NEC PC-9801VX – Video Output

6Dec/212

NEC PC-9801VX – Video Output

After getting the PC-9801VX up and running via the monochrome output, it was time to get the RGB signals properly-connected to a real monitor. Thanks to previously playing with Amigas, I actually have an LCD on-hand that can do 15khz via a standard VGA cable. What to do? Hack together a cable that would interface with the DB-15 port on the PC-9801.

As mentioned, this unit uses a DB-15 Analog port which looks exactly like a Macintosh Video port. Of course, it's not, so you can't just use a Mac adapter, but you can refit one, thanks to this blog. You can also just roll-your-own-cable, using the pinout here. Jaycar has the necessary DB-15 male port with housing and the cable above was built.

Plugging it in saw the picture work beautifully... but the fonts were still out. A-Train was also still scaling off the bottom edge?

Front-Panel Dip-Switches

Turns out there's display configuration options on the front-panel to configure resolutions and font-sizes. As seen above, the DOS installation screen looked OK for the most part, but the selectable menu options showed stretched/cut-off fonts!? A quick read of the the dipswitch descriptions pointed to items that may help.

SW1-1 sets the output resolution from 640x200 to 640x400 and this helped a bit, but the fonts were still wrong. I then also toggled SW2-3 and SW2-4 to enable both 80-character screen width and 25-character screen height. After this? Perrrrrfect!

Very similar to the version from the MSX.

You don't have a 15khz monitor?

No fret! This is the internet and someone has already done this before. To make this work, you'll need a GBS8200 video adapter. Once you have this in-hand, you'll need to either wire things in directly, or build a circuit such as antarcticlion's GBHV_RGBS_CONV adapter. I've had a few PCBs printed for me...

... and I intend to build them up and test them. If anyone wants one, then contact me here. I'll post again once I've built one and got it going.

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Comments (2) Trackbacks (1) ( subscribe to comments on this post )

Bright
January 3rd, 2024 - 18:29

This project is really cool and gives me so much hope for my PC-98! I have not used my NEC 9801 VX since my monitor blew out in the 90s and been looking for a way to connect my CPU to a PC monitor.

Is there a way you can help me acquire one like what you’ve put together? I’d appreciate it if you can make time to comment or drop me an email bowusuk (at) gmail (dot) com.

( REPLY )
- stevenh
  January 3rd, 2024 - 20:00
  
  Hi Bright! I wired up a VGA cable using the instructions here:
  http://rdstyle.cocolog-nifty.com/gm/2020/02/post-3324c0.html
  
  Luckily my monitor supports the lower refresh rates required. I do have the boards above, but never actually populated or tested them. If you don’t happy to have a monitor that can do the lower rates then I’d be happy to try and build one and get it to you. You’d still need to grab one of the following:
  GBS-8200/GBS-8220/HD9800V/HD-VC9900(RGBS)
  
  Steven.
  
  ( REPLY )