[This is a community wiki post. However, if you don't have enough reputation to edit it, or just don't feel like doing the work, feel free to post a comment on it linking to the source of a pinout that's not yet here and I will add it to the post --cjs.]
The page OLD Hard 旧機種情報 >>Connecter Infomation ケーブル情報 (Japanese) has a lot of information on various cables for Japanese computers, including a summary table at the bottom of that page covering CRT, keyboard, mouse, CMT (cassette), FDD and power (電源) connectors for 36 different computers. This information should be integrated into what's below.
This won't help with determining if there are any formal standards, but we can at least figure out if there are informal/accidental standards by looking at actual machines. MSX1 seems to be fairly consistent (excepting analogue RGB, which we're not covering here), so between that and the pre-MSX computers there aren't all that many of them. The listings below are in approximately chronological order of release.
In this answer we use the standard pin numbering, below looking in to the female jack on the computer. Note that some references are different, in particular some that give a numbering marked "male" are actually looking at the side of the connector you solder, which gives the same numbering as below. But check the reference carefully!
∪ ∪
7 6
3 1 3 8 1
5 4 5 4
2 2
The critical signals to make the interface work are in CAPS below; "extra" signals that shouldn't harm operation if left unconnected are in lower-case.
Cassette
Verdict so far: very standard.
1 2 3 4 5 6 7 8
NEC PC-8001 GND GND GND REC PLAY REM+ REM- GND
NEC PC-6001mkII cmt1 cmt2 GND REC PLAY REM+ REM- GND
Fujitsu FM-77 GND GND GND REC PLAY REM+ REM- GND
Panasonic JR-200 GND GND GND REC PLAY REM+ REM- GND
MSX Standard GND GND GND REC PLAY REM+ REM- GND
Notes:
- Cables almost invariably connect only pin 2 to sleeve on record/play plugs, 4/5 to tip on rec/play, and 6/7 to ring/tip on remote, leaving all other pins unconnected.
- There are a few exceptions to the standard for the MSX CMT interface. Most of them use a 5-pin port, and so are obviously different. It's not clear if the Frael Bruc 100, a not-quite-MSX computer, uses an 8-pin port, but at any rate it's Italian, not Japanese.
Sources: PC-8001 CMT, PC-6001mkII CMT, FM-77 CMT, JR-200 CMT, MSX CMT.
Composite Video
Verdict so far: two standards, one for non-MSX, one for MSX.
1 2 3 4 5
NEC PC-8001 ??? GND COMP ??? ???
Fujitsu FM-77 2MHz GND COMP hsync vsync
Sega SC-3000 audio GND COMP gnd gnd
MSX Yamaha/Victor sound GND a/v+5 nc/rf COMP
Notes:
- The MSX comp used by Yamaha and Victor may not be an MSX-defined standard. It's incompatible with other systems listed here. The
a/v+5
is used as a control signal to JP-21 (and possibly SCART) connections to set the aspect ratio.
- The Sega Master System/Mega Drive 8-pin video connector is also compatible with a 5-pin composite video cable using 2:GND and 3:COMP.
Sources: Sources: PC-8001 comp, FM-77 comp, SC-3000 comp, MSX comp.
Digital RGB Video
Verdict so far: very standard.
1 2 3 4 5 6 7 8
NEC PC-8001 vcc GND 14Mhz HSYNC VSYNC RED GREEN BLUE
NEC PC-8801 +12V GND ? HSYNC VSYNC RED GREEN BLUE
NEC PC-6001mkII 14MHz GND ccont HSYNC VSYNC RED GREEN BLUE
Sharp MZ-700 video GND csync HSYNC VSYNC RED GREEN BLUE
Fujitsu FM-7 +12V GND 2MHz HSYNC VSYNC RED GREEN BLUE
Panasonic JR-200 vblank GND comp. HSYNC VSYNC RED GREEN BLUE
MSX Digital +12V GND n/c HSYNC VSYNC RED GREEN BLUE
Sharp MZ-800 i GND unused HSYNC VSYNC RED GREEN BLUE
Notes:
- The NEC PC-6001mkII
ccont
signal is the 色相制御信号 or RGBI intensity bit.
- The Sharp MZ-700 RGB scan has been corrected by hand to reverse the original 3:HSYNC and 5:CSYNC description, which was presumably a typo. The MZ-800 RGB source also gives an MZ-700 pinout that matches the correction.
- National/Panasonic JR-200 includes DIN-5-compatible composite color output on the same DIN-8 connector used for DRGB.
- Warning: The MSX RGB pinout above applies only to those MSX computers that use digital RGB output, which is very few (Fujitsu FM-X, Pioneer PX-7, PX-V7 and PX-V60 according to the source). Most MSX computers seem to use analogue RGB outputs on 8-pin connectors, which are not compatible with digital and vary in the pins they use for which signals.
Sources: PC-8001 RGB, PC-8801 RGB, PC-6001mkII RGB, MZ-700 RGB (p.19), FM-7 RGB, MSX RGB, MZ-800 RGB.