3. JAMMA Video Standard

JVS is a communication standard designed for I/O devices in arcade macines. It typically operates over RS485 serial, however the framing format can and sometimes is used outside of this context.

This page documents the canonical JVS structure. Some devices, notably the 838-14971, use a slight variant on the format. The differences between the two formats are document here.

3.1 Escaping

Every packet sent begins with the SYNC byte, 0xe0. Read buffers should be continually flushed until this byte is observed.

As such, 0xe0 is reserved, and is escaped. A byte is escaped using MARK, 0xd0, followed by the byte’s value subtract one. While any byte can be escaped, the only two that require escaping are SYNC and MARK, escaped as D0 DF and D0 CF respectively. All operations on packets, such as checksums and length calculation, are performed on the unescaped packet.

3.2 Packet framing

SUM is calculated by summing all bytes in the packet, excluding SYNC and SUM itself. n indicates the number of bytes succeeding itself. Due to the existance of the checksum, n=0 is nonsensical.

3.3 Commands and responses

A request packet may contain multiple commands within it. D~0~ always contains the first command. It is followed by m bytes of data, typically a constant number per command but not always (see the command reference), at which point either we have consumed n-1 bytes and the next byte is SUM, or the next byte is another command and we repeat.

Response packets are structured similarly, however the CMD byte is not present, instead relying on the response to be ordered exactly the same as the request. The data of each command is preceeded by a report code, indicating that individual command’s status. The below diagram also includes the entire-packet status byte too for clarity.

3.4 Individual commands

Later.