ap@dde.uucp (Allan P. Petersen) (02/20/91)
Epson RAM card surgery ... or: All about RAM cards.
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by Allan Petersen
Dansk Data Elektronik A/S, Copenhagen
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Shortly after the introduction of the HP48sx it was mentioned here
on c.s.h. that Epson RAM cards possibly could be used in the 48 and
that HP RAM cards in fact were custom versions of Epson cards. It
was later pointed out that the standard Epson RAM cards should not
be used in the 48 because of different electrical specifications
than the HP custom version.
I followed this discussion with interest and later when I got my
48sx, I looked for further info on the specs of Epson cards. I
found some specs and studied them carefully; recently I found a
supplier of cards and asked for prices and delivery times.
I have now got a RAM card, have made some tests on it, and have
made some comparisons to the CMT RAM card and the HP Eq. Lib. card
(as mentioned recently by a colleague of mine Arne Christensen here
at DDE).
Here I will describe the test results; but first the prices:
In Denmark the price of the 32k Epson RAM card is 72 $ and the 128k
Epson card is 157 $. Both prices are based on single piece
quantities and are excl. sales tax.
These prices more or less correspond to the EduCalc prices of
HP/CMT cards but as the 32k/128k Epson card cost respectively 81%
and 56% of the 32k/128k HP card here in Denmark, I would expect the
Epson cards to be slightly cheaper in the States than the mentioned
prices from here.
However from a cost point of view there is not much to hope for,
the best advice seems to be: Buy from EduCalc.
Well, being of a curious mind, I could not resist the challenge and
finally got a 32k Seiko-Epson RAM card (type no. RBC032IE10, the
128k card is called RBC128IE10) hoping to be able to change the
voltage detect level and/or add some extra RAM chips to the card.
Other types (OTP EPROM/EEPROM/Flash EEPROM/MASK) are also available
from Seiko-Epson.
It was indeed somewhat of a challenge but so far I have managed to
change the detection level, which I will describe to you a little
later; first concerning the specs:
I can in every respect confirm that every piece of information on
the HP cards given here by Jim Dickie, Steve Harper and Preston
Brown (all from HP Corvallis) is in fact correct and
considered:
I have found that the standard Epson RAM card that I have clamps
the address and control lines to ground until the supply voltage
reaches 4.20 volts and again clamps the lines when the voltage goes
lower than 4.15 volts.
The data lines are not clamped.
These measurements were made with the card inserted in an original
Seiko-Epson card socket, not the 48.
The voltage supplied from my 48 to the card is 4.24 volts when the
48 is running programs (battery midt-life or so, version E
calculator).
This means that there is typically a chance that the card will
work if inserted, but the margin is very limited. As earlier stated
here the powersupply of the 48 is stabilized and not very dependent
of battery state; however when the batteries become almost empty at
some point the stabilizer wil not be able to sustain the needed
voltage and the RAM card will clamp its inputs long before the 48
stops operating. This has a very high chance of causing a memory
lost (or something worse...) as the clamps are rather effective: On
my card it takes about 10 mA per line to pull the line to 0,4 volt
and 25 mA to pull to 1 volt. This corresponds to about 40 ohm
on-resistance, which makes me guess that the controller uses
on-biased MOSFETs on the inputs in the clamp-state.
There is no way the 48 will be able to pull-up all these lines,
therefore: DO NOT USE A STANDARD EPSON RAM CARD IN YOUR HP48 !!
It may work for a time, but you are on very, very thin ice here !
The only way to safely use an otherwise standard Epson RAM card is
to change the clamp voltage detect level on the card and at the end
of this article I will describe how to do this; but if you do not
feel absolutely sure of what you are doing, I will strongly
recommend that you order a HP card from eg. EduCalc (I can highly
recommend EduCalc for their excellent service, I speak of personal
experience). The cost saving - if any - on changing a standard card
is not worth the change of a possibly damaged card - or damaged
calculator :-).
BTW: No, I have no connection in any way to HP, I just happen to
love their products and quality!
Some more specifications:
The card has of course the same mechanical dimensions as the HP
card, although it has no plastic handle to help in removing the
card from the calculator, it has 40 pin connections on a contact
pitch of 1.27 mm, pattern width 0.8 mm, pattern gap 0.47 mm,
contacts have min. 0.3 micrometer gold over min. 5 micrometer Ni,
and the shutter mechanism is guaranteed for 10,000 insertions. The
front and rear are covered by stainless steel panels to protect
against statics, noise and shock. The card and connector are keyed,
preventing incorrect insertion.
The data interface is 8 bits wide.
I have made some measurements of current consumption, but I will
not give you any figures, as there is often a factor of up to 100
between the max. RAM chip current over temp and the typical current
at room temp and it is in fact this current that will decide the
lifetime of the back-up battery; the current consumed by the card
at 4.4 volt (or other voltage) from the calculator is irrelevant
from a back-up point of view, although it in some cases may have a
small influence on the lifetime of the batteries in the calculator
itself.
The main part of the card's standby current consumption from the 48
is probably caused by the controller chip (gate array) in the Epson
card; the standby current is specified as max. 1.15 mA, but a
current this high is NOT taken from the back-up battery when the 48
is off. Only the RAM chip data retention current and a very small
current for the controller to keep its clamps on-biased is taken
from the Lithium battery.
The CMT card is claimed to use much less than the 1.15 mA, which is
very likely, as the CMT card is probably designed using standard
CMOS chips, but this difference does not need to have much relation
to the actual back-up currents from the Lithium battery.
I agree that the Epson card controller will consume a small current
to keep the clamp MOSFETs on and to bias the gatearray, but this
should not be that significant.
The standard Epson card is specified for a typ. data retention
periode of 4 years at room temp.
When you look down at the gold contacts and hold the battery end of
card away from you, the leftmost pin is pin number one.
The pin connections are:
PIN: Signal: PIN: Signal:
1 Power supply input 21 Card enable (active HIGH)
2 Batt. out/ EPROM Vpp 22 Output enable (active low)
3 Addr. 0 23 Data 0
4 Addr. 1 24 Data 1
5 Addr. 2 25 Data 2
6 Addr. 3 26 Data 3
7 Addr. 4 27 Data 4
8 Addr. 5 28 Data 5
9 Addr. 6 29 Data 6
10 Addr. 7 30 Data 7
11 Addr. 8 31 nc How are these
12 Addr. 9 32 nc lines connected
13 Addr. 10 33 nc in the 48?
14 Addr. 11 34 nc Could really be
15 Addr. 12 35 nc nice to have
16 Addr. 13 36 nc waitrequest here!!
17 Addr. 14 37 Write prot. out (act low)
18 Addr. 15 38 Card present out(act high)
19 Addr. 16 39 Device type (low for RAM)
20 Write enable (active low) 40 Ground
It can be seen that the card enable signal is active high; on
standard EPROMs the enable signal is active low, so if you try to
make an adaptor for an EPROM programmer, you need to invert this
signal in your adaptor.
Some signal lines are used to indicate card present, writeprotect,
and ROM/RAM type. If the writeprotect switch on the card is set to
protect position, the 48 will not be able to change the card data,
not even in a memory lost/lock-up situation.
The card battery voltage is measured via a 1M resistor.
And now to the exciting part: OPENING THE CARD.
Remove the battery and use a scalpel to very carefully lift and cut
loose the stainless steel panel on the upper side (the side
opposite to the side where the contacts and the shutter are
located). Be careful not to damage the inside of the card - or your
fingers :-).
Near one edge by the battery there is a metal spring used to
connect the two steel panels to keep them at the same electrical
potential. You will probably damage this spring when you open the
card (so did I). You must remember to put in a new spring when you
assemble the card again (I used a piece of conductive rubber) in
order to keep the protection against statics. Also be careful not
to damage the springs for the shutter mechanism; put a piece of
tape over them until you assemble the card again.
When you have removed the panel, you will (on the 32k card) see the
following:
A position for a 2016 Lithium battery (coin cell, not supplied with
the card) and close to this the write protect switch; positions for
4 pieces of 32k RAM chips, with one position occupied. One
decoder/controller chip (Seiko-Epson proprietary gate array), and
another smaller chip which is the voltage detector chip. Several
surface mount capacitors used for decouplings and some surface
mount chip resistors. Four gold plated test points.
The PCB is of good quality with gold plated contacts and bonding
pads. Also the plastic card frame and the shutter mechanism seems
to be of high quality, so in my opinion the card will stand up to
the usual high quality that we associate with HP products.
There is no plastic handle like the one found on the HP version, so
the card can be difficult to remove if inserted into the 48, but it
is easy to glue some kind of grip onto the card if you wish to.
Unfortunately the chips are fastened directly to the PCB, bonded to
bondingpads on the PCB and covered with drops of epoxy, so it is
not possible to use standard small-outline RAM chips to extend the
memory. The chips actually used are standard dies (you can see the
familiar bonding pad layout known from eg. 27C256 EPROMs with
quartz window).
You can't solder out the detector chip either :-(; but the test_
point located away from the three others happens to be the output
of the voltage detector.
If you cut the PCB trace (be careful...) from the smaller IC to the
testpoint, you can now via the testpoint access the clamp enable
input of the gate array!
This is very interesting, because you can now add a new voltage
detector IC and thereby change the detection level at your own
choice (provided you can get the correct detector IC).
The detector IC requires three signals: Supply voltage from the
card connector, ground, and a detector output, which is connected
to the clamp enable input of the gate array.
The three testpoints close together are ground (closest to the card
edge), some kind of detector checkpoint, and card supply voltage.
It is not possible to change the original detector level via the
detector checkpoint, but the detector hysteresis may be changed
(although this just makes things worse :-)).
I know of two types of voltage detectors that can be used:
Seiko Epson Corp. type SCI7701YHA (typ. 3.2 volt)
or Seiko Instruments Inc. type S-8053ALR-LJ-X (typ. 3.25 volt).
These are SOT89 surface mount packages and can be connected to the
required testpoints with thin, isolated wirewrap wire.
I do not know the price of the detectors, I just got a few samples.
I have placed the detector in one of the three empty positions for
RAM chips and with a fine solder tip I melted grooves in the
plastic frame to make room for the wires.
If you can get a detector IC, you will probably also be able to
find the pinconnections of the IC, so I will not go into these
details.
You may use a different voltage version of the detector (when using
an open drain version instead of a CMOS version a pull-up is
required), but the voltage should be between 3.2 and 3.9 volts for
best results. If the voltage is too low, you may end up in a
situation where the RAM chips may take power from the back-up
battery while they are active; this will exhaust the back-up
battery very fast and you may even risk to loose data memory when
your main batteries are down.
Under normal battery conditions this will not occur with a low
detector voltage as the supplied voltage from the 48 will drop
rapidly below the clamp voltage when the 48 is switched off; so if
you just switch off your 48 immediately when a low main battery is
detected, you can safely get away with using a detector voltage
down to about 2 volts. I have tried 2.6 volt with no problems.
After these changes carefully glue the panel in place again, glue
on a plastic handle, put on some nice labels on the card, install a
2016 lithium battery, set a battery alarm, and you have got
yourself a RAM card for your 48.
Sounds complicated? Well, then don't do it; but if you tried
something like the memory expansion of the HP28C as described by
Wlodek, I think you will find this very interesting also.
Some comments on the CMT card:
I have found that the CMT card does not clamp the signal lines, so
in this respect you won't get problems. However this could make the
pins more sensitive to static electricity, especially when there is
no shutter mechanism to cover the contacts. CMT also warns you to
keep the card in the anti-static bag when the card is not in the
48.
It has been claimed that CMT uses tranzorbers on the signal lines
for protection; sounds good, that will give you adequate protection,
but then why does CMT put so much emphasis on the protective bag
issue? I have not tested the card with voltages outside the
specified normal range, and I have not seen the inside of the card
(Arne would not let me open his CMT card :-) :-)) so I can not
really judge how protection is made.
On the other hand I have not yet seen commercially available
tranzorbers for surface mounting - but I may be wrong here; if
someone will give me a CMT card, I certainly will make a closer
examination :-).
The mechanical dimensions of the CMT and the HP cards are somewhat
different: the CMT card has a thickness of 2.0 mm at the contact
area and a thickness from 2.42 to 3.0 mm on the body, where the
measures of the HP card are 1.80 mm and 2.3 mm. Thus the required
insertion force for the CMT card is rather high.
It is very difficult to see if the CMT card corresponds correctly
with the connector keying or not, but when inserted in a separate
card connector you can see that the contact fingers are not fully
depressed, there is still a clearance of about 0.2 - 0.3 mm behind
them, so I assume that the contacts themselves will not be damaged
by inserting the CMT card, but I can't see if that is also the case
with the sockets in the 48 itself, as I have not taken the 48 apart
to see if the sockets are standard Epson sockets.
The contacts of the CMT card does not seem to be of the same quality
as on the original cards; the PCB traces have blurred edges, not
straight and sharp lines as you usually see on PCB's for professional
use, but if the traces on the board are intact, you may argue that
these are only cosmetic flaws.
After all, I do not see any really serious reasons why you should
not use a CMT card in your 48 calculator as long as you mainly keep
the card in your calculator instead of using it as an exchange
medium; but I don't like the high insertion force. If you are going
to make a card for the 48, why not make it the same mechanical
dimensions as the original cards?
I think the price difference is not enough to justify the electrical
and mechanical differences.
What is the next project here? Well, some form of programmer
for Epson EPROM or EEPROM cards, I guess. I have noticed that the
HP Equation Library card - and therefore also the Epson EPROM
cards, I hope - do not clamp the signal lines on a low supply
voltage. It will then be easy to design a programmer for these
cards and use them in the 48.
Some time ago HP released a very useful MSDOS program USRLIB that
enables you to build libraries for the 48.
However I do not have a PC, so I was very, very pleased to see
Frank Ochoa's TOOLS.48 that Friedrich Schroeder sent to c.s.h.
recently. Very nice. I can recommend the >LIB function. Now we can
build libraries in the 48 itself. Great. Then we just need a
convenient way of programming EPROM/EEPROM cards (I know that CMT
has a programmer, but it is priced at 280 $ ..., I also know that
EduCalc has a programming service, but I would like to do this at
home).
I'll try to work something out, so that I can make my own library
cards, so stay tuned.
Now, after going through all this, what would I really recommend
you to do concerning RAM cards? In my opinion it goes like this:
- You CAN modify standard Epson cards but it is hardly worth the
effort, so buy the HP cards and buy them where you can get them
at the best price (I can recommend EduCalc). This will give you
the best value for your money in the long run.
Disclaimer: Don't blame me if something goes wrong if you use
some of my ideas; these opinions are mine alone, but
you may have them if you want to :-).
I do not speak on behalf of anybody else and have no
connections to HP, Educalc or Epson.
Even my employer may have opinions different from
mine and can not be held responsible either.
Allan
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Allan Petersen HP25/HP19C/HP15C/HP28S/HP48SX/HP?? ap@dde.dk
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