HP-71B Memory Modules shouldn't rattle, should they?

[Mike]

(Just kidding... I know they are not supposed to rattle)

I heard this strange rattle from one of my many HP-71B. I traced the sound to my card reader memory module. It was a Hand Held Products 96K RAM. I opened the module and found this little critter floating around in the package.

But what is it, where does it go and what does it do? I should know this but alas, I don't. The odd thing is the only place it looks like it was connected was on the ferite-like material. There doesn't appear to be any connection points for the coil wire. Strange (at least to me). You can see what appears to be one end of the coil but it doesn't appear to have been connected anywhere, as it's stuck to the resin and not near a solder point. Don't have a clue where the other end is.

I have located the place where it was mounted. You can still see the footprint impressions made by this (what I'm calling) ferite-like material.

Anyone know the purpose of this device? There are 3 boards and only the center board has the Transistor, surface mount component and this device. I have tested the RAM by loading program to it and it seems to work fine. I wonder, if this might have something to do with making a 64K partition. I have heard that these have such a partition. But when I freeport these, I can only make 32K IRAM. Might this be because this part is missing? Should the wire coil be connected somewhere? I ask this because the dark gray parts were connected to bridge the gap (shown in photo).

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This part is also on one of my RAM/EPROM Modules but it's on the EPROM board and none on the RAM board, making me think, once again, that this might have something to do with the 64K chunk I was talking about.

Here it is on the RAM/EPROM board. This is just an example of another board that has this component. My board that has the fallen off component is RAM only

Anyone have any suggestions or ideas about this? I'll probably try to reattach it this weekend, if I can determine what it's for.

Edited: 26 May 2006, 1:31 p.m. after one or more responses were posted

[David Smith]

The device is an inductor (aka choke). It is most likely used as a noise filter in series with a power line. You can probably just replace it with a jumper or very small value resistor (measure one of the other ones).

The only other likely reason fo such a component would be in a DC-DC converter for generating an EPROM programming voltage or substrate bias generator, but it does not look like there are not enough other components on the board for that. You could probe one of your good boards with a scope and see if you can see a switching signal on one end of it. If so, you would have to replace it with a proper inductor... finding a suitiable value would be fun...

[J-F Garnier]

Hi Mike,

Regarding your question: the coil and the transistor are used to switch off the supply of the EPROM. I think this was designed in this way to allow use of NMOS EPROM (32k CMOS EPROM were not so commun on 1984, when the HHP module was designed). I always used CMOS EPROM (27C256) and experimented problems at the time due to the supply switching with some Japanese EPROM, so I bypassed the transistor on my two modules (see pictures). I guess that the coil is an attempt to limit the inrush current when the transistor is switched on.

Following the last thread on this subject, I spent a few hours of reverse-engineering on my HHP modules. I confirm the use of the jumper: it selects either VCC or A14 on pin 27, to allow using either a 27128 (16k) or 27256 (32k) EPROM. Again the reason may be that HHP though that 27256 EPROM would be difficult to find, or too expensive for small applications.

I investigated to see if it could be possible to hack the module and use 64k EPROM (27C512). Unfortunately, the EPROM pin 1 (which is A15 on the 27C512) is hardwired to Vcc. Maybe the HP chip can control a 64K ROM, but the HHP PCB doesn't provide any support for such a configuration (sigh..).

J-F

Edited: 26 May 2006, 1:23 p.m. after one or more responses were posted

[Mike]

Quote:

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Regarding your question: the coil and the transistor are used to switch off the supply of the EPROM.

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Except that in my case, there isn't even an EPROM in the module that had the broken off component. So, I don't think it has anything to do with an EPROM, at least on my RAM module. I think it might be some sort of "selection" as you suggest, but in a general way.

My gut feel is that this has something to do with making a connection that enables a 64K block, instead of them all being 32K blocks. Just as you suggest it is used for "selection" of EPROM sizes, in your module. I do know that these modules can be configured as one 64K block and one 32K block. With this part off, I do get three seperate 32K blocks. I'm going to put a jumper on tonight and see what difference it makes. I'll also try to buz out this part of the schematic and post it.

I have also been sent some other photos of others that have the 96K RAM and theirs don't even have the component or transistor on them. Wierd! It's probably a multi-function (read configuration) board that can be wired many ways.

Here is a photo that someone sent me of their 96K RAM. You will notice a jumper on the center board but no transistor. I suspect I can just put a wire between the two points this component came from. I suspect that if my hunch is right, I will see a 64K IRAM, instead of all 32K. We'll see.

Edited: 26 May 2006, 1:32 p.m.