Upgrading by yourself the RAM of your HP48G to 256K

This document is based on the file 48GS256K.ZIP. The original document describe how to upgrade your HP48G(S) to 256K with standard DIL ram packages. I would like to thanks Thomas Otten and Thijs Kaper for their initial works.
You can download the file 48gs256k.zip
In this document, I explain how to use SMD (surface mounted chip) ram chips to do the same upgrade, but for HP48G only. If you want to upgrade the HP48S, you should refer to the original file. Moreover, you'll find some pictures showing how to open your HP without damages, and what looks like the upgrade.

WARNING:
Do the upgrade only if you know you're qualified to do it !
Watch out for any Static-discharges !
Of course this upgrade is at your own risk, you lose all waranty, if you had any left......

Parts needed:
You need also a thin soldering iron. The soldering tip should be less than 2mm (1/10 inch) wide.
You'll need as secondary accessories:
Provide yourself also with a lot of patience!

Let's start!

The numbers inside brackets are the size of the document with pictures.
  1. Open your HP.
    This part was originaly written by Paul Smith. All pictures are my work, as the html translation.
    1. Are you sure? Ok, backup your memory!
    2. Remove carrefully the overlay
    3. Remove 10 platics rivets
    4. Pull appart the two parts of the case
  2. Look inside
  3. Upgrade
    1. 32k to 128k (easy)
    2. 128k to 256k (more difficult)
  4. Close & Test 
Usefull datas:

Opening the case

Disassembling the hp48
Despite the fact that that's pretty scary, I will entail what I know here. Thanks to David Holmes for taking the first step and helping make my attempt more informed.
None of the information that follows came from hp, so if what I say starts to sound rather official, it's not.
For the few folks out there who've actually had to send their hp48 back to hp for service, you probably ended up with an entirely new unit. I suspect the reason for this, aside from some labor cost savings on the part of hp, is that they really aren't much fun to take apart. You can draw your own conclusions as to why it was designed that way.

 

Backup your memory.

You will need to remove the batteries for a while to disassemble the unit, and you may have them out long enough to cause memory loss (although in my case, even 2 days didn't wipe anything out).

Moreover, you will remove the original ram chip! If you manage not to loose the memory, you are not born on this planet!!

Remove the tin key overlay.

The overlay is attached with double- sided tape of some sort. A little care and patience will allow you to remove it only slightly warped, and it is fairly easy to straighten out later. I took a small jewelers screwdriver and started prying near the "ON" button, and worked upward toward the screen.


When you get near the screen, try to preserve the shape of the overlay because that portion is a bit more difficult to properly straighten later. Once the overlay is removed, put it in a container or somewhere away from dust. The double-sided tape should stay with the overlay (mine did, at least) and will gather dust and stuff making it less sticky if left out in the open. 48sx owners will need to remove the little hp logo insert above the screen as well.
I used a hair dryer to warm a little bit the glue. It's then easier to remove the overlay. But you must be very carrefull not to over heat the HP, especially the LCD display!! I placed a cardboard above the LCD and blew hot hair only in the opposite direction. You will not manage to heat all the overlay, but at least until the second or third row of keys it'll be easier! - SG

HP-original HP-original
Before After

Defeat the 10 plastic rivets.

You can see - under the thin metal overlay you just removed - 10 plastic rivets.You can see on this picture their location.

rivets'location

Believe it or not, these rivets aren't all that critical to holding the unit together, so you can be as careful or as reckless as you want (I prefer reckless, because I hate plastic rivets). A good way to carefully remove the rivet heads is to use a flat head screwdriver that is the same width as the recessed rivet hole (4mm?) and "drill" the head away by hand with the screwdriver.
(I used a real drill to do this operation - SG).

 Drilling the rivets
 drilling plastic rivets
The same operation, but in 2D and black&white!
 shema of operation

The head is about 2mm deep, so stop "drilling" there to leave the keyboard material shoulder intact for easier reconnection later. There are 6 rivets near the number keys and four above the screen. These rivets above the screen perform more of a holding function than the others, and you may want to consider using some screws and small washers to replace them when you re-assemble.

drilled plastic rivetsThis is a close-up of a drilled rivet.

Separate the lower half from the upper half.

The upper half of the calculator contains almost all of the components except for the piezo "beeper" element, and there are no interconnecting wires between halves.
plastic hook

The only obstacle is the snap-together "hooks" that David Holmes refers to. These hooks are positioned near the [A], [F], [SIN], [1/x], [ON], [+] and [.] keys (the one by the [.] key is a real bugger!). Refer to the cutaway diagram below for details.













Key overlay here ___
                    \     ____________
                     \   |            |
             ____________|    ______  |
                             |   _  | |  <--- Upper section.
                             |  | | | |
             ________________|  | | |_|
                             |  | |___  
  Circuit board and other    |  |     |
  components (affixed to     | /      |  <--- Lower section.
  upper section)             |/__     |        Has a lip which 
                             |   |    |        engages with metal 
           Metal "hook" ---> | / |    |        "hook" from upper 
                             |/  |    |        section.
                             |   |    |
             ________________|   |    |
                                 |    |
To separate the two sections, you will need to push the lower section out and down around the hooks. You can't do this from the outside because the upper section hangs over the lower, so you have to go from the inside. Luckily, the six slots in the keyboard (used by the separate user-overlays that fit over the original overlay) near the [MTH], [ENTER], [blushift], [NXT], [backspace] and [-], come in handy for this. These slots are not exactly lined up with the hooks, but are close enough. You can insert something in these holes (I used a jewelers screwdriver, flat head) at an angle that is mostly down and somewhat out, to a depth of about 5mm, and come in contact with the lower section. Push the lower section out about 2mm (this will take a bit of force), while wedging something in the outside gap to separate and hold the sections apart while working on the other hook positions ( a wooden matchstick works). I recommend starting with the [A] or [F] positions first, working down whichever side you started with, then do the other side, and leave the [.] position for later (there is no helpful slot there).

 opening  closeup

 

Remove the battery cover and the batteries.

 The two battery contacts which come through the case will need to go back through the case when the sections separate. The upper contact is hooked on a plastic boss, and needs to be freed from it. Just pop it off with your finger or a screwdriver.
 plastic boss closeup

Very carefully pull the two sections apart at the top

(above the screen).
If you have freed all the hooks, the case should sort of hinge at the bottom edge. This is due to the last hook (near the [.] key). Some careful twisting and working of the sections should free this hook as well. You should now be able to completely separate the two sections. I recommend at this point that you take a pair of pliers and flatten that last hook so that it will not hold next time. It is really not needed anyway, as the other six hooks hold the unit together just fine.

Ok, your are inside!

Now you can poke around and explore things. Be careful what you touch, I'm not sure how sensitive the insides are to static discharge from your fingers, etc.
If you're interested in adding stuff, some open real estate can be found in the areas between the battery compartment and the card receivers, and in the cavities in the lower section below the tin shielding on either side of the card ports (this is where I put some jacks). If you own a 48s, you may find a lot more space.

If you want to disassemble the unit further, like removing the circuit board from the upper section, you have a considerably tougher job on your hands, and you're on your own. Also, if you separate the screen from it's circuit board, you will disturb the rubber conductors (there are two) which provide electrical connection to the lcd rows and columns (zebra strips). Upon reassembly of the screen, you may find that you have lost some rows or columns in the display (not permanent, just a zebra strip alignment problem) and will have to keep reassembling and perhaps cleaning until it comes out okay. I haven't done this to my hp (yet) but I have done it to other cheap calculators and it wasn't fun.

What's what (128k)

The ram chip is the original 32k ram. You can see on the left the four pads waiting for the new 128k ram chip! The PCB of the 48G is the same as the 48GX! It differs only in the fact that in the G ther's missing some chips. Especially the 74HC00 (bottom of the picture) used to manage the port 1, and the 74HC174 used for the bank switching mechanism used at port 2.

The Coil is used to generate regulated 5V (for the chips) and +/- 12V (for serial IO) from the 4.5 V delivered by the 3 batteries.The black capacitor maintain the ram while you are removing the batteries. You will also find there a good +5V source.

Port 1 is on the right, port 2 on the left (for the pins that are not the same for the two ports).

The CPU's name is Saturn. A nice processor with 64 bits internal registers, but with a data bus of 4 bits!!!

Doing the upgrade

Ok, now that you have opened your HP, let's start with the upgrade.

32K -> 128K

 You must desolder the original 32K ram chip. I used a needle to bent up the pins while I was heating with the soldering iron. As you can see, the RAM chip is the left one (the LCD display being up and the battery case down). There is 28 pins to desolder. We don't need this chip any more, so you can throw it if you want.
Once this is done, check the absence of soldering bridges. Put some fresh solder on all the pins (there must be 32 pins, the first 28 used by the old ram, and the last 4 wich were free), it will be easier to solder the new ram.		  desoldering 32K

 128K  Solder the new ram, 128K x 8bits static SMD. Take care at the orientation of the ram: The first pin is down near the ROM chip, near the battery case. There's a dot on the chip near one corner of the ram locating the pin number 1.
It allready works!! Close the HP (see how to do it) replace the batteries and check if the HP turns ON... If it doesn't, then you are in troubles... Reopen your HP, check all your solders, check for soldering bridges, try to bend up the pins to check if they are good soldered,...
If you have done it carrefully, there should be no problems. At this point, you have 128K. You may want to stop now, there's no problem. If on the other hand you are brave enough, you can try to do the second part of the upgrade: adding 128k more for a total of 256k!

128K -> 256K

You allready have a 128K ram HP. Let's go for 256K!
You have to solder the second chip above the first. Warning: It's a hard work... Usually, the pins are a little too short to manage an easy soldering. You have to solder these pins together with the help of a tiny piece of wire...

Solder the second ram chip

All the pins have to be soldered on the first RAM except 3 of them: You have to bend up pins # 22, 24 and 30. They have to be connected to some signals present at the place where the card connector is usually soldered.
Take a look at the layout of the card connector and also at the pinout of the rams.
The pins 2,21,37,38 and 39 of the card connector are devided for the two card slots. For all those pins you have to use only the row of pins that has more distance from the battery case. This is the row for card slot 1.

 pin 22:  connect it to pin 40 of the connector
 pin 24:     "            " 22           "
 pin 30:     "            " 21           "
There's some additionnal connections to do :
Here's the result: 256Kthe same picture at high resolution (232k)

Put now some adhesive tape on the metal shield to prevent shortcuts when you'll close the HP. Close the HP, and ready for the first test!

Closing the HP

This is covered in the text by Paul Smith. Pay attention to the "plus" battery connector: it has to snap back into its place or you will get a nice shortcut!

Reassembly-
Putting it back together is much easier. You may need to clean up the remains of the rivet heads so they will easily reinsert into their holes. Make sure the battery connections align with their respective holes, and snap the unit together, applying pressure where the six (or seven, if you left that one near the [.] key alone) hooks are. screw

You will probably want to hold the case together at the top with one or two of the rivets there. If you can find a wood screw with a flat or thin head, you can screw it into the body of the rivet and let the head hold the upper section surface. You may need a small washer for this. A machine screw will probably work as well, but will strip the plastic easier and not hold as well. I have only one screw holding mine together and it works fine.

You will probably need to re-shape your tin overlay, as it probably took a beating during removal. I removed the sticky tape from mine, but it's probably better not to (unless it really wont stick anymore). Put wax paper over the tape and put the overlay face down on a hard cover book. Grab a hammer with a smooth and somewhat flat head, and pound away. DONT pound away on the part that surrounds the screen (beveled part). You can probably use your fingers and a little massaging to fix that area. With a little care you can end up with an overlay that looks like new. Press the overlay in place and hope it sticks. If not, a little rubber cement wouldn't hurt.
Have fun!
Paul Smith

Testing

WARNING:
If, at startup time, the normal display doesn't show up (either blank screen or strange patterns), REMOVE RAPIDLY THE BATTERIES!!
Reopen to check for soldering bridges and for bad soldered pins or wires...
I have had many times this problem... It didn't burned my HP, but I can not say it will not destroy yours!

To test new memory inside the HP48G, press ON, then ON-D to enter test mode, and after the vertical line(s) apear press the "k" key (arrow up). After this the display should say "RAM1 OK 128K" and something like "RAM2 90000". The first "OK" just stays on my display a very short time. Press "k" again to test it another time if you missed the message... Leave test with ON-C.
To test the new internal 128K (the previous test was an external port-mem test only), press ON, ON-D, and then EEX. Now wait for a message stating "IRAM OK 128K". Leave the test with ON-C. Don't be afraid if you see a "fail 100" or "fail 180" mesasage at the end ! This message was already present before you upgraded your calculator, and as far as i can see the "fail 180" means that both port-1 and port-2 memory are missing, and in the same way "fail 100" indicates the missing port-2 memory (you only installed port-1). So if everything is Ok, you should see the "fail 100" message after doing the upgrade.

To get rid of the "Invalid card data" message, type PINIT, or just start using the port.
I wish everybody good luck on doing the upgrade !
HP-256K HP-original
There's now 256K of ram! "You know what? I'm happy..."
On the left: Jean-Marc (the pictures are his HP)
On the right: Myself

Pin layout of the 128K RAM chips

                __                  __      __
     Vcc A15 CE2 WE  A13 A8  A9  A11 OE  A10 CE1 D8  D7  D6  D5  D4
  ____I___I___I___I___I___I___I___I___I___I___I___I___I___I___I___I____
  I  32  31  30  29  28  27  26  25  24  23  22  21  20  19  18  17   I
  I                                                                   I
   \                                                                  I
    I                                                                 I
   /                                                                  I
  I   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15  16   I
  I---I---I---I---I---I---I---I---I---I---I---I---I---I---I---I---I---I
     NC  A16 A14 A12 A7  A6  A5  A4  A3  A2  A1  A0  D1  D2  D3  Gnd
And this is for the built in 32K chip. In my G it's labeled M5M5256BFP and is located just above the battery case (the left one of the two chips located there before you inserted the HC00).
                 __                  __      __
             VCC WE  A13 A8  A9  A11 OE  A10 CE1 D8  D7  D6  D5  D4
           ___I___I___I___I___I___I___I___I___I___I___I___I___I___I____
           I  28  27  26  25  24  23  22  21  20  19  18  17  16  15  I
           I                                                          I
            \                                                         I
             I                                                        I
            /                                                         I
           I   1   2   3   4   5   6   7   8   9  10  11  12  13  14  I
           I- -I---I---I---I---I---I---I---I---I---I---I---I---I---I--I
              A14 A12 A7  A6  A5  A4  A3  A2  A1  A0  D1  D2  D3  Gnd
As you can see the layout is nearly the same. Not for the pin numbers but for the pin places if the chips are right assigned.
Be sure to isolate everything. I glued some paper to the back side of the RAM chips to keep it away from the shielding. A little sheet of paper between pin 20 of the 32K and it's former connection keeps these pins apart.

Pin layout of a 512k RAM chip

This chip can be used if you want to add more than 256k to your HP48G(X).

Here is the 512k ram pinout:

         |-------\_/-------|
         |                 |
   A18 - | 1            32 | - Vcc
   A16 - | 2            31 | - A15
   A14 - | 3            30 | - A17
   A12 - | 4            29 | - Negative WE
    A7 - | 5            28 | - A13
    A6 - | 6            27 | - A8
    A5 - | 7            26 | - A9
    A4 - | 8            25 | - A11
    A3 - | 9            24 | - Negative OE
    A2 - | 10           23 | - A10
    A1 - | 11           22 | - Negative CS
    A0 - | 12           21 | - D7
    D0 - | 13           20 | - D6
    D1 - | 14           19 | - D5
    D2 - | 15           18 | - D4
   VSS - | 16           17 | - D3
         |                 |
         |-----------------|

Layout of the card connector

(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.

Credits: