Reproduction ACI Artwork

Apple Cassette Interface Notes

click here to go to  Mimeo 1 kit information page

The ACI Parser Bug

I don't know if this is really a bug or a compromise in the design needed to get it into 256 bytes of code space.  This code is really tight and there is really no free space to do more.

Always enter all four characters of 16 bit hex address when entering addresses into the ACI parser.  The parser doesn't clear out preexisting addresses, so unless you enter all four address characters, you will use whatever was already present in memory mixed in with the new address.

EXAMPLE:  to load Hamarabi enter the following

004A.00FFR 0400.0FFFR

Cassette Interface LED Operation

The LED on the Cassette Inteface is set to light at about 1.2 volts.  This voltage is way too high, so don't expect the LED to operate correctly on your ACI.

Cassette Interface Reliability Improvement

I have discovered that the Apple II cassette interface is far more reliable than the Apple 1 was. I did spice simulations of both and quite a bit of fooling around with the actual ACI in order to see if I could figure out the problem. The best improvement I could come up with, was to add a low pass filter on the input. This was done by adding a .01uF cap between the input side of the comparator and ground.

After experimenting extensively with the ACI, I finally discovered the real difference in the design that makes the Apple II so much more reliable. The Apple II uses a .1uF capacitor for input coupling versus a .01uF on the ACI. When I originally looked at this, I must have forgotten to make this change when evaluating the designs with spice. This change provides a lot more noise immunity in the comparator inputs.

Here is the ACI input signal with the stock .01uF input coupling capacitor. The source of the signal is the 1HZ tone (all ones) at the front of Wendell Sanders Hammerabi clip being played from an iPod with the equalizer set to “treble reducer”. All signals are displayed at .5 volts/division on this page.

both sides of  .01 cap

In the image, above, the input is the bottom signal and top signal is how it is presented to the comparator input after passing through the capacitor. Note how the signal spikes and then returns to the value set by the 10K resistors.

both sides of .1uF cap

Above is the same input and output signals with a .1uF capacitor. The signal follows the shape of the input waveform much more closely. The bottom trace is input signal and is the same as in the first image. The top trace is input to comparator after passing through cap. Note how the extra capacitance prevents the resistors from quickly returning to the base value set by the resistors.

comparitor inputs with .01uF cap

This image shows both inputs to the comparator with the stock .01uF capacitor. Spiky looking trace is input signal to comparator (same signal as seen in first image). Other trace is reference signal with hysterisis. The reference signal is pulled up and down slightly as the comparator switches state to prevent instability. This results in the square wave.

In the picture above, the output switches state when input signal (more spiky looking signal) passes the level of the reference signal as can be plainly seen in this image. The problem with this design is the limited amount of room between the input and the reference, any input noise with an amplitude of around .2 volts or more may be enough to trigger a switch in output levels.

Below are the same two inputs to the comparator with the .1uF cap. This wave form increases noise immunity to almost a volt, about 5 times improvement over the apple 1 stock implementation.

comparitor inputs with .1uF cap

Unless you are interested in maintaining complete commonality with the original Apple 1, I highly recommend using a .1uF capacitor in your replica cassette interface instead of the .01uF used in the original design. Keep in mind that the folks at Apple recognized this improvement before coming out with the Apple II, as that design includes .1uF cap for this application.

Since I was using a clone ACI, built with components based on the schematics, I wondered if Apple actually shipped with .1uF caps on the ACI. I exchanged a couple of emails with Wendell Sander. He confirmed that the schematics are correct and the Apple ACI did indeed ship with a .01uF caps. He also confirmed that he had independently come to the same conclusion regarding the reliability improvement that could be attained with a .1uF cap.

Apple recommended tape recorder

Back in the old days, the Apple recommended tape recorder was a Panasonic RQ2102.  Believe it or not, they are still available from Panasonic.  This recorder works much better than the period Radio Shack recorder that I was attempting to use.  I highly recommend you find a RQ2102  to go with your Apple 1.  

Schematics Errors

ACI schematics
Like usual there is an error in the schematics - this time only 1 error.  Pin 5 of the voltage comparator is identified as being connected to +5V.  Instead pin 8 is connected to +5V and pin 5 is left open.


Even with the RQ2102  recorder and the .1uF capacitor change don't expect perfect operation, you'll still have to play with the volume control and may have trouble loading programs if everything isn't working just right.

Reproduction ACI Boards

I've layed out a reproduction ACI board.  The image at top of this page is artwork from this layout.  My blog details some of the steps I taken in this process.

The price is now $100, with the exception of those who buy a Mimeo 1 kit at the same time. Those who buy a Mimeo kit at the same time they buy an ACI card, will get the special $75 price.  This is the original price of an ACI card.

Send me Email  at for ordering information.

A  downloadable copy of the ACI build and operations manual is now available for downloading.  This manual contains a reproduction of the original Apple ACI manual and an annotated listing of the firmware as appendixes.

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