first. This means that the most significant bit of the least significant byte of each word transmits first. All tables in this wiki list bytes in transmission order with the least significant bit (LSB) as the first byte.

When ASCII text or a byte stream is transmitted, the most significant byte is the first character of the 4 character sequence in each word. This means that the byte order of each word needs to be flipped before parsing the payload as a character or byte array. The size of an ASCII payload section is pre-determined based on the command. No NULL termination byte is supplied at the end of the string, and spaces (0x20) are used to pad out remaining characters at the end of the string.

Packet Data Format

A packet consists of the following data.

• Frame: 1 32-Bit Word
• Payload: 0 to 255 32-Bit Words
• CRC: 1 Byte

Frame Word

The following is how a frame word is broken down into its 4 parts.

Byte 0 (LSB)	Byte 1	Byte 2	Byte 3 (MSB)
Number of Words in Payload	Sender Address	Recipient Address	Command

example:

Addressing

The following addresses are used for all components on the bus.

Player Number	Host	Main Peripheral	Sub-Peripheral 1	Sub-Peripheral 2	Sub-Peripheral 3	Sub-Peripheral 4	Sub-Peripheral 5
1	0x00	0x20*	0x01	0x02	0x04	0x08	0x10
2	0x40	0x60*	0x41	0x42	0x44	0x48	0x50
3	0x80	0xA0*	0x81	0x82	0x84	0x88	0x90
4	0xC0	0xE0*	0xC1	0xC2	0xC4	0xC8	0xD0

*When the main peripheral responds with its sender address, it also sets the bits corresponding to which sub-peripherals are attached. For example, if sub-peripherals 1 and 2 are attached to player 1's main peripheral, the main peripheral will set its sender address to 0x23. This informs the host what else is attached. The host should still set the recipient address to 0x20 when sending data to this peripheral though.

In testing, there have been cases where a peripheral will respond with a sender address as if it is player 1. As such, the host should ignore whatever the upper 2 bits that the device uses as its sender address.

Commands

Below shows command values and their meanings.

Command Value	Description	Communication Direction	Number of Payload Words	Expected Response**
0x01	Device Info Request*	Host->Device	0	0x05
0x02	Extended Device Info Request	Host->Device	0	0x06
0x03	Reset	Host->Device	0	0x07
0x04	Shutdown	Host->Device	0	0x07
0x05	Device Info	Device->Host	28	-
0x06	Extended Device Info	Device->Host	48	-
0x07	Acknowledge	Device->Host	0	-
0x08	Data Transfer	Device->Host	2..255	-
0x09	Get Condition	Host->Device	1	0x08
0x0A	Get Memory Information	Host->Device	2	0x08
0x0B	Block Read	Host->Device	2	0x08
0x0C	Block Write	Host->Device	3..255	0x07
0x0D	Get Last Error	Host->Device	2	0x07
0x0E	Set Condition	Host->Device	2..255	0x07
0xF9	AR Error	Device->Host	0	-
0xFA	LCD Error	Device->Host	0	-
0xFB	File Error	Device->Host	0	-
0xFC	Request Resend	ANY	0	Last Sent Command
0xFD	Unknown Command	Device->Host	0	-
0xFE	Function code Not Supported	Device->Host	0	-

*Most peripheral devices won't respond to any other command until device info is requested for the device.

**This is the expected response when device didn't experience an error

Payload

The structure of a payload is dependent on the command used in the frame word.

Device Info Payload Structure (cmd 0x05)

Word 0	Words 1-3	Word 4	Words 5-11	Words 12-26	Word 27
Supported function codes mask*	Function definitions for up to 3 devices**	Byte 3 (MSB): Region code Byte 2: Connection direction code Bytes 1 and 0: first two characters of description ASCII string***	The rest of the description ASCII string***	Producer information ASCII string**	2 most significant bytes: standby current consumption 2 least significant bytes: maximum current consumption****

*The supported function codes mask in device info responses will contain the bitmask for 1 or more devices ex: a VMU will have a mask of 0x0000000E for Timer, Screen, and Storage.

**The first word in this set is meant for the most significant bit that is set to 1 in the function codes word

***Refer to the word format section about how to parse ASCII strings.

****Current values in (x / 10) mA.

Extended Device Info Payload Structure (cmd 0x06)

Word 0	Words 1-3	Word 4	Words 5-11	Words 12-26	Word 27	Words 28-47
Supported function codes mask*	Function definitions for up to 3 devices**	Byte 3 (MSB): Region code Byte 2: Connection direction code Bytes 1 and 0: first two characters of description ASCII string***	The rest of the description ASCII string***	Producer information ASCII string***	2 most significant bytes: standby current consumption 2 least significant bytes: maximum current consumption****	Version information and/or capabilities ASCII string***

*The supported function codes mask in device info responses will contain the bitmask for 1 or more devices ex: a VMU will have a mask of 0x0000000E for Timer, Screen, and Storage.

**The first word in this set is meant for the most significant bit that is set to 1 in the function codes word

***Refer to the word format section about how to parse ASCII strings.

****Current values in (x / 10) mA.

Data Transfer Payload Structure (cmd 0x08)

Word 0	Words 1..255
Function code	Data - device dependent structure

Get Condition Payload Structure (cmd 0x09)

Word 0
Function code

Get Memory Information Payload Structure (cmd 0x0A)

Word 0	Word 1
Function code	Location word

Block Read Payload Structure (cmd 0x0B)

Word 0	Word 1
Function code	Location word

Block Write Payload Structure (cmd 0x0C)

Word 0	Word 1	Words 2..255
Function code	Location word	Data - device dependent structure

Get Last Error Payload Structure (cmd 0x0D)

Word 0	Word 1
Function code	Location word

Set Condition Payload Structure (cmd 0x0E)

Word 0	Words 1..255
Function code	Condition - device dependent structure

Common Payload Word Types

The following word types are used in some of the above payload structures.

Function Codes

The below are function codes which are used to address functionality in some payloads.

Code / Mask	Description
0x00000001	Controller
0x00000002	Storage
0x00000004	Screen
0x00000008	Timer
0x00000010	Audio Input
0x00000020	AR Gun
0x00000040	Keyboard
0x00000080	Gun
0x00000100	Vibration
0x00000200	Mouse

Location Word

Below defines a location word which is used to address blocks of memory in some peripherals.

Byte 0 (LSB)	Byte 1	Byte 2	Byte 3 (MSB)
Block LSB	Block MSB	Phase	Partition

• Block: Memory block number index
• Phase: Sequence number (starts at 0 and counts up)
• Partition: Partition number (normally 0)

CRC

CRC byte transmits last, just before the end sequence is transmitted. It is the value after starting with 0 and applying XOR against each byte in the packet.