Here we go.... (48G fixing and MEMORY UPGRADE)

[Paul Berger (Canada)]

(05-25-2017 06:44 AM)lemontea Wrote:  The pinouts are a bit different. Will it be OK?

The original RAM chip has only 28 pins, so I am guessing the extra 4 pins on the 32 pin chip will be ignored?

I got the new part info from

http://www.finetune.co.jp/~lyuka/interes...hp48g.html

(Note that he has a typing mistake on the Web page for the Part # of the RAM chip. I checked the photograph of the chip itself for the Part #).

But the datasheet for the Cypress says it is NOT suitable for CMOS level, and only TTL level.

The existing RAM chip (on the 48G that I opened) is a Winbond CMOS chip.

UPDATE: The Datasheet on the Winbond says it is TTL compatible even though it is a CMOS chip. So, should be good.

The extra 4 pins provide the extra address lines they are all pre-wired on the 48's circuit board, you will note that on the 32 pin 128K chip, the position that was Vcc for 28 pin chip is not an active high chip enable, which makes it very convenient to upgrade, no wiring changes required. Some care ie required when desoldering the old chip. I used a needle to lift away the pins as I heated them one by one.

I think that there are a few documents about upgrading memory in the HP48 file section on hpcalc.org, it seems to me that is where I found information when I upgraded some of them a long time ago, I have one here that I not only upgraded the base RAM to 128K I also added 128K port 1 and a further 512K for ports 2 to 5.