First post, by clb
Hey all,
When IBM originally created the Feature Connector for their VGA adapter around 1988,
the maximum pixel clock that video would output from the connector was 28.2 MHz. (e.g. in the 720x400 text mode).
Later in August 1989, VESA took on to standardize this Feature Connector in a specification VS890803: VESA Standard VGA Pass-Through Connector, (VSVPC, yeah what a mouthful), although unfortunately this standard is lost in time and no scanned copies of it exists (not even at VESA - we asked, and their Executive Director Bill Lempesis replied saying they couldn't find it anymore).
The earliest graphics adapter that I have that actually adopts VSVPC is this Acumos AVGA 2 card from week 7 of 1992:
In the absence of the standard, we don't actually know for 100% what was the maximum pixel clock requirement that would have been standardized for this connector.
However, in a later document from March 1994 that VESA kindly shared, VESA standardized a follow-up to the VSVPC, the VESA Advanced Feature Connector (VAFC). This was an expansion of the 8-bit VSVPC connector to a 32 bit wide bus, that reuses the SCSI connector. Unfortunately the VAFC standard never caught on, and almost no vendor implemented this connector. The only one I have in my lab is this rather obscure Compaq card:
In this VAFC standard, VESA mentions the maximum rated pixel clock bandwidth of this new 32-bit connector to be 37.5 MHz:
So it is fair to say that the earlier VSVPC connector would have had a max. pixel clock at most 37.5 MHz, and at least 28.2 MHz.
These figures go well in hand compared to ATA33 -> ATA66 transition. ATA33 is a flat ribbon cable like the VSVPC connector is, and is rated to max. 33 MHz. To get up to 66 MHz, the ATA cable had to add a dedicated ground pin to each signal line, which the VSVPC connector does not have.
Alright then, presumably max 37.5 MHz it is. What that means is that the maximum rating for the VSVPC connector would be to be able to display up to the "first" SVGA Mode 6Ah, 800x600@56Hz video mode:
which runs at a pixel clock of 36.1 MHz.
With CRT Terminator out the door, we have gotten more familiar with how this connector behaves. So now it is time to !!overclock!! this connector. 😀
With a good quality VGA adapter, the 37.5 MHz limit is not a limit at all, but it is possible to display higher resolutions through the Feature Connector. In the next posts (given the Vogons 5 pictures per post limit), I'll share images of the absolute S tier of graphics adapters that I have found to work best with CRT Terminator, along with the output resolutions that they are able to reach.