After RESET the baudrate is determined by measuring the length of the start bit of an incoming serial character. This means any character is usable where the first data bit is the opposite of the start bit. The KIM-1 User manual suggest RUBOUT ($7F 1111111) but ENTER ($0D 0000 1101) also works fine. SPACE ($20 0010 0000) for example does not work, any character with an odd value is OK.

0612   1C2A A9 FF               LDA   #$FF       ; COUNT START BIT
0613   1C2C 8D F3 17            STA   CNTH30     ; ZERO CNTH30
0614   1C2F A9 01               LDA   #$01       ; MASK HI ORDER BITS
0615   1C31 2C 40 17    DET1    BIT   SAD        ; TEST 
0616   1C34 D0 19               BNE   START      ; KEYBD SSW TEST
0617   1C36 30 F9               BMI   DET1       ; START BIT TEST
0618   1C38 A9 FC               LDA   #$FC
0619   1C3A 18          DET3    CLC              ; THIS LOOP COUNTS 
0620   1C3B 69 01               ADC   #$01       ; THE START BIT TIME
0621   1C3D 90 03               BCC   DET2
0622   1C3F EE F3 17            INC   CNTH30
0623   1C42 AC 40 17    DET2    LDY   SAD        ; CHECK FOR END OF START BIT 
0624   1C45 10 F3               BPL   DET3
0625   1C47 8D F2 17            STA   CNTL30
0626   1C4A A2 08               LDX   #$08
0627   1C4C 20 6A 1E            JSR   GET5       ; GET REST OF THE CHAR, 
0628   1C4F                                      ; TEST CHAR HERE

What is happening here?

– bit 7 (PB7) is tested until it becomes 0 (BIT SAD and BMI DET1 loop)
– the time is counted and kept in CNTH30 and CNTL30
– bit 7 is tested for becoming 1 (LDY SAD and BPL DET3)
– the rest of the character is read in by jumping into GETCH , the actual character received is not tested.

1006   1ED4             ;		
1007   1ED4             ;       DELAY 1 BIT TIME   
1008   1ED4             ;       AS DETERMINED BY DETCPS
1009   1ED4             ;
1010   1ED4 AD F3 17    DELAY   LDA   CNTH30     ; THIS LOOP SIMULATES 
1011   1ED7 8D F4 17            STA   TIMH       ; DETCPS SECTIONS AND WILL DELAY
1012   1EDA AD F2 17            LDA   CNTL30     ; 1 BIT TIME
1013   1EDD 38          DE2     SEC   
1014   1EDE E9 01       DE4     SBC   #$01
1015   1EE0 B0 03               BCS   DE3
1016   1EE2 CE F4 17            DEC   TIMH
1017   1EE5 AC F4 17    DE3     LDY   TIMH
1018   1EE8 10 F3               BPL   DE2
1019   1EEA 60                  RTS
1020   1EEB             ;		
1021   1EEB             ;                          DELAY 1/2 BIT TIME   
1022   1EEB AD F3 17    DEHALF  LDA   CNTH30     ; DOUBLE RIGHT SHIFT OF DELAY                              
1023   1EEE 8D F4 17            STA   TIMH       ; CONSTANT FOR A DIVE 2 
1024   1EF1 AD F2 17            LDA   CNTL30
1025   1EF4 4A                  LSR   A
1026   1EF5 4E F4 17            LSR   TIMH
1027   1EF8 90 E3               BCC   DE2
1028   1EFA 09 80               ORA   #$80
1029   1EFC B0 E0               BCS   DE4

The actual delay routines use the same logic as DETCPS. IN DEHALF the delay time is divided by 2 and jumped into DELAY.

SPEED CONTROL OF KIM-1 TTY PORT
KIM User notes Vol 06

If you are having problems trying to use a high speed terminal with the KIM YTY port the following information might be useful. To start with, the Reset/ Rubout sequence activates a subroutine called DETCPS (1C2AH to IC4EH)
which determines two constants CNTL30 (17F2N1 and CNTH30 (12F3H). These are used to time the serial TTY port via subroutines DELAY (1E4DH to IEEAH) and DEHALF (1EEBH to 1EFDH). DELAY and DEHALF are called by GETCH (
1E5AH to 1E87H) which inputs one character, and OUTCH (1EA0H to 1ED3H1) which outputs one character.

CNTL30 and CNTH30 are the whole key to trimming up the TTY port speed. You can change the baudrate without going through the Reset/Rubout sequence simply by changing one or both of these constants.
The following list gives the values of CNTL30 and CNTH30 for several speeds as determined by my particular KIM-I Reset/Rubout sequence.

BAUD   110  150  300  600  1200 1800 2400 4800 9600 
CNTL30 85H  D8H  EBH  74H  38H  24H  1AH  06H  03H 
CNTH30 02H  01H  OOH  00H  00H  00H  00H  00H  00H 

Now for the interesting part. At lower speeds (110 through 1800 Baud) everything works fine with these values. But, at higher speeds problems arise.
Repeated Reset/Rubout sequences kept producing CNTL30 = 1AH for 2400 Baud. This value did work for most functions like examine and fill memory. however, for tape dump (Q command) intermittent characters were lost.
This problem was eliminated by substituting either 18H or 19H In CNTL30. All functions worked perfectly for either value.
At 4800 Baud I was unable to use the terminal at all. It acted more like it wanted to work at CNTL30 = 07H rather than at 06H, but no value between 05H and OAH would make it work.
At 9600 Baud the slow functions like examine and fill would work, but tape dump (Q) resulted in severe loss of characters.
The problem at these speeds is that the quantization level between allowed speeds (i.e. integer values of CNTL30) is so large that unless you are lucky you will not hit close enough to your terminal speed.
Since CNTL30 as used in a software timing loop to count instruction sequences (thus machine cycles) the high Baud can be fine tuned with the system clock.
You should be able to use the technique described by R. H. Burhans (page 10, issue #5. May 77, KIM-1/6502 Users Notes) to perform this fine tuning.

For casual use, 2400 Baud (or maybe even 110 Baud) is satisfactory with no hardware modifications.
However, if you have a dedicated high speed terminal you may find it worthwhile to fine tune either the KIM-1 clock or the terminal clock.
This will allow you to utilise the KIM-1 software (like GETCH and OUTCH) at high speeds. After you become accustomed to it. 9600 Baud is nice. 2400 ok, 1200 a bit of a drag. and anything less unbearable.
Finally, since CNTL30 • 03H (greater than zero) for 9600 Baud you should be able to fine tune for 19,200 Baud. and maybe (but probably not) for 38,400 Baud. Dwight D. Egbert

Jim mcClahanan notes

The PAL-1 (just like the KIM-1) uses a ‘soft UART’ or ‘bit banger’ for its serial I/O. I’m not a fan of this approach, but at the same time it demonstrates what could be accomplished with a minimial amount of hardware. The PAL-1 automatically figures out the appropriate delay between bits of the serial character when you press enter after a reboot. I have found that decreasing the value actually significantly improves the odds of an error-free load of larger punchtape format files. Below is a table for values found and suggest for $17F2.

Baud    Found   New
300     $EA     $E8
1200    $37     $35
2400    $1A     $18
4800    $0B     $0A

I haven’t tried to optimize the delay values. Right now I’m using 5 ms between characters and 500 ms between lines when doing 8K transfers and with the modified values I usually am successful. With the default values, it seemed like even with longer delays things would slip out of synchronization at some point more often than not on large transfers.

OUTCH sends one character via the serial output vai bitbanging
8N1: All 8 databits are sent. One stopbit.

A destroyed, Y=FF, X preserved.

0979   1E9E             ;       PRINT 1 CHAR   CHAR=A
0980   1E9E             ;       X IS PRESERVED, Y RETURNED = FF
0981   1E9E             ;       OUTSP    PRINTS 1 SPACE   
0982   1E9E             ;
0983   1E9E A9 20       OUTSP   LDA   #$20       
0984   1EA0 85 FE       OUTCH   STA   CHAR
0985   1EA2 86 FD               STX   TMPX
0986   1EA4 20 D4 1E            JSR   DELAY      ; 10/11 BIT CODE SYNC
0987   1EA7 AD 42 17            LDA   SBD        ; START BIT
0988   1EAA 29 FE               AND   #$FE
0989   1EAC 8D 42 17            STA   SBD
0990   1EAF 20 D4 1E            JSR   DELAY
0991   1EB2 A2 08               LDX   #$08
0992   1EB4 AD 42 17    OUT1    LDA   SBD        ; DATA BIT
0993   1EB7 29 FE               AND   #$FE
0994   1EB9 46 FE               LSR   CHAR
0995   1EBB 69 00               ADC   #$00       
0996   1EBD 8D 42 17            STA   SBD
0997   1EC0 20 D4 1E            JSR   DELAY
0998   1EC3 CA                  DEX   
0999   1EC4 D0 EE               BNE   OUT1
1000   1EC6 AD 42 17            LDA   SBD        ; STOP BIT
1001   1EC9 09 01               ORA   #$01
1002   1ECB 8D 42 17            STA   SBD
1003   1ECE 20 D4 1E            JSR   DELAY      ; STOP BIT
1004   1ED1 A6 FD               LDX   TMPX       ; RESTORE INDEX
1005   1ED3 60                  RTS

A subroutine that sends out an 8 bit character via the serial TTY output followed by one stop bit via bitbanging to PB7. Contrary to GETCH this allow 8 bits characters.
The receiving device should be set to 8N1 (8 databits, no parity, 1 stopbit)
Timing is done with the 1 bit DELAY routine.
A is lost, Y = FF, X preserved.

What is happening here?

• The character to send is stored at zeropage CHAR (984).
• save X
• A 1 bit delay, just to be sure (986)
• set PB7 to the start bit 0 (987-989)
• a 1 bit delay (980)
• get current value of PB7 (LDA SBD and AND #$FE) (982)
• shift databit into carry (LSR CHAR) (994)
• set carry to PB7 (ADC #$00 and STA SBD) (985)
• delay 1 bit (987)
• repeat for 8 databits

(998-99)

• set stop bit 1 (1000-1002)
• delay 1 bit, leave PB7 in rest state (1003)
• restore X

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 storing 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 chaos.
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:

RS232/TTL to the KIM-1 20 mA interface

The 20 mA current loop TTY interface of the KIM-1 may look alien. It is not, very simple to interface.

To RS-232C is requiring a negative power supply voltage (-5V – – 12V) to be reliable. Here two examples of transistor based (any general purpose transistor will work)
Using GND instead of -12V may work with some USB RS232 connectors, UGREEN types are known to work, many others are not.

My circuit, used from 1978 on. 9600 baud without issues.

From KIM Hints

A modern approach would be to use a special IC that generates the right power lines itself.

Corsham KIM-1 I/O board

TTL USB to to the KIM-1 20 mA interface
Since this is 5V only it can be simple. The output of the KIM-1 needs an inverter (a simple transistor emitter follower will do as seen in the Corsham I/O board ) , the input can be connected straight away.