Before MIDI came around, the electronic music world was a melting pot of different formats, signal strengths, polarities, speeds, and connector types. One of the many ways that manufacturers tried to get the consumers to buy more of their products is that they would come out with some specialized way of synchronizing their drum machines and sequencers together so that the consumer would only stay true to that family of products. Throughout the years, many devices had (and still have, to some extent) come out that try and bridge the gap between some of these oddball formats.
Two products that did this quite well and are worth noting are the MiniDoc and the Dr. Click, both made by Garfield Electronics.
The MiniDoc (pictured above) is a double space rackmount unit which generates clocks of different speeds than the input clock so that other devices from other manufacturers can be synchronized. The input options are DIN Sync or various clock pulse code formats. The output selections are similar to that of the input. There is no MIDI on this device - but it will perfectly synchronize many older drum machines, such as the Sequential Circuits Drumtraks or Tom, LinnDrum, Oberheim DMX, and more. The DIN sync will work with any of the famous Roland TR- drum machines.
Dr. Click is similar to the MiniDoc in that it has several formats of input and output clock formats. One main thing that the Dr. is missing is a DIN sync input - it only has an output. It does, on the other hand, offer a rather sophisticated clock extractor where you can run audio into the device, set a threshhold, mask off unwanted triggers, and set the master tempo to the tempo of the audio track. This works best when the audio track is a steady beat or song or metronome/click track. It also should be noted that it usually takes the Dr. a measure or two to get perfectly in time.
One thing to point out about the Dr Click is that in addition to its multiple clock outputs, it has 2 channels, each complete with its own subdivision tempo and a simple AR envelope generator. These envelopes can be used in a slow manner to create tempo-matched LFO modulations, or they can be used as rhythm generators for various analog equipment.