THE SIMPLIFIED THEORY

1) An serial RS232 port type in  full duplex at 9600 bauds max:

Male connector with  4 pins spaced by 2 mm ( The female connector is difficult to get)  hidden in front of  hp .

The signals are the ground signal, TX send pin, Rx receive pin and the shield pin for EMC protection (Electromagnetic Compatibility)- By the way, you will notice on the official wire HP/PC (wire with Subd9/25 IBM PC: HP 82208A, MAC: 82209A), a cylinder greater in diameter than the wire, it is a parasite attenuator).

The transfer rate (who matches the  bit fréquency)  can be set up to 1200/2400/4800/9600 bauds, with 1 bit start, 8 bit de données , 1 or 0 parity bit (odd,even,none, mark or space) and  at least one stop bit . The voltage V between (Tx/Rx) pin and the signal ground pin should  vary inside  +3/-3V range to  +12/-12V  range to respect  RS232C normalization. You should observe that the logic is negative with  Rs232 signals: a  bit  at  "0"  logical level correspond to  an analogic voltage  +V whereas a bit at "1" logic state is represented by  an analogic -V voltage..

The input buffer can store a maximum of 255 bytes. It is possible to send and reveive at the same time  to differents bytes on the serial port.

2) An Infrarouge Half duplex port  at 9600 bauds max

The reception and the emission can't be done at the same time. Depending on the bit to send, the   infrarouge diode,  protected by the ambient light lumière with the utlraviolet cache, provides a glitch or not. The features of the glitchs (high time, time between two succesive glitchs) should be very precise .( at the  microsecond) to allow the correct reception of the IR frame on the other side.

The maximum transfert rate is given by the relation   Speed_in_Bauds/11.375 indicating that ther is an exchange of 844 bytes/seconde at 9600 bauds). The distance at the emission is about d'en 2 à 3 meters although the reception, is limited to 20 centimeters.(This is due to  some little genious guy  who has exchanged answers by infrarouge with the A version of the HP48SX during examinations. Unfortunately for them, they have been catched and HP in france must have been forced to rectify future versions by Educationnal French Institute.) - Only the A version of  HP48S(X)  are not limited in receiving, for the others , it is required to open the hp and change a resistor (I have never done it as  the screen is so fragile);

3) Two extended slot cards (only SX/GX):

When a card is inside one of the two ports,the batteries of the hp replace the backup button of the card (type 2016). It is then possible by building a false card to know if the hp is on or off. The signals of the adress/data bus  and  some control signals are available. I give below the circuits to build a card by yourself   .( I decline all responsabilty of the bad use of this and of any damage that may occur).

As express demand of Asterix86, I have make it clean  Layo, alls the drafts one of my friend gave me.

This is the copper view. The coper tracks are green and the coponents red. U1 is a 128k*8 Static RAM (pinout AFP) with a  SOP package of 32 pins which should be weld on the copper side . BT1: is a battery button of 3V for RAM memory's backup. Drill a hole at the diameter of the battery  and weld  wires on it to connect to the board   (take care of the orientation) R1,C1,D1,D2,R2,C2 has be welded  to the copper side if you got  the SMT elements else it is on the component side (in this case, hole should be made) S1:  3 state DIL switch The red lines represent straps on the component side (holes required)

To avoid damages caused by the bad orientation of the card, I have build  a chipless card with only connectors :

Let's take the temperature of the adress bus (16  white plots ) and data bus (8 black plots) ...

to: a) observe a voltage greater (between 2 and 40 = 4.91V) than all the batteries combined (4V)  when the hp is on. I bet it is due to LCD display. b) to locate pinouts orientation -pin 1 is on the right -pin 40 is on the left with the reference the back of the hp.

Here are the  les plans if you want to build your own 128k extension card. The print.bat file will allow you to print PCB of this board. I have added my Layo files (PCB designer)  hpram3.lmc (component location) et hpram4.lmc(copper side only) with redefintion of connectors hpram.smb.

Reférence:
Hp48 I/O Interface Guide (C) Hewlett Packard 1990