Cassette read and write indicator

KIM Kenner 10 March 1980, Dutch article by Hans Otten, English translation Hans Otten, 2021

Having an indication of how the reading and writing to the cassette recorder is going is nice. With the Micro Ade routines the LED display is used for this. the standard KIM-1 routines are mute.
This small hardware circuit gives either a visual or audible indication with the standard KIM-1 routines.

How does it work?
An indication for for the read process is the output signal of the PLL circuit, a stream of digital pulsed delivered at PB7 of the RRIOT 6530-002. During the reading pin PB7 is rpogrammed as input, during the wirte it is an output. Via a buffer the output signal signal is brought to Audio-Out and Audio-Out-High.
The use of PB7 as input and output during the read and write process means that during a read the decoded signal appears on Audio Out, and as it is meant to be, during writing also has the output signal. Collision is prevented by blocking the input signal during writing. So this means that during reading and during write the signal is present on Audio-Out. This enables a indication of both read and write processes.

The circuit

The signal on Audio-Out (application connector A-M) is brought to a transistor (general purpose type) and the alternating signal makes the LED blink quite fast. A miniature loudspeaker can be switched in and the signal can be heard, it is in the hearable audio range. The LED is not very informative, the loudspeaker is, but also can be annoying, so the switch is welcome.

Do not use a lower value than 33 ohm for R2, it will damage the transistor. The circuit does not represent a heavy load, so you can use the KIM-1 power supply.

A piece of stripboard is sufficient. See the figure below for the connections of transistor and LED. Red, green, yellow can all be used.
Other usage
By programming the output PB7 music or signals can be heard.

Reading tapes from other KIM-1

KIM kenner 6 1978 Hans Otten Dutch, translation to English Hans Otten 2021

Reading tapes recorded on another KIM-1 or bought like Tiny basic, Microsoft KB-9, Miccrochess and Micro Ade was not that easy. Many many errors, especially the hypertape format.

Reading and writing of my own tapes, after having configured the PLL following the guide in the User manual, and keeping the cassette recorder clean, now is flawless. With the motor control (see Micro Ade manual appendix) it is a pleasure!

I read and tried all advices, like adjusting the azimuth of the recorder head, cleaning etc. Maybe the variations in tape speed between recorder and my player and the resulting frequency shift was a problem?
What really helped was looking with an oscilloscope at the output of the PLL (pin 7 of the LM311) we can see clearly the problem:

No real problems with noise or dropouts, just deviations between different recordings, so the PLL does not operate correctly. Bought tapes showed the pictures Wrong 1 and Wrong 2. And that leads to reading problems, the software makes the wrong decisions on ‘0’s or ‘1’s.

The following procedure was developed to read the bought tape, store it and save the program on my own tapes:

  1. Start reading the problem tape, 1873G
  2. Connect a simple voltmeter to pin 7 of the LM311 or Expansion connector-X
  3. Now change the PLL variable resistor to a reading of 2.7V. A scope would show ‘Correct’
  4. Start the read again
  5. Save the program
  6. Repeat with your own recording or the user Manual PLL set procedure

This helped me to read the problem tapes. Only for Microsoft Basic I had to play with the output volume, large variations in the recorded signal. But it worked in the end and now I have my own recordings on tape, several stored and working copies. One can never have enough backups!

PLL Test

This program write alternate two low frequency or three high frequency tones to the tape recorder.
Record this for a couple of minutes.
When reading this tape the PLL should convert this to a symmetric block pulse.

This program shows on the first two digits on the KIM-1 LED display the number of ‘1’s read, on the second two digits the number of ‘0’s read.
When all is well this should 80/80 but anything between 60/A0 and A0/60 is fine.

ASCII parallel keyboard to KIM-1

Kenner 2 page 4 July 1977 Co Filmer, translated to english Hans Otten, 2021

A parallel ASCII keyboard is a nice addition to the KIM-1.

Connect to the PIA A of the KIM-1 on the Application connector.

Some keyboards may require a -12 V power supply.

The address 1700 now shows the ASCII keyboard code of (the last) key pressed. The high bit (PB7) is the strobe signal, the other 7 bits the ASCII code.

A program should poll the strobe bit, if activated wait for the strobe bit to be deactivated, and then read the key and return.
Testing for keypressed such as a break test means checking the strobe bit, if activated stop processing, etc..

Code fragments by Hans Otten

   ; example of break test
 BPL GOON ;     WHILE NOT keypressed
 JMP MONRET ;      
GOON continue processing

Cassette reading problems KIM-1

Another article from the Dutch KIM Kenner magazine:

RECORDING PROGRAMS WITH THE KIM-1 AND THE CASSETTE RECORDEROriginal Uwe Schroeder, KIM Kenner 1, March 12 1977 Translation June 2021 Hans Otten

Recording programs with the KIM-1 and the cassette recorder

Original Uwe Schroeder, KIM Kenner 1, March 12 1977  Translation June 2021 Hans Otten


A large number of KIM-1 users seem, like me, to have problems recording computer programs on the cassette recorder. For unknown reasons the KIM-1 refuses to read a program, while before it went well with the same tape. These problems have led me to study the KIM-1 system and I hope to have finally discovered the cause of the problem.
This article serves to aid other KIM-1 users to solve also these problems.

Part of the PLL circuit, including input impedances

Analysis of the KIM-1 FSK system
Signals are stored on the cassette tape with FSK (frequency Shift Keying). By consecutive high and low frequency sounds (on the KIM-1 3.6 kHz and 2.4 kHz). These high and low frequency sounds are generating not with much hardware, but with software. Reading programs is done by analyzing these sounds with the LM565 IC (a Phase Locked Loop, see User manual page 31 and fig 3.8). The fact if the sound was high or low frequency is determined after some amplification and filtering via a LM311 comparator to ‘0’ and ‘1’ and offered to I/O port PB7 of the second 6530 RRIOT).

The problems arising at the reading are probably caused by not correct functioning of the circuit around the PLL. The average cassette recorder appears to supply sometimes during a very short period a dropout to let the PLL function correctly.

Where and how things can go wrong with the PLL

  • When we record on the cassette recorder a constant tone of 3.6 kHz and listen to the recording and examine it with an oscilloscope, we see and hear the sound volume fluctuate or even disappear for short periods, we call this dropouts of the tape. These dropouts will mean a fluctuation of the sound available for the PLL to detect 10 to 100 times lower volume and cause the detection to fail. By measuring the PLL level I have seen 10 to 100% more signal than required, so that ca mean PLL malfunction.
  • If a tape is passing the head misaligned/tilted of the tape recorder head, higher frequencies are in the disadvantage and weaker. This head misalignment will cause problems with recordings from other tape recorders, bought or from other users.
  • Suppose we use a perfect +5V power supply, then VCC can be considered ‘Ground’ When we send on Audio In a AC current of 550 mV, then resistors R8 andR14 reduce the signal 1/11 of 550 Mv = 50 mV supplied to the PLL. Measurements indicate the PLL requires at least 40mV to sync the PLL and see it as a ‘high’ frequency sound.
    Remark: replace the R8 with 1K to give the PLL 250 mV instead of 50mV.
  • Suppose we use a perfect recorder with a very low output impedance. And suppose the power supply has a noise level of 600 mV. Fig 2 shows, after some calculations the noise level results in 40 mV on the PLL input. If we reduce the resistor to 1K , the noise level becomes 230 mV.

The specifications of the PLL indicate the correct functioning of the PLL at an input level of nominal 2, maximal 20 mV. The fact that the measurements indicate the PLL only operates at levels of 40 mV indicate the noise levels are at the same level as the input signal coning from the recorder. A better noise reduced may help, but other sources of interference are possible. Therefore extra amplification of the signal is preferred instead of altering the KIM-1 hardware.

Solve the tape recorder problems
Since the problems with reading of tapes on the KIM-1 seems to be related to fluctuating signal levels:

  • Record the signal as loudly as possible, use a recorder without automatic level adjustment and record so that the tape is saturated.
  • When using the loudspeaker output, experiment with the volume. But too loud means distortion and may also lead to failures.
  • Build an amplifier for frequencies 2-4 kHz 10 to 20 times, short dropouts should be well amplified.
  • Use C60 instead of C120 types. Normal cheap ferro is fine, Chrome has more dropouts.
  • If the KIM-1 reports a reading error (FFFF in the display) and you want to know how much of the tape has been read, location 17ED and 17EE contain the first address not read yet.
    Make a copy of finished programs on another cassette and check this copy for readability. Do not use this copy anymore and store it. When using a cassette often, this may lead to problems, like the mangling of the tape in the drive.

Here is a procedure to work around reading tape problems:

  1. Check if the recorder is connected to the KIM-1
  2. Check Volume and Tone control ((max high)
  3. Press Reset.
  4. Set location 17F9 to 0.
  5. Set location 00F1 to 0.
  6. Inspect location 1742. Here the information of I/O pin is shown. The display shows 1742 87
  7. Start the recorder. The middle bar of the 8 now will blink, if not : you have Error 6A (see below).
    Stop the recorder, remove the cassette and start the recorder. Now the middle bar of the 8 should not blink, else you have Error 6B (see below)
  8. If the Volume knob of your recorder controls the strength of the output signal: start the recorder and determine in which setting the blinking of the bar changes. If you have not enough headroom, see Error 7.
  9. Check of the correct detection of the high frequency.
    Type in the next program and start it (the program writes a constant tone of 3.6kHz to the recorder)
    Record this tone on the recorder for several minutes.
    Rewind the recorder and start playing. Now the display should show no middle bar 1742 07
    The bar should not blink at all, every blink indicates a dropout or such. See Error 7 and Error 8.
  10. Read User Manual C and E

Oscilloscope test
If you have an oscilloscope, do the following measurements.

  1. Attach the scope to Audio Out, e.g the negative side of C4 (user manual page B-1).
  2. If you have dual channel scope, connect the other input to the top of resistor R8, that is the PLL input.
  3. Set the timebase to 1 ms, and connect Audio-Out-High with Audio-In.
  4. Start a dump of memory with 1800G

The scope will now show figure 3.

Stop the dump program , remove the connection between Audio-In and Audio Out-High and connect the cassette recorder to the KIM-1. Start reading the tape (1873G) and move the tape to a problem area. You need to start the reading program to avoid the interference of the display. When all is right you should see the same nice picture on the scope as before.
Now increase and lower the signal level of the cassette recorder to see, if or when, there are problems with the PLL. Dropouts are visible with a image that is unstable or noise peaks. Dropouts are best studied with the 3.6 kHz recorded signal. They are visible as negative peaks on Expansion connector PLL-Test E-X. A high frequency tone is on this pin a +5V, a low as 0V.

Error 6A
The PLL is not functioning, sounds are not detected. This can be caused by:

  • No +12V power supply
  • The signal of the cassette recorder is not reaching the KIM-1
  • The signal is way too weak
  • The PLL is broken or not properly configured.
    Configuring of the PLL can be done with the program PLLCAL in Appendix I page 13
    Connect Audio-Out-High with Audio-In and start the test program on 1A6B. Inspect pin E-X PLL test on the expansion connector with a voltmeter. Adjust the variable resistor on the KIM-1 so that you see 2.5V. A small adjustment can lead to 0 to 5V, as expected.

Error 6B
The PLL is active while no input signal is present.

  • Noise signals picked up
  • Defective PLL or misconfigured of the display shows: 1742 07

Error 7
Your cassette recorder is delivering a too weak signal and you will get reading problems. See the amplifier below for a solution.
Error 8
Essential for the correct operation is the correct high frequency 3.6 kHz.

  • Dirty cassette recorder heads. Clean with a quality product
  • Unaligned head. If you are lucky there is aa small screw next to the tape head allows to adjust it, listen to a high pitch
  • Tape head is worn out, replace the cassette recorder

Amplifier between KIM-1 and the cassette recorder.

When some amplification is required the following circuit may be useful.

The amplification is controlled with variable resistor P1 from 3 to 100x. A second order Butterworth filter lowers frequencies below 2 kHz to remove mains noise.
Note the shielded cables in the drawing. Be careful to use the indicated ground point, never make a groundloop !
Place the amplifier away from noise sources and the KIM-1, noise will be amplified too!

Insert a 22 nf (22kpF in the drawing) on resistors R33 and R34 (see figure 4 right bottom and Appendix B page B1).
Adjust P2 variable resistor for Vu (output 741 opamp) = 6V
P1 controls the amplification.
The opamp (741) can be any standard general purpose audio type.
Test the amplifier as described above. Adjust for optimal volume. Record a program on tape with lower as usual volume. Try higher volume only temporarily if an error occurs.

Junior display overlay and PSU PCB scans

The Junior Display print came with a transparant overlay.

Scan here on the Junior Hardware page.

Also PSU scans by Dick Blok.


Unilab Three Chips clone

Stephen Crozier made a clone of the Unilab Three Chips main board.


Das EMUF Sonderheft 2

Thanks to Mathias Ohlerich for the scan of the Sonderheft 2


MOS KIM-1 Reproduction documents added

Dave Williams, who designed and builds the MOS KIM-1 Reproductions, sent me three worthwhile documents:
– Schematic
– Board Layout
– Bill of Materials

You can find those on the MOS MOS KIM-1 Reproduction page.