Monday, October 12, 2020

Fix for Tseng ET3000AX SVGA card displaying monochrome

This is a Micro-Labs VGA Solution, an ISA super VGA card based on the Tseng Labs ET3000AX. It seems to be a reference design because other brands of cards also exist with the same board layout. It displayed in monochrome on the SVGA monitor.

At first I thought the switches on the back were set incorrectly. They tell the board what kind of monitor is connected and whether it's primary or secondary. They're documented here, but basically they all need to be off. That tells it a that it's the primary card, a "multisync analog monitor" is connected, and a secondary monochrome card may or may not be present. Up, away from the circuit board, is off.

The switches seemed to make no difference. So, I thought the board was broken. But no, the VGA connector standard changed. Those switches only talk about colour and monochrome regarding TTL monitors, using the DB9 connector. The VGA monitor is detected via monitor ID bits according to the old VGA connector standard. The current standard doesn't include those and instead uses an EEPROM providing information about the monitor. It seems that based on what the board detects on those ID bits, it can go into monochrome mode. All the palette entries get transformed via gray-scale summing, making everything black and white. This even happens if you try to set the palette via int 10h.

A software solution is, which uses int 10h functions to switch the card to colour mode from DOS. Another solution is to boot with the monitor disconnected. This doesn't only mean power-on, but also if you reboot via the reset button or control-alt-delete. I opted for a hardware solution, cutting the circuit board trace between those connector pins and the rest of the card:

One last thing. The old VGA connector standard has a key at pin 9, meaning no pin there in the male and no hole there in the female. Now that is +5 V DC supplied to the monitor for the identification EEPROM. You might want to drill a hole there in the connector to avoid breaking pins on more modern VGA connectors.

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