Suppress KIM-1 echo

Hans Otten, May 1979, March 2021
Original article: KIM Kenner 17 page 14, Dutch, 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 took the idea and implemented the software.

Hardware echo

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 = $1E5A ; KIM TTY Getch routine 
0006   1000             ;
0007   1000 AD E2 17    EGETCHAR LDA echoflag ; if not echoflag 
0008   1003 F0 08         beq normal ;  then normal echo 
0009   1005 AD CE 06      LDA SBD  ; else set TTY bit PB0 to 0 
0010   1008 29 FE         AND #$FE  
0011   100A 8D CE 06      STA SBD ; 
0012   100D 20 5A 1E    normal JSR GETCH ; get character from input
0013   1010 48            PHA ; save
0014   1011 AD CE 06      LDA SBD ; set TTY bit PB0 
0015   1014 09 01         ORA #$01 
0016   1016 8D CE 06      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?

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.

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 = $1E5A ; KIM TTY Getch routine 
0006   1000             ;
0007   1000             ; no echo when reading character
0008   1000             ; 
0009   1000 AD E2 17    EGETCHAR LDA echoflag ; if not echoflag 
0010   1003 F0 08         beq normal ;  then normal echo 
0011   1005 AD CE 06      LDA SBD  ; else set TTY bit PB0 to 
0012   1008 29 FE         AND #$FE 
0013   100A 8D CE 06      STA SBD ; 
0014   100D 20 5A 1E    normal JSR GETCH ; get character from input
0015   1010 48            PHA ; save
0016   1011 AD CE 06      LDA SBD ; set TTY bit PB0 
0017   1014 09 01         ORA #$01 
0018   1016 8D CE 06      STA SBD 
0019   1019 68            PLA ; restore received character
0020   101A 60            RTS 
0021   101B             ;
0022   101B             ; no echo only at wish if reading character
0023   101B             ; note that using tape I/O will leave PB5 low
0024   101B             ; 
0025   101B AD E2 17    DGETCHAR LDA echoflag ; if notechoflag 
0026   101E F0 05         beq dnormal ;  then normal echo 
0027   1020 AD CE 06      LDA SBD  ; else set TTY bit PB0 to 
0028   1023 29 FE         AND #$FE ; PB0 low
0029   1025 29 DF       dnormal AND #$DF ; PB5 low
0030   1027 8D CE 06      STA SBD ; 
0031   102A 20 5A 1E      JSR GETCH ; get character from input
0032   102D 48            PHA ; save
0033   102E AD CE 06      LDA SBD ; set TTY bit PB0 and PB5
0034   1031 09 21         ORA #$21 ; high
0035   1033 8D CE 06      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, allwoing 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.