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Full Version: HP38G Hardware - HC74/ HC112 , Connector
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Could someone recommend a site or document that describes the HP38G I/O connector pinouts?

Also, what is the function of the HC74 and HC112 components on the main board?

Thanks in advance,


This webpage should fit your needs:

Thank you, but... . ....

This page does not describe all 10 pins of the HP38G calculator,


describe the HC devices within the HP38G.


The HC74 and HC112 devices function as dual flip-flops. The hardware is very similar to the HP 48G/GX.


Thanks for your response. I understand they are flip flops, but what I would like to know is what is their purpose is in the HP38G? These devices are not present in a HP48G or HP48GX.

I'm hoping there is a schematic available of the HP38G, but that does not appear to be the case.

Thanks again,

I thought there were some flip-flops in the 48GX, though maybe not in the 48G. If there aren't dual flip-flops in the 48GX, was there a hex or octal flip-flop, something like a 74HC174, 74HC273, or 74HC374, perhaps? My recollection was that some flip-flops were needed to handle the size of the internal ROM as well as the addressing for memory cards. Since the 38G doesn't support memory cards, it wouldn't need as many flip-flops.

I don't have the actual I/O pinouts [1] but I can tell that they serve two purposes:

The first is as an RS232 serial port to allow you to connect the 38G to a PC running the HP supplied communications software. This allows aplets to be transferred from and backed-up to the PC.

The second is to drive an overhead projector display. (Part no F1212-90002). The intention here is to allow a teacher to show a class of students what they should be seeing on their own calcs.

[1] Actually I don't think they were ever officially documented.

Sorry, I don't know why I thought you had wanted to make a cable. Well, I would be interested too if someone could tell what the 10 pins of the "serial" connector are. Four of them are used for RS232 and the six others are used by the overhead projector.

The FAQ just tells the name of the six mysterious pins, but there is no detailed description.
http://www.hpcalc.org/hp49/docs/faq/#ss1.11. It would be fun to use these pins as an output port for transmitting data at very high speed. I guess some reverse engineering is required...