A page on suppressing the KIM-1 echo of TTY input, read non-blocking and make the TTY input deaf.
Problems with the KIM-1 TTY character input
- The KIM-1 hardware is hardware echoing incoming serial characters to the output, no echo in software involved, so you cannot influence what appears on screen. Very annoying!
- The KIM-1 GETCH routine is blocking, no way to check for a character coming in, like a Break. waiting.
Also quite annoying if porting other software to the KIM-1 or you want the program interruptable.
- While a program is running something CPU intensive and you type something the program is not really waiting for, the characters appear on screen. Because the KIM-1 does hardware echoing of TTY input, this is unavoidable it seems
Here I present solutions for these problems in software, made possible by the genius hardware design of the KIM-1 TTY I/O.
Are they perfect? Maybe not, it is still bitbanging the incoming serial signal. It can miss the correct starting point for the incoming character bit stream.
If you want a perfect solution, you will need interrupt driven ringbuffered serial I/O with a dedicated IC like the 6850, 6551 etc.
Without this extra hardware you can achieve acceptable results with these routines.
Background in (updated) original article KIM Kenner 17 page 14, Dutch, Hans Otten, 1980
In the KIM Kenner 1 Siep de Vries, founder of the Dutch KIM Club mentioned how in Focal-65 for the 6502 a trick was built in to suppress the hardware echo by manipulating the TTY out bit PB0. I took the idea and implemented it on my KIM-1 in 1980 without seeing the Focal code, as I did not have a Focal binary yet then!
I examined in 2003 how it was done, from the Focal disassembly I made then:
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
How to suppress the hardware echo to TTY out or making the TTY input deaf
The hardware echo of incoming serial signal to outgoing TTY output 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.
PB5 (audio TTY control) 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 for incoming signals.
The genius designers of the KIM-1 used NAND gates in the TTY I/O!
Non-blocking input
The KIM-1 GETCH routine detects an incoming character by looking in a loop for the start bit to appear. It then reads the character.
By first doing that loop of looking for the start bit and returning if not yet, then we have the check for a key pressed and a character coming in.
If a character is incoming we have to call as fast as possible the GETCH routine.
;
; KEYPRESS
; check character coming on character non-blocking
; - carry set if char coming in
; - follow up with GETCH or EGETCH as fast as possible if you want echo or no echo
;
KEYPRS LDA SAD
BMI NOKEY ; If bit 7 is set, the line is idle, no char
SEC
RTS ; Carry set if key pressed, A is key
NOKEY CLC
RTS ; carry clear, no key
As argued above, this is not foolproof. It is easy to miss an incoming character, as there is no buffering of the input.
Echo suppress
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, the incoming serial character. So nothing is echoed.
In this routine the standard KIM-1 GETCH routine at $1E5A is encapsulated in a subroutine that prevents the echo by setting PB0. Note that this is not a complete block of the echo, it is only active when the program calls the blocking EGETCHAR. The calling program is now responsible for the echoing or otherwise.
;
; EGETCH from TTY without echo (Y returned FF due to GETCH)
;
EGETCH LDA SBD
AND #$FE ; Set PB0 to U26 low to suppress the echo
STA SBD
JSR GETCH
PHA
LDA SBD
ORA #$01 ; Set PB0 to U26 high to enable the echo
STA SBD
PLA
RTS
To make the TTY input really deaf you can use PB5. Calling the deaf routine hardware blocks any incoming TTY signal.
;
; TTYDEAF
; call this to block any incoming character
;
TTYDEAF LDA SBD
ORA #$20 ; Set PB5 to U26 high to block input
STA SBD
RTS
;
; TTYHEAR
; call this to restore incoming character via GETCH or EGETCH
;
TTYHEAR LDA SBD
AND #$DF ; Set PB5 to U26 low to allow input
STA SBD
RTS
Using TTYdeaf/hear in combination with KEYPRS and EGETCH works quite well to prevent most unwanted screen display of characters.
Example program of suppressing echo and non-blocking
I wrote a litle program demonstarting the non-blocking and no echo facilities presented here.
Download sources, binary, papertape here.
This is the console output of the program:
KIM
0000 0200
0200 A2 G
Demo of echo suppress and non-blocking input Hans Otten, 2026
Normal get character, until ESCAPE
1 31 2 32 3 33 4 34 5 35 6 36 03 1B
NOECHO get character, until ESCAPE
31 32 33 34 35 36 03 1B
If run on KIM-1 Simulator: set in Settings Non-blocking or Focal-V3D
Non-blocking no echo until ESCAPE
31
key pressed
32
key pressed
33
key pressed
03
key pressed
Demo of echo suppress and non-blocking input Hans Otten, 2026
KIM
0200 A2 _
If you have a KIM-1, PAL-1, PAL-2 or Micro-KIM, these routines may help you.
The Corsham KIM Clone does not support PB5, and no deaf input on that one. Echo suppress works!
These routines also run on the KIM-1 Simulator. The non-blocking routine requires a Setting in the Simulator.
In versions before 2.3.1 check Focal-V3D, the later versions check Allow non-blocking.
The TTYdeaf routine does not work yet on the Simulator, PB5 is ignored.
Settings in 2.3.0
Settings 2.3.1
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.
