(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.