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Using a 1M RAM card in 48GX slot 1 - Printable Version

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Using a 1M RAM card in 48GX slot 1 - Joe Horn - 12-23-2019 10:47 PM

Calling all 48 GX hardware experts...

Background: The 48 GX can access all the memory in an HP 1 Meg RAM card in slot 2, because the GX's slot 2 has special pins which are dedicated to bank switching those RAM cards. The same pins in slot 1 are used for display output, for use with the old overhead projector LCD panel. That, of course, is why the LCD panel can only be plugged into slot 1 of a GX, and why HP 1M RAM cards are only supposed to be plugged into slot 2. (The 48 SX used those pins in both slots for display output).

Question: Can strategic use of the display on a 48 GX allow the banks of an HP 1M RAM card to be accessed when it's plugged into slot 1? In other words, can the bank-switching signals of slot 2 be faked by strategic display manipulation when the card is in slot 1?

Please note, I'm only talking about the large-capacity RAM cards which were designed for slot 2 of the 48 GX, which created multiple "ports" when plugged into slot 2. I'm not talking about the older multi-bank RAM cards which were designed for the 48 SX and which could only access one bank of 128K at a time, using special bank-switching software which you had to install.

Also, yes I know that plugging a large-capacity GX RAM card into slot 1 is a Bad Idea. I'm just wondering whether it's *possible* to access its banks by bit-twiddling the display.

Thanks in advance for any insights!

RE: Using a 1M RAM card in 48GX slot 1 - BruceH - 12-24-2019 07:37 PM

No is the short answer.

Looking at the pixel display signals using an oscilloscope, there is quite a lot of glitchy noise so an all blank (all pixels off) screen isn't an all zero voltage signal and, vice versa, an all black (all pixels on) screen doesn't give a clean all +ve signal.


The above shows the display for an all blank screen (blank PICT, no menu, just the cursor cross-hairs showing). The top line (blue) is the 'new row of pixels' trigger pulse while the bottom line (yellow) is the row of pixels for one half of the screen. The right hand pulse are the pixels for (one row of) the cross-hairs cursor but I don't know what the half-voltage left hand spike is. It's there all the time so if a glitch then a pretty stable one. Perhaps another timing signal.

Whatever it is, it means that an 'all off' display is not clean enough to attempt to drive a RAM switching card.

RE: Using a 1M RAM card in 48GX slot 1 - Joe Horn - 12-24-2019 10:38 PM

Awesome analysis! Thanks, Bruce!

RE: Using a 1M RAM card in 48GX slot 1 - acoto - 12-24-2019 11:35 PM

Schematics here:


Port 2 Pins 31, 32, 33, 34, 35, are connected to Address lines A17, A18, A19, A20 and A21.

While Equivalent Port 1 Pins, are connected to EA17, EA18, LCDclk, LCDsync, LCDd1.

Not enough details to tell if LCD lines can be disabled by software. Probably no. They will be continuously changing state.

One possibility is to cripple the card by cutting the lines 33, 34, 35 and probably 36 ( Not sure on what the BEN signal is for), leave the connector side open, ground them internally, but I will not recommend it.