Another article from the Dutch KIM Kenner magazine:

About small SBC systems
Another article from the Dutch KIM Kenner magazine:
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
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:
Troubleshooting
Here is a procedure to work around reading tape problems:
Oscilloscope test
If you have an oscilloscope, do the following measurements.
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:
Error 6B
The PLL is active while no input signal is present.
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.
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.
The Junior Display print came with a transparant overlay.
Also PSU scans by Dick Blok.
Stephen Crozier made a clone of the Unilab Three Chips main board.
Thanks to Mathias Ohlerich for the scan of the Sonderheft 2
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.
Decapped 6530-004 TIM photo’s by Frank Wolf
Didier form aida.org was approached by his friend Erik about a malfunctioning MPS-65.
By studying the material on this page and doing measurements on the board it was decided the PROM on the board was malfunctioning.
So Didier designed a GAL replacement for the PROM and the board came back to life!
Here the story how the design looks like.
Original article: KIM Kenner 17 page 14, Dutch, Hans Otten. Translation 2021 Hans Otten
Problem: the KIM-1 hardware is echoing incoming serial characters to the output, no echo in software involved. Very annoying!
In the KIM Kenner 1 Siep de Vries, founder of the Dutch KIM Club mentioned how in Focal for the 6502 a trick was built in to suppress the hardware echo by manipulating the TTY out bit. I examined later how it was done, from the Focal disassembly:
34AF E6 76 L34AF INC $76 ; random number? 34B1 2C 40 17 BIT H1740 ; check if character is incoming 34B4 30 F9 BMI H34AF ;=> wait until startbit 34B6 AD 42 17 LDA H1742 34B9 29 FE AND #$FE ; clear PA7 34BB 8D 42 17 STA H1742 34BE 20 5A 1E JSR H1E5A ; KIM-1 input 34C1 48 PHA 34C2 AD 42 17 LDA H1742 34C5 29 FE AND #$FE ; isolate PA7 34C7 09 01 ORA #$01 ; set PA7 to 1 34C9 8D 42 17 STA H1742 34CC 68 PLA 34CD 18 CLC 34CE 60 RTS
Hardware echo
I took the idea and implemented the software (without knowing then in 1980 the Focal disassembly!).
The echo of incoming serial to outgoing is shown in the next figures (from the KIM user manual and the KIM Circuit poster).
The TTY KEYBD signal goes via a transistor and NAND gate U15 to PA7 port of the 6532. That signal also goes to pin 10 input of NAND gate U26 which is the TTY out line. This is the hardware echo. When the KIM-1 sends out a character it comes from PB0 to pin 9 of of NAND gate U26 and so comes out to the TTY Out line.
Note that PB5 is connected via an inverter to NAND gate U15. The other input is TTY IN. Making PB5 high will make the TTY input PA7 deaf.
Note PB5 is also Audio out.
Suppress echo in software
The solution to suppress the echo is making output PB0 low. The NAND gate out will now stay high, ignoring any changes on the other input, which is the incoming serial character.
Only when receiving a character PBO should be made high. Also any incoming character will now not be echoed unless the program wants to receive a character!
Example program
In this routine the standard KIM-1 GETCH routine at $1E5A is encapsulated in a subroutine that prevents the echo by setting PB0. Note that this is not a complete block of the echo, it is only active when the program calls the blocking EGETCHAR. When the program sends out charactersto a dispaly, anything typed at the keyboard will also appear at the display.
The calling program is now responsible for the echoing!
0001 1000 echo .org $1000 0002 1000 ; 0003 1000 echoflag = $17E2 ; flag: 0 normal echo 0004 1000 SBD = $1742 ; KIM 6532 PIA B data register 0005 1000 GETCH = $1E5A ; KIM TTY Getch routine 0006 1000 ; 0007 1000 AD E2 17 EGETCHAR LDA echoflag ; if notechoflag 0008 1003 F0 08 beq normal ; then normal echo 0009 1005 AD 42 17 LDA SBD ; else set TTY bit PB0 to 0 0010 1008 29 FE AND #$FE 0011 100A 8D 42 17 STA SBD ; 0012 100D 20 5A 1E normal JSR GETCH ; get character from input 0013 1010 48 PHA ; save 0014 1011 AD 42 17 LDA SBD ; set TTY bit PB0 0015 1014 09 01 ORA #$01 0016 1016 8D 42 17 STA SBD 0017 1019 68 PLA ; restore received character 0018 101A 60 RTS 0019 101B .end 0020 101B tasm: Number of errors = 0
Does EGETCHAR work on the KIM-1 clones or SImulator?
Micro-KIM and PAL-1: yes, the hardware is identical, IC numbers are different
Corsham Technology: yes, though the hardware for audio is not there, there is still a NAND gate IC17C coupling PA7 and PB0.
KIM-1 Simulator V1.16 and higher: yes.
Enhanced solution: always deaf for input
If you study the hardware shown above you see PB5 also blocks the echo. The following routine tries to use this to make the input permanent deaf.
0001 1000 echo .org $1000 0002 1000 ; 0003 1000 echoflag = $17E2 ; flag: 0 normal echo 0004 1000 SBD = $1742 ; KIM 6532 PIA B data register 0005 1000 GETCH = $1E5A ; KIM TTY Getch routine 0006 1000 ; 0007 1000 ; no echo when reading character 0008 1000 ; 0009 1000 AD E2 17 EGETCHAR LDA echoflag ; if not echoflag 0010 1003 F0 08 beq normal ; then normal echo 0011 1005 AD 42 17 LDA SBD ; else set TTY bit PB0 to 0012 1008 29 FE AND #$FE 0013 100A 8D 42 17 STA SBD ; 0014 100D 20 5A 1E normal JSR GETCH ; get character form input 0015 1010 48 PHA ; save 0016 1011 AD 42 17 LDA SBD ; set TTY bit PB0 0017 1014 09 01 ORA #$01 0018 1016 8D 42 17 STA SBD 0019 1019 68 PLA ; restore received character 0020 101A 60 RTS 0021 101B ; 0022 101B ; no echo only at wish if reading character 0023 101B ; note that using tape I/O will leave PB5 low 0024 101B ; 0025 101B AD E2 17 DGETCHAR LDA echoflag ; if notechoflag 0026 101E F0 05 beq dnormal ; then normal echo 0027 1020 AD 42 17 LDA SBD ; else set TTY bit PB0 to 0028 1023 29 FE AND #$FE ; PB0 low 0029 1025 29 DF dnormal AND #$DF ; PB5 low 0030 1027 8D 42 17 STA SBD ; 0031 102A 20 5A 1E JSR GETCH ; get character from input 0032 102D 48 PHA ; save 0033 102E AD 42 17 LDA SBD ; set TTY bit PB0 and PB5 0034 1031 09 21 ORA #$21 ; high 0035 1033 8D 42 17 STA SBD 0036 1036 68 PLA ; restore received character 0037 1037 60 RTS 0038 1038 .end 0039 1038 0040 1038 tasm: Number of errors = 0
Note that using tape I/O will leave PB5 low, allowing echo, only set high when the program calls DGETCHAR.
Does DGETCHAR work on the KIM-1 clones?
Micro-KIM and PAL-1: yes, the hardware is identical, IC numbers are different
Corsham Technology: no, PB5 is not used.
KIM-1 Simulator: not yet
On the Utilities page I have two programs to convert to MOS Technology papertape format: KIMpaper, a command line utility, and ConvertHexFormat, a GUI app.
All in Freepascal/Lazarus source format, and tested on Linux (Raspberry PI OS) and Windows 10 64 bit. So the programs will run everywhere Lazarus is available (MS DOS, WIndows, Linux Mac OS).
KIMPAPER is written at the time the Micro-KIM appeared. CLI utility. Supports Binary to/from Papertape. Still runs fine on all platforms supported by Freepascal (Windows, MS DOS, Linux etc) after a recompilation, source available.
ConvertHexFormat is a more recent GUI utilitilty with many more 8 bit hex formats as input and output.
There were some bugs of course in older versions. V2 added the ability for multipart hex formats, records having a non-consecutive load address. That seems to wok fine since V2.1
In 2.2 a bug in MOS Papertape format for bigger files is fixed, the end-of-file record (record type 00, total line count) had a bug in the checksum calculation. KIMPAPER is and was correct in the calculation.
But in ConvertHexFormat it was wrong (as it still is in the well known srec utility in the Unix world!).