The RP-100 Rhythm Programmer is an odd, obscure little device, produced around 1983/84 by Korg just prior to the transition from the Poly series (Six/61/800) to the DW series. Apparently it was only produced for about a year, and there is next to no information on it online aside from a quote from a scanned promotional pamphlet. The RP-100 is part of a series which included the even more obscure MP-100 Chord Programmer and possibly the SQ-8 MIDI Sequencer. The RP-100 itself is essentially a programmable metronome, with some simplistic drum machine features added in. The button format and case is the same for the RP/MP-100 and the SQ-8, although the SQ-8 is a far more complex piece of equipment, and its' two little sisters each have a different color scheme. The functions of the RP-100 are very simple, with a basic metronome with variable tempo from 40 to 208 BPM, selectable only in 4 BPM increments (an odd shortcoming for a metronome), a functionality to 'step write' a bar of blips one beat at a time (capable of adding 1 to 8 blips per beat, and also of subtracting individual blips with the rest function), a 'chain play' which allows you to play back the steps you've written in a time signature ranging from 1/4 to 8/4, and a 'swing' mode which is unfortunately not applicable to the chain play mode, and simply allows you to use the basic metronome with one of 8 drum machine swing settings. There is also an A440 tone generator, and a tap-tempo input. A curious feature of the RP-100 is a DIN Sync I/O port, which allows it to either drive another DIN Sync instrument (48ppn) or apparently to accept a "tap" input. Unfortunately, the only available sounds are four harsh analog blips consisting of the 2 basic metronome blips plus a click and an accent, output through either the internal speaker or the phones port. The output volume is loud enough that it may be capable of driving an analog arpeggiator, which when taken in tandem with the DIN I/O would open up some extremely interesting possibilities given the swing function and the ability to program blip-numbers on a given beat.
If anyone has any information, or better yet, is interested in selling an MP-100 unit, I'd be very interested in buying it. As best I can figure, it's a simple MIDI sequencer similar to those which were being included in home keyboards in the early 80s, and possibly something similar to the internal sequencer of the Korg PSS-50 accompaniment machine (which itself does not have any way of synchronizing with any external equipment, which was a very odd shortcoming for a device produced alongside the Poly-800 and the DDM-110/220).
By the way, these units really aren't worth anything. Unless you'd like to experiment with the internals of it, or just have a rare display piece, they're not worth seeking out. I paid about ten dollars for mine, plus shipping. They're a great example of a device which is rare not because it was unknown, or because it has become sought-after, but because it's really just not very interesting or useful outside of extremely specific applications.