Eirik Lie
  E-mail: eirikli@notam.uio.no
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  The Yamaha CX5M was made around 1985 and was based on the MSX standard, which 
  was a great flop. It had a Z80 CPU and ran, apart from ROM BASIC, an operating 
  system equivalent to MS-DOS 1.0, if you can imagine. However, it also had a 
  built-in 4-operator FM synth (as opposed to the DX7's 6 operators), soundwise 
  equivalent (and patch- compatible, I think) to the DX21, DX100, and FB-01. The 
  original CX5M could only be played with a proprietary non-velocity sensitive 
  Yamaha keyboard, and not via MIDI. It could be upgraded, however. If you take 
  a look under your CX5M you will see the bolted-on synthesizer unit. If you can 
  read 'Yamaha FM Sound Synthesizer Unit II' and 'Model SFG-05', you have the 
  upgraded version. There also was a later model called the CX5M II/128. These 
  two latter versions were velocity-sensitive playable via MIDI. You cannot use 
  an RGB monitor with the CX5M, you only have an RF output to go into the antenna 
  socket on your TV, and a 'composite video' output, also found on many TV sets 
  and old PC monitors. Software for the CX5M came in the form of plug-in cartridges. 
  If you have the original CX5M, the only cartridges of any use would be: 1. The 
  YRM-303 MIDI Macro & Monitor, showing MIDI messages on screen, very handy 
  for MIDI system troubleshooting, and: 2. A DX7 editing program of which I can't 
  remember the name, making it possible to edit DX7 sounds on screen instead of 
  the DX7's LCD display. If you have one of the upgraded versions, however, you 
  will be able to use it as an external MIDI sound device. You then also will 
  need the 'YRM-502 FM Voicing Program II' cartridge to edit the internal sounds. 
  The CX5M is 8-voice, 4-channel multitimbral without dynamic voice allocation. 
  Storage of sounds is on cassette tape. Any tape recorder can be used, but you 
  need a special DIN-to-minijack cable, originally included with the CX5M. There 
  was also an external disk drive available, but it was terribly expensive and 
  very difficult to find. When you boot the CX5M, it defaults to the ROM BASIC, 
  with no access to the synthesizer unit. You have to type 'call music' to get 
  access to the synth, 'call fmv' to access the FM Voicing program, etc. I still 
  use my CX5M in my MIDI rig at home, both as a sound module, and as a patch and 
  sequencer library for my ESQ1 synth, a program I wrote myself. (And I still 
  play "Chuckie Egg" regularly!) I hope this information has been of 
  some help for you. Feel free to email me if you have more questions.