The KIM-1 has two methods of loading programs:
– from audio files on the audio interface
– from papertape from a papertape reader connected to the teletype terminal

(Photos by Dave Wiliams with the MOS KIM-1 Reproduction)

The papertape method is the preferred way available for KIM-1 clone owners, since audio input input hardware is either not present or quite inconvenient and using terminal emulators is already the way we use these computers.

Now papertape format is a special MOS Technology format, already used in the TIM-1. See the KIM-1 user manual for a technical description.
This is for example the papertape output captured with the KIM-1 S command for the memory test program in the Fist Book of KIM

;1800000000A900A885FA8570A2028672A50085FBA601A57049FF850B27
;1800187191FAC8D0FBE6FBE4FBB0F5A672A50085FBA570CA1004A20FF6
;1800300291FAC8D0F6E6FBA501C5FBB0ECA50085FBA672A571CA100F73
;18004804A202A570D1FAD015C8D0F0E6FBA501C5FBB0E8C67210AD0F29
;0B0060A57049FF30A184FA4C4F1C05CE
;0000050005

Load address, data and checksums are in the records.

What the dump above does not show that the KIM-1 inserts in front of every record a series of NULL characters (a character with value 0), to give the papertape device time to do its mechnica work and also helps the slow KIM-1 load routine to do its work after a line end of a record.

A part of a real dump:

Papertape format is therefore a readable text file, but when captured from a KIM-1 output contains NULL characters.

So if we could send the papertape formatted test file to the KIM, we can load programs.

This requires solutions for the following:

Make a papertape file
The PC utilities section has programs to produce MOS papertape from binaries or other common 8 bit hex formats produced by assembler such as Intel hex, Motorola S-Record

Send a text file
Many terminal emulators that have support for serial allow to capture the serial output to a text file or send a text file to the serial input.
Good examples are nowadays Teraterm for Windows or Minicom for Linux.

Compensate for timing
The KIM-1 character routines are quite primitive and not rebust : bit-banged, not interrupt driven, no hardware ,handshake so no buffering and it is CPU intensive.
When you sent characters quite fast to the KIM-1 (and that means any baud rate from 1200 to 9600, and the KIM-1 also has to do some processing like processsing the record just received, it is to be expected the KIM-1 will be too late reading the next record, skip a record and sync at the next and leave the program received in chaso.
So we need to give time to the poor KIM-1.
1200 baud, 20 ms character delay, 200 ms line delay is conservative but reliable for me. It is slow ..

An example for Teraterm is shown here:

Decimal mode
The 6502 NMOS version is in unknown state after reset regarding decimal mode.
Most programs start with the CLD D8 instruction, but not all. Microsoft KIM-1 Basic v1.1 is one of those.

A section from the KIM Hints:

A number of KIM-1 customers have reported difficulty in achieving correct results for the sample problem shown in Sec. 2.4 of the KIM-1 User Manual. In addition, some customers have experienced problems in recording or playback of audio cassettes. (Sec. 2.5 of the KIM-1 User Manual). In all cases, the problems have been traced to a single cause: the inadvertent setting of the DECIMAL MODE.

The 6502 Microprocessor Array used in the KIM-1 system is capable of operating in either binary or decimal arithmetic mode. The programmer must be certain that the mode is selected correctly for the program to be executed. Since the system may be in either mode after initial power-on, a specific action is required to insure the selection of the correct mode. Specifically, the results predicted for the sample problem (Sec. 2.4) are based on the assumption that the system is operating in the binary arithmetic mode. To insure that this is the case, insert the following key sequence prior to the key operations shown at the bottom of Page 11 of the KIM-1 User Manual.

[AD]
[0] [0] [F] [1]
[DA] [0] [0]

This sequence resets the decimal mode flag in the Status Register prior to the execution of the sample program.

The same key sequence may be inserted prior to the key operations shown on pages 14 and 15 for audio cassette recording and playback. These operations will not be performed correctly if the decimal mode is in effect.

In general, whenever a program is to be executed in response to the [GO] key, the programmer should insure that the correct arithmetic mode has been set in the status register (00F1) prior to program execution.

Design by Timothy Alicie

Demonstrates his design for a cassette interface for the Micro-KIM single board computer from Briel Computers (a replica of the 1970’s KIM-1 SBC). The original KIM-1 has a built-in cassette interface, but the Micro-KIM replica does not, so I designed and built my own.

The design uses a single PIC micro-controller, is very reliable, supports all HyperTAPE speeds, and has the ability to save and play back recorded data into the KIM-1.

From the source of the microcontroller:

All design files (PCB gerbers, source and hex files PIC microcontroller) in this archive.

/*******************************************************************************
* Micro-KIM Cassette Interface, PIC16F6/27A/28A/48A Implementation
*
* The original KIM-1 uses a PLL and a comparator to implement the receive
* path of the cassette interface. These two parts have been replaced by
* this implementation which runs on a PIC microcontroller.
*
* The KIM-1 cassette format encodes each bit using two tones:
* 1) A high-frequency tone of 3623.188 Hz (1.5X the low-frequency tone)
* 2) A low-frequency tone of 2415.459 Hz
*
* Each bit is broken up into three periods, each of which is 2.484 ms long,
* during which either the low frequency or the high frequency tone is played.
*
* The high frequency tone is always played in the first time period, and the
* low-frequency tone is always played in the last time period. A bit is encoded
* as logic ‘1’ by playing the low-frequency tone in the middle time period, and
* a logic ‘0’ by playing the high-frequency tone in the middle time period:
*
* Logic 1: encoded as HiFreq-LoFreq-LoFreq
* Logic 0: encoded as HiFreq-HiFreq-LoFreq
*
* The KIM-1 cassette interface uses a PLL tuned to distinguish between the
* high and low frequency tone. The output of this PLL is then fed into a
* comparator to generate a logic ‘1’ whenever the high frequency is detected,
* and a logic ‘0’ whenever the low frequency is detected. This logic signal
* is analyzed by the KIM-1 to reconstruct the bit-stream stored on the cassette
* tape. Each bit begins with a low-high transition, and the bit value can be
* determine by the timing of the falling edge generated by the high-frequency
* to low-frequency transition within each bit.
*
* The job of the PIC KIM cassette interface implementation is to perform the
* same function of the original PLL and comparator: analyze the input signal,
* and generate a logic ‘1’ output whenever the high frequency is detected, and
* a logic ‘0’ output whenever the low frequency is detected. The implementation
* is rather simple: analyze the timings of zero-crossings detected in the input
* signal, use this information to determine the frequency of the input signal,
* and generate the output signal based on if the input signal is closer to the
* high frequency, or closer to the low frequency.
*
* The original KIM-1 tape algorithm uses a bit period of 7.452 ms, which is
* three periods of 2.484 ms each. Within each sub-period, exactly 9 cycles of
* the high frequency tone can be played, or exactly 6 cycles of the low
* frequency tone can be played.
*
* Jim Butterfield popularize an alternative called HYPERTAPE, which reduces
* these periods to speed up the data by a factor of 2X, 3X, or 6X. The only
* difference between the HYPERTAPE implementation and the original implementation
* is that the sub-bit periods are reduced. The 2X and 6X sub-bit periods are
* reduced such that a non-integer number of cycles of the high and low frequency
* tones are played within each sub-period. Thus, the PIC detects the frequency
* based on half-cycles to fully-support HYPERTAPE.
*
* Interesting facts:
*
* Original KIM-1 cassette bit-rate: 134.2 bits/sec (402.6 baud, 3 symbols/bit)
* HYPERTAPE 6X bit-rate: 805.2 bits/sec (2415.5 baud, 3 symbols/bit)
*
* Each data byte is represented as two ASCII hex digits, so the effective data
* transfer rate is 8.4 bytes/sec for the original speed, or 50.3 bytes/sec for
* HyperTAPE x6.
*
* High frequency tone: 3623.188 Hz, or 0.276 ms/cycle
* Low frequency tone: 2415.459 Hz, or 0.414 ms/cycle
* Center frequency: 2898.551 Hz, or 0.345 ms/cycle
*
* Frequency detection based on half-wave zero-crossings:
* High frequency tone: zero-crossing every 138 us
* Low frequency tone: zero-crossing every 207 us
*
* Separation between high-frequency and low-frequency zero-crossing: 69 us
* Threshold between high and low-frequency zero crossing: 172.5 us
*
* Implementation Notes:
*
* The bi-color LED lights red when an input signal is present, but it does
* not contain the correct frequencies to be a valid KIM-1 bit stream, for
* example, voice input. The bi-color LED lights green if the input signal
* contains the correct frequencies to be a valid KIM-1 bit stream. While
* receiving data, the green LED should be lit solid to ensure reliable data.

* Comparator 1 (CMP1) is used to detect signal zero-crossings by adding a bias
* to the input AC signal of Vcc/2. The bias is used as the V- comparator input.
*
* RB4 is used to monitor the PB7 line to/from the KIM-1. In audio-output mode,
* (dumping to a tape), the KIM-1 drives this line, so we can use it to detect
* audio-output mode and light the LED when the KIM-1 is dumping data.
*
* Timer 0 (TMR0) is used to light the LED indicators for a certain time period.
*
* Timer 1 (TMR1) is set up to count at 1.0 MHz, and it is used to precisely
* measure the time between zero crossings.
*
* Copyright Timothy Alicie, 2017, Timothy Alicie
*******************************************************************************/