Yeh the keyboard I found is only compatible with the Yamaha CX5M computer, it has no standalone capabilities. The computer itself is an MSX machine with a synth module in its expansion bay, apparently it is the Yamaha DX7 Synthesiser (that was current at the time) melded with an MSX machine. It was fiendishly expensive at the time, about 800 quid with the decent keyboard. I saw the one on ebay for 75 quid, thought that was somewhat steep. I bought the computer alone and it cost me AU$57 so about twenty quid. Struggled to find good technical information on it to be honest, but I did find out that the synth module came in two variants. The SG-01 was the original module and it was a severe limitation, the system could only be played via the attached Yamaha keyboards, via that 20 way connector, it could not be played by midi. It outputted midi but could not use anything sent to the midi IN ports. Apparently a lot of people were very angry, they rightly assumed that as the device has a midi IN port that you could actually use it. The later synth module was the SG-05 which was what people were expecting, true midi IN and OUT functionality. Actually the 01 and the 05 are idenical except for the ROM inside, the 01 has a 16KB rom and the 05 has a 32KB rom, so they are upgradeable with a bit old soldering. The only site I could find claimed that if there isnt a sticker on the bottom claiming its an 05 then its not, mine had no stickers but having looked inside I can see the modification has been done, also that the ROM is an official Yamaha ROM. So either this was sent back for the upgrade, or the mod was standard from new at a certain point in production.
If you have the 01 version then it is useless without either an YK-01 or a YK-10 keyboard that connects to the twenty way port, if you have the 05 version then it will work either via midi or with a yamaha keyboard as its input.
When you boot the machine it comes up with the standard MSX-DOS blue screen and prompt, to switch it into synth mode you have to type "call music", it then boots to the synth rom. If you have a screen full of text options then you have the 01 version, if you have the graphical representation of the keyboard then you have the 05 rom version.
There also was a range of Yamaha music carts for this machine, one of which was a synth voice editor, one of the weaknesses of the Yamaha DX7 full synth was that although you could edit the voices it only had a 2 line LCD screen which made it very hard work. With this machine and the Synth cart you could interface with that keyboard via midi and use the computer screen to do the modifications graphically. Typing "call synth" booted to that application apparently.
This site has the rom dumps of most of the software carts (and the synth module) http://www.rgp.nl/cx5m/soft.htm
. If could just find the cct diag for the carts I could build my own.
The one on ebay for 75 quid has no indication whether its an 01 or an 05, if its an 01 its pretty useless without getting the upgrade done, if its the 05 then its a tidy bit of kit, tho still overpriced at 75 quid. Still the one I have give composite video out so it would be easy to ask the seller to power it up and type "call synth" and see what the screen looks like.