(Taken from the file 48GS256K.ZIP)
If you have a look on the circuit board you will see a row of
pins that connect to the chip card slots of the 48GX. Most pins
are connected in the G, too. If the battery case points to you
the pins are numbered RIGHT TO LEFT. You can verify that if you
know that the pins 2,21,37,38 and 39 are devided for the two card
slots. These are the pins.
PIN: Signal: PIN: Signal: 1 VCC (only if HP48 is ON) 21 Card enable (active HIGH) 2 RAM Battery measure 22 Output enable (active low) 3 Addr. 0 23 Data 1 4 Addr. 1 24 Data 2 5 Addr. 2 25 Data 3 6 Addr. 3 26 Data 4 7 Addr. 4 27 Data 5 8 Addr. 5 28 Data 6 9 Addr. 6 29 Data 7 10 Addr. 7 30 Data 8 11 Addr. 8 31 Video (port1) / EA 0 12 Addr. 9 32 Video (port1) / EA 1 13 Addr. 10 33 Video (port1) / EA 2 14 Addr. 11 34 Video (port1) / EA 3 15 Addr. 12 35 Video (port1) / EA 4 16 Addr. 13 36 Video (port1) / EA 5 17 Addr. 14 37 Write prot. in (act low) 18 Addr. 15 38 Card present in (act high) 19 Addr. 16 39 Card type (low for RAM) 20 Write enable (active low) 40 Ground
Notes:
1. In the G the pins 31 to 36 (Extended Adress pins) are connected
to an empty chip place on the board. They are needed to address
the larger RAM cards (up to 4MB) and provide additional card enable
signals. Carl Raffa told me there is a HC174 in the GX but if
you don't plan to connect a RAM expansion box to your 48 or have
really small RAM chips you don't need it. The G with ROM version
K will not be able to use that at all since the command PINIT
is missing in that release (thanks to James H. Cloos, Jr. for
that info).
2. Pin 22 leads to a second empty chip place (on the right side
above the battery case). Thanks to Carl Raffa I now know that
this place has to be filled with an 74HC00 (costs ca. 50 cents)
to decode the OE (output enable) pin for simulating a RAM card
in the 48G.