NEC GB-1 EGA Card (Tseng ET2000)

[GloriousCow]

Picked up this NEC GB-1 recently, with the CMII daughtercard.
The NEC GB-1 is pretty much a rebadged Tseng EVA/480. The "480" refers to the ET2000's 640x480x4bpp support.

These are the only two cards I am aware of that used the ET2000 chipset.



Yes, that's a Z80.

The CMII module was a $50 add-on that promised full backwards compatibility with CGA.



The CM11 module has a 2KB 6116 SRAM. The main BIOS ROM contains both 8088 and Z80 code.

I was able to track down the DOS driver and utility diskette on an old FTP archive - it is attached.

It includes several soft fonts and the FEDIT/FLOAD utilities Tseng bundled with their cards up through the ET4000.
There is an EMODE utility that doesn't do much but let you choose between 40/80 columns, unless you have the DIP switches set for enhanced mode.
The GB-1 utility performs board diagnostics. My board appears to immediately fail with an error "06", but seems to work otherwise. The utilities will run through many of the board's extended graphics modes, none of which my rgb2hdmi likes very much at all.

Drivers include a Windows 3.1(?) driver EGAHIRES.DRV, an enhanced ANSI.SYS called 'EANSI.SYS', and a resident AutoCad driver DSGB-1.EXE.
The last two utilities, CMON and CMOFF, enable or disable the CMII module.

 

[GloriousCow]

I'd like to test out the capabilities of the CMII module.
I can't really think of a lot of CGA titles that straight up didn't work on the EGA, at least off the top of my head. I know that the EGA didn't offer the CGA's third palette.

Just for fun I tried running Area 5150 - most of the effects cause the rgb2hdmi to lose sync , but some of the static ANSI art scenes look fine.
8088MPH loses sync immediately and then seems to hang.



Can anyone think of some good CGA titles to test?

 

[dieymir]

Windmill Software Digger, for example.

 

[GloriousCow]

Windmill Software Digger, for example.

Digger seems to work fine in EGA, other than using the wrong palette.

 

[GloriousCow]

On the GB-1 in compatibility mode, not only does Digger have the correct palette, but the third CGA palette is available too.
Unfortunate this card has some rendering issues.

 

[GloriousCow]

I'm suspecting that one of the DRAM chips on plane 1 is bad.
This causes corrupted character attributes, and in CGA compatibility mode, vertical lines every 8 pixels since 4 pixels are rendered from plane 0 and the next 4 from plane 1.
Curious that the VRAM test in Checkit 2.0 said that all the RAM is fine, though. I'm not sure how much I trust that determination.

 

[GloriousCow]

The CMII compatibility card was intriguing me so I reproduced the board in KiCad.



The Z80 address space is mapped as so:

0x0000-0x3FFF - ET2000 maps this range somewhere in the BIOS - there's probably a page register on the main card that controls where.
0x4000-0x4800 - 2K SRAM
0xC000-0xFFFF - ET2000 register file (memory-mapped)

four IO addresses are decoded - IO ports 0 and 1 reset flip-flops used for bus timings. Ports 2 & 3 are 8-bit address latches. The address is latched when the CMII board receives the ~BUSACK signal, and it looks like an 8-bit jump table index is passed to the Z80 this way.