I don't know why '80s sound chips also handle some of the i/o  but it's the same with the C64 - the SID also handles the paddle input.

I'd noticed that the MSX AY sound chip (or Programmable Sound Generator, PSG) also handles joystick(s). I'd spotted that the PPI (see previous log) handled cassette motor and cassette *out* (possibly just cassette write signal, I'm not sure yet) but I'd struggled to find where the cassette *in* action happens.  Eventually I found it on an MSX schematic - it's handled by the AY sound chip.  So the PSG is an important part of the system; it does more than produce the sound.

I've had an Ed Brindley YM/AY sound card for RC2014 for a long time. It's a favourite module of mine, not least because of its two bidirectional i/o ports. 

So I thought that my MSX PSG needs were sorted. But Ed himself has been in touch since I started this project to let me know that the version of the board I had (rev 5) wasn't capable of being jumpered for MSX ports and that I'd need a rev 6 module. 

I'll keep my original board (top) jumpered for the standard RC2014 ports ($d0 & $d8) because I have the pt3 player along with lots of pt3 files on my CF card, as well as other music stuff that I've written. It now has an AY-3-8912 in an adaptor because that's the only spare chip I have. (Currently no use for i/o because the ...12 chip lacks an i/o register, and the adaptor board covers my i/o headers). Below it is the new board, jumpered for MSX ports. I haven't fitted the oscillator because it isn't necessary, it can use the clock signal from the bus and divide it if necessary. 

I've tested the new card (register write port $a0, value write $a1, value read $a2) and that all seems to work fine. Thanks to Ed for his work on this module.

A kit for the rev 5 board is available at https://z80kits.com/shop/ym2149-sound-card/ 
Files and information for the r6 board are at https://github.com/electrified/rc2014-ym2149