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.
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!
This text was originally written by Paul Smith. All pictures are my work,
as the html translation.
++++++++++++++++ start forwarded text ++++++++++++++++++
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.
You will need to remove the batteries for awhile 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).
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 careful
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
![]() |
![]() |
Before | After |
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). 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.
This is a close-up of a drilled rivet.
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.
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], [blueshift],
[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).
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.
(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, now that you have opened your HP, let's start with the upgrade.
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 which were free), it will be easier to solder the
new ram.
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 already 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 carefully, 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!
You already have a 128K ram chip. 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...
------- second ram |I-------I| wire | ------- | first ram |I-------I| ------------------------------------PCB board
Well... Sorry, I haven't got the time to make a better picture... Hope you understood!
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 divided for the two
card slots. For all those pins you have to use only the part of that pins
that has more distance from the battery case. This is the part for card
slot 1.
pin 22: connect it to pin 40 of the connector pin 24: " " 22 " pin 30: " " 21 "
There's some additional connections to do :
Here's the result:
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!
To test new memory inside the HP48G, press ON, then ON-D to enter test
mode, and after the vertical line(s) appear 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" message 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!
![]() |
![]() |
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 |
(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
divided 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) / RAM Bank switching 12 Addr. 9 32 Video (port1) / RAM Bank switching 13 Addr. 10 33 Video (port1) / RAM Bank switching 14 Addr. 11 34 Video (port1) / RAM Bank switching 15 Addr. 12 35 Video (port1) / RAM Bank switching 16 Addr. 13 36 Video (port1) / RAM Bank switching 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 are connected to an empty chip place on the
board. I assume 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.
__ __ __ 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.
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. 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