PC-9801 extensions capabilities

[Dr.Wily]

I notice there is a lot of documentation on later models of PC-98 like PC-9821 or all those following the integration of the 386 CPU.

But early models, with Nec V30 CPU, are poorly documented on their bus (other than C-BUS), vidéo chip, and ram upgrade. For example, the range of PC-9801V / VM / U / UV / UX has no documentation on how to upgrade it.

There is some slots on motherboard. One is used for sound card and it's "not" C-BUS but more like than a large grey IDC connector. Is this connecor can host others extentions than a sound card ?

How to expand RAM ? early PC-9801 models has a limited amount of RAM. Often 256 or 384Ko. I notice there is some free IC sockets maybe for RAM, upgrade or option...

I try to answer myself to these questions in the following lines, but I hope someone with more knowledge can help

Now the photos :

The beast : fully functional PC-9801 with V30 CPU. I received this unit with no boot disk, HDD or screen. Few days after having suffer to booting up the thing, through floppies, I decide to install a SCSI HDD and a CD-ROM (DVD-ROM here)

- Front of this model : DIP switch is particularly tricky and model specific. Bad setup == no boot. On this model (PC-9801UV) DIP are from the left to the right :

assuming open (0) dip is up and closed (1) dip is down

SW1 :
1 - display resolution
--> open == 640x200
--> closed == 640x400

2 - superimpose mode (way of displaying japanese characters)
--> open == disabled
--> closed == enabled

3 - digital display
--> open == no
--> closed == yes

4 - IDs of floppies drives
--> open == Internal have ID 1 and 2 and external 3 and 4
--> closed == Internal have ID 3 and 4 and external 1 and 2

5 and 6 - RS-232 transfer protocol configuration
--> open - open == asynchronous transfer
--> open - closed == synchronous transfer with clocking
--> close - open == ST2 synchronization
--> close - close == BCI synchronization

7 - VF key
--> open == disabled
--> closed == enabled

8 - graphics capabilities
--> open == only 8 colors
--> closed == only 8 colors or 16 color among 4096

SW2 :

1 - LT mode (for PC-98 laptop)
--> open == normal display
--> closed == LT mode enabled (change the way of displaying graphics on screen)

2 - N88 basic operation
--> open == basic mode (normal operation)
--> closed == terminal mode (act as dumb terminal like TTY)

3 and 4 - text mode resolution (best is 80x25)
3 : columns
--> open == 40 columns
--> closed == 80 columns
4 : lines
--> open == 20 lines
--> closed == 25 lines

5 - erase RAM at reset
--> open == yes
--> closed == no

6 - internal HDD enabled (I don't know if it's for SCSI, SASI or whatever type of HDD interface)
--> open == yes
--> closed == no

7 - floppy drives rotation control (I think CRC control...)
--> open == no
--> closed == yes

8 - graphic controler frequency
--> open == 2.5Mhz
--> closed == 5Mhz

SW3 :

1 and 2 - type of floppy format dring boot process
--> open - open == auto detection for internal drives and assume 1.2mb for external drives
--> open - closed == auto detection for internal drives and assume 640kb for external drives
--> close - open == force to 1.2mb
--> close - close == force to 640kb

3 - DMA channels
--> open == enable DMA channel 0 only
--> closed == enable DMA channel 0 and 1

4 - type of display with LCD screen (not sure, but it can invert color for high contrast display on LCD screen)
--> open == LCD Reverse / PDP Normal
--> closed == LCD Normal / PDP reverse

5 - DMA clock
--> open == auto
--> closed == always high speed

6 - amount of base memory to use (this switch tell to OS what is the amount of base memory)
--> open == normal 512 or 640kb
--> closed == force base memory to 384 or 256kb

7 - unknow. On this PC-98 it is open.

8 - CPU type
--> open == NEC V30
--> closed == x86


- Back of this model : there is standard 25 pin D-sub RS-232 com port. Specific connector for mouse, floppy and printer. The analog RGB output can handle both progressive (480p) and interlaced mode (240p) depending on SW1-1.

B/W connector is for monochrome display. And an interesting "Digital RGB" connector. I wonder which type of signla it delivers...


- inside this PC-98 : there is 3 removable boards. CPU board, sound card, and C-BUS hub. Back in the days, this is a beautiful piece of hardware beside western PC market with huge motherboard and no builtin video or sound.

- The motherboard with CPU board
- The motherboard without CPU board


Unknown C-BUS card : maybe a clever guy can find what is the purpose of this piece of hardware. I think this card may can drive some ECM machines...

- the board  itself
- solder side
- connectors on the back


- The floppy drives : 3'5'' 360rpm 1.23mb drives. These give me a lot of headache. The best way to transfer data on an 1.23mb 3'5'' floppy is to find a 3 mode floppy drive. You can follow this useful thread.


- SCSI B55 IV C-BUS card : purchased this card after successfully booting from a floppy disk. New storage device... new headaches ! This card works, but after a lot of trial and errors. There is no diagnotic or setup included in the BIOS of this card. But after set the dip switch and boot option on PC-98 side (DIP SW2-6) I manage to boot from HDD and use SCSI DVD drive. You can download the DIP setting that I find after some experiment.


The C-BUS hub board. It working like ISA bus. Here there is 2 slot, but you can add more devices if you install a hub board with more than 2 slots. Unfortunately, the form factor is too small to up over 2 cards.

- C-BUS hub board
- C-BUS hub board solder side


The sound card based on YM2203 FM+PSG. No PCM. This chip have fantastic sounds and this is really pleasant to listen the ton of musics there you can found on this support. Specficaly FMP Music discs.

- the YM2203 sound card
- solder side

I have some examples recorded directly from this PC-98 :

- Gradius
- Eve Burst Error : nice bass with only FM, no PCM at all.
- Castlevania
- Popful Mail

This sound card use a special connector. I think this is just a C-BUS with specific pinout. Can we plug others devices than sound card on this connector ?


The CPu board. This is an interesting thing. I wonder if it possible to upgrade this one. This board contain CPU + RAM. RAM bank can be unplugged and upgraded but can't find ones on the Web... there is an empty socket, maybe for FPU...

- CPU board with RAM
- CPU board without RAM
- RAM modules


Let's talk about upgrade capabilities :

So, there is 3 way of extending possibilities.

- C-BUS : there is memory/CPU upgrade through this one ? The speed may too slow to upgrade RAM or CPU through it...

- Sound card connector : If it only C-BUS with another connector shape it's useless. It's a proprietary bus, than can accept device who upgrade this machine...

- CPU board connector : I think this is the best way to upgrade this model of PC-98. I found other CPU board like this. I think it's compatible but the form factor is not the same as present CPU  board (plus there is no RAM connectors). On the other hand connector seem to be the same.

Voila ! If any guy have more info on how to extend the PC-9801 models (V30 to 286) give your tips !