HP-41 Input/Output-Board

Interfacing extern measurement and control hardware to HP-41 handheld computer using the HP 82166A IL-Converter.

by Christoph Klug

Most people use the HP 82166 A IL-Converter to adapt a standard parallel printer to HP Interface Loop. Alternatively you can use the powerful IL-Converter to connect electronic hardware to the Interface Loop, controlled by the HP-41 handheld computer. I developed an Input/Output Board for the HP 82166 A IL-Converter, designed as a mainboard with four 16 bit extension slots.

HP-41 Interface Loop Controller

Best HP-41 IL-Controller is the powerful CX model with intern X-Functions, X-Memory and Time-Module. Additionally you need the HP 82160 IL-Module plus EXT I/O-Module to control the Interface Loop. Instead of EXT I/O-Module you can use the Development-Module. For maximum performance you can complete the handheld with the German CCD-Module. Hard- and software of I/O-Board are developed to work optimal in combination with other Loop-Devices like Cassette Drive, Thermal Printer or Video Interface. For that reason you can configurate extensive IL-Systems.

Software for EXT I/O-Module and Development-Module

For controlling I/O-Board with HP-41 you must alter some internal status register of the HP 82166 A IL-Converter. Therefore you need the EXT I/O-Module or alternatively the Development-Module. For EXT I/O-Module 695 Bytes (= 100 Registers) of software programmed for the HP-41 to control I/O-Board, or 811 Bytes (= 116 Registers) of software for Development-Module.

I/O-Board Software for EXT I/O-Module

Controlling the I/O-Board with HP-41 CX and EXT I/O-Module is easier and faster than with Development-Module ! The I/O-Module uses the ALPHA-Register to transfer data to IL-Converter. Avoiding problems with loss of leading null bytes in ALPHA-Register, EXT I/O-Module generally works with leading dummy byte D. For controlling the I/O-Board with HP-41 CX and EXT I/O-Module, 24 commands are provided for power up/down, initialisation, addressing, manual service request, interrupt, clear, 16-bit I/O-transfer by ALPHA-Register and by X-Register and 8 bit I/O-transfer by X-Register. For 8 bit I/O-transfer the software provided needs the X-Memory Module for storage.

I/O-Board Software for Development-Module

Controlling I/O-Board with HP-41 CX and Development-Module is more complex and slower than with I/O-Module ! The Development-Module generates a Buffer to transfer data to IL-Converter. An advantageous feature of the Development-Module is the INTR-routine, which starts HP-41 automatically from an active MSRQ-signal ! For controlling the I/O-Board with HP-41 CX and Development-Module, 22 commands are provided for power up/down, addressing + initialisation, manual service request, interrupt, clear, 16-bit I/O-transfer by X-Register and by ALPHA-Register, 8 bit I/O-transfer by X-Register.

I/O-Board and Time-Module

You can use Time Module commands for automatic control of I/O-Board. For example power up the system using an alarm function and start some activities like measurement, storing data in main memory or cassette drive, then set new time alarm and put the I/O-Board to sleep waiting for next cycle.

I/O-Board and CCD-Module

For controlling I/O-Board you do not require the German CCD-Module, but this helpful module expands HP-41 with 100 powerful commands programmed in machine-language. Some are fantastic for use with I/O-Board and make work easy : Synthetic programming, binary-functions for logic bit operations, matrix- functions for data collection applications.

I/O-Board and Zeprom-Module

The Zeprom-Module is a EPROM housed in a HP-41 plug in modul case, produced by Zengrange (London) who also produced the well known HP-41 Zenrom-Module. Permanent storing complete I/O-Board control command set inside Zeprom is done by adapting module to small plug in burning converter tool and using corresponding Programer-Rom software. Using Zeprom-Module, HP-41 main memory is complete free for other program applications and I/O-Board software is protected against memory lost. 51 commands controlling I/O-Board plus 12 utility functions now belonging to CAT 2 and assigned to three User keyboards, which selected by softkeys. Zeprom-Module sum up extension modules and I/O-Board software in only one HP-41 plug in port.

Extern Interrupt

Inside HP-41 case you can mount a small hardware extension circuit for switching handheld computer from sleep-modus to run-modus by extern interrupt signal. As response to extern event HP-41 starts automatic control of I/O-Board. Because interrupt circuit works with an opto-coupler element there is no risk for damanging HP-41 ! During a running program extern interrupt signal can not switch off handheld computer.

HP-41 and PC

Normally I/O-Board data processing is done completely by HP-41. But it is possible to transfer HP-41 data-files to a PC. Use a HP-IL * PC Interface Card and Link Plus or Trans41 software. Link Plus only runs on 286/16 MHz PC´s and on slow 386SX/25 MHz PC´s and emulates two Cassette Drives, a Video Display, a Printer and a DOS file. Trans41 runs on modern PC´s and emulates a Printer, a Cassette Drive and a DOS file. Collect data by HP-41 and I/O-Board in the field. Now transfer HP-41 data-files to PC and DOS area. Than import data to EXCEL or other powerful software package for post processing.

I/O-Board Hardware Feature List

The board uses a basic application for handshake timing from Gary Friedman, described in his book "Control The World With HP-IL". Some more helpful circuits added to the board to improve I/O performance. You find hardware solutions for port addressing, Power On reset, MSRQ (Manual Service Request), Clear function, Power Up/Down function, MSRQ Lock when Power Down and a Reference Voltage. Power supply is external : A special 6V DC-supply unit with optional solar powered rechargeable cell for modular applications. Output voltages are +5V and + 15V for digital- and analog circuits. Stand by current of DC-supply (Power Down) is lower than 10 mA !

I/O-Board Port Configuration

The I/O-Board supports four 16 Bit slots. Every slot is divided into two 8 Bit ports A and B. All slots are connected by a common bus with power supply, handshake lines and data lines. The logic of I/O-Board hardware maximally addresses two 16 Bit input modules and two 16 Bit output modules plugged in at same time ! Address setting is done by jumpers on the modules, not by slot position. Inserted 8 Bit modules need additional software addressing. The slot architecture of the board, hardware and software addressing allows some port configurations for digital and analog input/output.

I/O-Board Plug In Modules

I/O-Board uses 8 bit or 16 bit add-on modules. On next page is the list of the basic modules for completing motherboard hardware. Furthermore some special modules exist like analog filter modules an RMS measurement module or a computer controlled sine wave generator module. The modular hardware concept of the I/O-Board makes it possible to configurate the interface system with I/O ports to every task you want. You can also develop special modules for your own applications and hardware projects. The I/O-Board is a good basic IL-Device to implement your ideas quickly !

Quality Tests

Correct and bug free function of software and interface hardware tested with some HP-41 Controllers, some IL-Converters, and eight samples of the I/O-Board which I have built ! Measurement tools and software programs for testing mother board hardware and plug in module circuits exist.

I/O-Board Manual

The English language manual for I/O-Board (16 chapters and over 300 pages) includes circuit descriptions, circuit diagrams and software documentation for HP-41 CX including barcode plots. The manual is a rich source for users, who want to use the IL-Converter in their own electronic equipment, and for those, who want to learn step by step something about interface technology (read and write bytes to external hardware devices, voltage conversation to the analog area). Test programs included for checking hardware functions. Some practical tips for building, installing and starting with I/O-Board complete the manual, including advanced programming tips and reference lists.

Furthermore manual includes a program for printing HP-41 barcodes on HP DeskJet 500 printer. This needs an HP-41 with HP-IL module, Extended Functions (on a plug-in module or built into an HP-41CX), a CCD-Module plus an HP 82166A IL-Converter to connect the DeskJet 500, or an HP-IL PC interface card to print the barcodes to a printer connected to a PC.

Also manual contains information about HP-41 hardware modifications like building Double X-Memory Modules, interrupt circuit for waking up handheld by extern signal and a extern battery power supply unit. Own chapters describe burning Zeprom-Modules and realizing data transfer or upload from handheld to PC by using the HP-IL*PC Interface Card with Link Plus or Trans41 PC software and improved HP-41 software for transfer main memory register-blocks, X-Memory data- or ASCII files or CCD-Module matrix files to DOS files.

For giving the reader an practical application and imagination about flexibility and amazing performance of HP-41 & HP-IL & I/O-Board one chapter contains the complete hardware and software solution for realizing audio frequency response measurement, including advanced data processing by working with CCD-module matrix files, data storage by X-Memory and Cassette Drive, data output by IL-Printer and IL-Plotter and data transfer to PC !

The manual is completed by a HP-41 & HP-IL reference list, a photo-story about history of developing I/O-Board system and some more photos and details about IL-Devices for measurement, printing&plotting and data-storage. The manual is written for use with I/O-Board, but the manual can be read on it´s own for the details and programs it gives. Describing the advanced development, manual continues Gary Friedman´s book *Control The World With HP-IL*.

Source of Equipment

The HP 82166 A IL-Converter is the key part for I/O-Board. Some kits for building this unit including IL-Terminal, IL-Transformer and IL-Chip exist. Printed circuit boards exist for the I/O-Board but presently with only single-sided layout. You must make a lot of additional connections in wire wrapping or soldering technique. That is easy for electronic professionals. For people who have no complex measurement equipment I can align some critical components. Electronic novices can get a completely finished and tested version of I/O-Board including DC Power Supply. Building plug in modules for I/O-Board is not complex and can done by newcomers to electronic construction.


Today the HP-41 is a nostalgic handheld computer. With added accessories like plug in expansion modules you can bypass hardware limitations of the system. Using the IL-Converter and I/O-Board you open up the interesting field of controlling and developing external hardware. Now you can realize measurement and control from your HP-41, a task normally done with larger systems. Interfacing the handheld to modern PC you can also use modern software tools to analyse the data.

For more information about HP-41, HP 82166A IL-Converter, Input/Output Board and belonging I/O-Board manual write to :

Christoph Klug Körnerstraße 47 B 31141 Hildesheim Germany

I/O-Board Manual Contents

CHAPTER I Introduction
CHAPTER II HP-41 Controller
CHAPTER III I/O-Board Hardware
CHAPTER IV DC Power Supply
CHAPTER V I/O-Board Modules
CHAPTER VI Practical tips
CHAPTER VII Advanced Programming
CHAPTER IIX References
CHAPTER X HP-41 Barcodes
CHAPTER XIV I/O-Board Data Transfer
CHAPTER XV HP-41*PC Data Transfer
CHAPTER XVI Audio Measurement

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Page last modified : 10th March 2000