interfacing hp41 with hp71b by hp-il


I read on this forum that it is possible! For instence, somebody said that the OUTA function of the hp 41 hp-il module works to put a string on the hp 71! How do you do that? Since I don't have any doc, I wondered if somebody could help me. Thanks in advance.


This was a message, I emailed to some other person earlier (last millenium) -----------------------


It took me five minutes, so I did it now:

HP-71 and 41 are the only devices in the loop


HP41 to 71 HP71 procedure:


connect both calculators


(note the SPACE between ENTRT and ":", it's important)

hp71 waits ...



the HP 71 receives and displays the string


HP71 to 41 HP71 procedure:


connect both calculators

OUTPUT :LOOP;"ASDFGHJKL" ! hp71 stores the string in a buffer and returns to the blinking cursor.

HP41 procedure: 1 XEQ [ALPHA]INA[ALPHA]

alpha register contains ASDFGHJKL


This might also apply to numeric values, if you change the variables to numeric variables.

Execute RESET HPIL or CONTROL ON (without an 41 in the loop) to re-gain LOOP control for your HP-71. RESTORE IO afterwards could help in any case.

More tips or explanations needed? Just ask!


Thanks a lot Reinhardt! It works beautifully! So I'd like to get more tips from you.... Thanks in advance, Thib


Could you be a little more specific ? What tips exactly. I don't even realize, what of my knowledge could be useful and what is simply useless.

The HP-41 interfacing was rather fast to accomplish, I really don't have a HP-41 knowledge.

I'm a die-hard 71B user from the first hour, I never had an 41 in the golden calculator days. The 71B was my first HP and I purchased it in the age of 15. Rather much money for a pupil.

Anyway, just ask and I'll try my best to answer. I'm rather busy sometimes (help, help :-) ), but I'll try.


I read your solution with interest. I have a 71B and a 41CV with IL. I attempted to run your solution but was unsuccesful. When I did the RESTORE IO @ CONTROL OFF on the 71, I got an IL Error: Loop Broken. Any ideas? I tried with and without the wires connected and with and without the 41 connected and on or off. I connected the 71 to an IL printer to make sure it was working and it printed.

What do you think? Thanks for the help.


Guess my solution was wrong somehow.



you could try a RESET HPIL without HP-41 and with a cable connected between IN and OUT of the 71 to test your module and cable.

You might check all your modules/cable too. Try to print something or read from a tape drive.

Now some theory: The IL loop consists of a controller and a lot of Talker/Listener or inactive devices. There's only one controller allowed on the loop.

Your 41 is a loop controller. I don't see any means to change that. Maybe the IL development module provides such a possibility, but I don't own one and I don't know anything about it's capabilities.

Your HP-71 after a INIT:1 - INIT:3 (memory lost) or after a RESET HPIL is per default a loop controller. RESTORE IO tries to detect any device on the loop (It's unimportant, if the 71 is controller or not, RESTORE IO works anyway). I'd execute a RESET HPIL @ RESTORE IO when I have problems with a peripheral (don't do RESET HPIL, if there's another controller on the loop).

RESET HPIL doesn't send any commands via the loop. It resets the settings of the IL module to their default state, making the 71 a controller.

When a device doesn't return a byte, sent by a controller (token, for you IBMers), there's a loop broken (there's surely a similar error message for the 41).

A controller generally never forwards a message to the next device. So, when controller 1 sends a byte (InterFaceClear for example, that's what RESTORE IO does, when the 71 is controller), it's never forwarded by a controller 2 and controller 1 will not get its message back. Thats a sign for a broken loop for controller 1. Additionally the loop might be in some half-initialized state, which can be dangerous.

Because the 41 can't give up it's controller role, the 71 must execute a CONTROL OFF.

RESTORE IO is for security reasons, just to keep everything o.k. I've made the experience, that a RESTORE IO too much never does harm. (You may experiment and omit it too).

ENTER and OUTPUT are simple commands to send and receive a string (or number) over the loop. The 71 buffers up to 65 bytes, so you can OUTPUT something, before the controlling 41 wants it (it's sent, after the 71 gets the right message from the controller). As fas as I remember, the 71 waits for this buffer to empty, if you send more than 65 bytes without getting them out on the loop.

Hope, this is sufficient for now. Asking further questions will bring further answers.


Thanks. I'll give it a try tonight. I just recently acquired the 71 with the IL and am trying it out. I don't have the IL manual for the 71, so I'm not familiar with those commands. Thanks for the lesson. I just recently received the HP Museum CD's and noticed that the Dual IL module doc is on the CD, so that should help me. I have the Ext IO and DevIL ROM's for the 41. I believe both of these will allow the 41 to give up control. I'm still experimenting with those.

By the way, I believe you contributed the HP41 Service Manual to the HP Museum CD's. Thanks for that! That is a very interesting document. I just recently acquired the Diagnostics ROM for the 41 so now I can make sense of how it works. Any other Service Manuals laying around for either the 41 or 71?


Your correction worked beautifully. Thanks again!


I have done it right but forgot to correct!! Thanks once again, thib

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