|Re: Output from HP48|
Message #9 Posted by Andrés C. Rodríguez on 29 Apr 2000, 11:58 a.m.,
in response to message #1 by Tal
If you would like to send commands to a simple (CMOS logic, for instance) device, there are some options apart from the serial port.
1) Piezo buzzer. The tone commands of the -48 series allow the user to define frequency and duration, so you could send a rather large variety of tones. Such tones could be seen as pulse trains, you define the frequency and the number of pulses come from the duration multiplied by the frequency
In example, a 400 Hz tone, 0.5 seconds will output 200 pulses.
Such pulses could be coupled to external devices via a small condenser microphone (I do not like it because the many distortion or noise factors going around). It would be better to connect the piezo buzzzer leads to the outside. I have done this successfully from an HP41 around 1980 (see my article "Calculator Memories from Argentina" in this Museum). Alas, the -41 was "openable", and the -48 is not (or is it?)
Steve suggestion on using an optocoupler is a very good and recommnended idea on almost every case.
2) Infrared Port: You may just output some info via the IR port (ie: printing commands), pick up the IR signal with a phototransistor near the IR LED, and go from there on. It would be difficult to decode the information sent, but it is very simple to detect "some" IR activity on this port.
As long as you can do with a simple code (ie: one signal, ON; two signals, OFF), you may get what you need with some simple CMOS logic, and one or two simple timers. And you will not have to open the calculator or to risk electrical connections that may affect it.
You may think of it as a "rather sohisticated optocoupler".