Ken Self replied to the topic 'ST4 vs pulse guiding?' in the forum. 10 months ago

ST4 is even more ancient than that. It has its roots in the old hand controllers used for manually guiding. In those days you would watch a guide star against the cross hair reticle and push the N/S/E/W buttons on the hand control to correct. ST4 autoguiding replaces the manual button pressing which is why it has 4 wires plus a ground - one wire for each of the N/S/E/W buttons on a hand controller. So a 100ms pulse is the same as holding down a button for 100ms. You could still guide manually via the ST4 port with a set of 4 push buttons if you wanted to. Each button grounds the relevant wire which causes the mount to move in the appropriate direction while the wire is grounded.
Pulse guiding uses two way communication between the mount and the autoguider. That way if something fails in the mount is is usually detected pretty quickly. With ST4 there is no feedback from the mount. Also, the ST4 cables are notoriously unreliable. If one wire fails, especially if intermittently, it is difficult to detect what is going wrong.