The SYNC standard was developed by Roland as a way of syncronizing drum machines and sequencers. It has 2 main signals, "clock" and "star/stop". The signals are TTL compatible, so 0V is low, and +5V is high. It uses a cable like midi, but its pinout is different:
If the "Start/stop" line is low, the device is stopped, or should be stoped. If it is high, then it should be started/running.
The Clock signal is either 24 pulses per quarter note, or 48 pulses per quarter note. Most Roland gear is 24ppq, whereas most Korg gear is 48ppq.