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HP 50g RS-232 Serial Cable


The HP 50g is advertised as having an RS-232 serial port, but HP never provided the required cable to use this port. It is not possible to connect standard RS-232 devices directly to the calculator, because the port on the calculator has a physically non-standard connector and the voltage levels are completely different from those required by RS-232. This means that a special third-party cable is needed to use the serial port.

We have spent several months designing and testing a special serial cable for use with the 50g. Our solution has a custom two-layer circuit board with two ICs (including a microcontroller) and other devices inside the D-sub shell.

There have been postings in the comp.sys.hp48 newsgroup of plans for designing a serial cable, but the simple designs posted will not work properly, as they usually consist of little more than a MAX232. There are special design considerations needed to make a proper cable, because the 50g drives the RX line when not in use, and attempting to drive more than 3.3 volts into the calculator could damage the calculator. We managed to overcome all of these hurdles with our design.

This cable was designed with low power consumption in mind. The level shifters needed for RS-232 signal levels consume power, so they cannot be left on all the time -- a naïve approach would run the level shifters at full power, even when the calculator is turned off! When in use, this cable increases the power consumption of the calculator by about 10%. When not in use, it increases power consumption by less than 1%, and when off it consumes approximately zero power.

Our cable has a standard male 9-pin connector for connecting directly to serial devices with no gender changer needed. While it is possible to connect it to a computer by using a null modem adapter, we feel that most users would prefer to use a USB cable for a PC link, allowing us to not need an adapter when connecting to serial devices.

The cable has been tested with a wide range of serial devices and surveying equipment, from a cheap Garmin eTrex GPS to an expensive survey grade GPS. Speeds from 1200 bps to 115,200 bps all work flawlessly. You may also use two cables in conjunction with a null modem adapter (sold separately) to connect two 50g calculators, or you may use one serial cable to connect the 50g to a blue 49G or the old 48S/SX/G/G+/GX in conjunction with a serial cable designed for one of those calculators (not included).

The 50g does not normally offer 1200 bits per second as a choice. If you need to connect to equipment at 1200 bps, please read the following from Claudio Lapilli:

Write the following program; it requires library 256 to be attached:

<< 16. * 48000000. / INV 0. RND 1. - R->B ->H 11. 18. SUB #07500028h SWAP POKEARM >>

[replace the <<, >>, -> symbols with the proper ones]

I'll assume it was stored with the name 'SETSPEED'.

To use it, simply set all parameters to establish a serial communication at any speed, then go back to the calculator stack and execute:


(where SETSPEED obviously calls the program given above)

NOTE: Manufacturing these cables is a labor-intensive process, and while we attempt to keep sufficient numbers in stock, large orders may require a few extra days before shipment.

Physical Characteristics

Technical Specifications

Optional Accessories

Null Modem Adapter

More Photos

Part of the HP Calculator Archive,
Copyright 1997-2023 Eric Rechlin.