Category: Lexitron VT202

This is a long technical post. There’s probably only one reason to read it: you have a Lexitron with a broken keyboard. I’m not going to polish it much, but I leave it here just for you!

I spent a lot of time creating a teensy replacement for the microcontroller in the keyboard.  One of the great challenges was to work out the key codes sent from the keyboard to the 8085.  This was very time-consuming and there were some that I never really cracked.

With the arrival of working keyboards, I could just swap out the original for one of the working units. Buuut the teensy solution actually provides a mechanism to move data from a modern PC to the Lexitron and that’s handy.  Also, I guess I’d like to just close the loop!

I can now sniff the communication between the 8085 and the keyboard microcontroller.  Because the keyboard appears to be just another device on the 8085 bus the data is transmitted as a parallel byte.  This would be easy to sniff with a logic analyser, but not so easy with an oscilloscope.  I thought my Analogue Discovery 2 might be sufficient for this task.

Two Different Keyboard Types

One of the VT202 units uses the same Cortron Keyboard as the VT1303.  The other uses a Digitran keyboard which looks quite different but seems to be entirely compatible with the Cortron.  The Digitran sound is not as loud.

Revisit Operation

Keyboard Pinout

Pin	Name	Function
1	+5V
2	+5V
3	GND
4	GND
5	NC
6	GND
7	Strobe_L	This line indicates activity on the keyboard.  It is asserted for 5us when a key is pressed and for 10us when a key is released.   The strobe is not asserted when a front panel switch is changed.  It is not asserted when the shift key is pressed.  It is asserted for spec shift.
8	D5
9	D6
10	GND
11	D7
12	D4
13	Kbd_Read_L	This line causes the keyboard to put data on to the 8085 bus.   The read occurs about 100us after the strobe is asserted. A0 is low.  Scan code?   A second read occurs about 8us after the first.  A0 is high. Switch state?   This strobe is asserted a couple of times every 10ms (approx.)
14	Screen On	This also appears in the status register.
15	A0	This selects the data that is read by the 8085 – either the keycode or the status of the front panel switches.
16	Kbd_Write_L	This strobe is asserted about 5us after Read is asserted in response to the Strobe.   This strobe is asserted every 10ms (approx.)
17	Reset?
18	GND
19	D3
20	D2
21	D1
22	D0
23	-15V
24	-15V
25	+15V
26	+15V

Character Register

Bits 6:0 = Character number

Bit 7 = Shift

Status Register:

Bits 1:0 = Print Spacing

Bit 2 = Pitch

Bit 3 = Screen Spacing

Bit 4 = Not used (input pulled up)

Bit 5 = Key pressed

Bit 6 = Screen On/Off

Bit 7 = Autorepeat (although it doesn’t seem to be used)

Command Register:

Bit 0 = Key click

Bit 1 = Type Through LED

Bit 2 = Select LED

Bit 3 = Continuous Typing LED

Bit 4 = Write LED state (Bits 1, 2 & 3)

Bit 5 = Acknowledge

Bit 6 = Bell (Not supressed by volume control)? Under CP/M this is sent when Select is pressed.  Select causes an alarm chime (not volume controlled). 

Bit 7 = Unknown

Adjustable volume applies to key strokes.  Alarms have a fixed volume.

Word Processing Program Behaviour

Periodic

Once every 10ms or so the program reads the status register and writes the Command Register. 

Keystroke

When a key is hit the keyboard alerts the uP by asserting Strobe_L for 5us.  The program responds by reading the character, reading the status register, and then writing 32 to the Command Register.  This seems to be an acknowledgement. If there is a LED change or sound then that will be sent a few milliseconds later.

The key release is also acknowledged with a command write which can update the LED state and sound a bell but I think does neither.  

The unit only autorepeats on space, backspace, dash, underline, and full stop.  This auto repeat is convoluted.  It involves resending a specific character after the wait time has elapsed.  This triggers the software to autorepeat.  It looks like the bit 6 of the status word is set at the same time.  This is set by an output from the microcontroller (Pin 29).  Although the original keyboard does this it does no appear necessary.

Character	Original Output	Autorepeat Output
Space	80	112
Dash	103	119
Underline	231	247
Full stop	26	122
Backspace	110	126

I can’t see any obvious pattern in these numbers.   They just are.

CP/M Behaviour

Periodic

Once every 10ms or so the program reads the status register. 

Keystroke

When a key is hit the keyboard alerts the uP by asserting Strobe_L for 5us.  The program responds by reading the status register, reading the character, and then writing the Command Register – it can acknowledge, set LEDs, or make a sound.

The key release is also acknowledged but the LED data is not repeated.  There is no periodic write The key release is also acknowledged with a command write which can update the LED state and sound a bell but I think does neither.  

CP/M autorepeats on most keys but not Spec Shift or Sel. It initiates a beep on each auto repeated character.   

CP/M politely ignores the autorepeat shenanigans that the keyboard does for the word processing program.

In CP/M lots of keys are defined as function keys that can be changed using the config program.  The config can also control auto repeat and key click. 

Select is a caps lock key.

My Current Keyboard Code

The current routine that I have for sending a character to the 8085 sets up the character and then takes then asserts the strobe (indirectly) for 1ms.  It then sets up another character (usually zero).

The real thing asserts the strobe for 5 or 10us.  Both the key register and the status register are read within about 100us of the strobe.  The Command Register is then written.  This may be the trigger to clear the key but maybe not.  I previously concluded that the key needed to be nulled but I’m not sure why it does.

My implementation autorepeats on all keys except “mode” keys like Spec Shift and Margin Set.  The actual unit does not autorepeat, although the software does under some circumstances.

I’ve treated Spec Shift and Margin Set as special cases because they are used in combination with other keys.  Certainly, autorepeat doesn’t apply.  CP/M treats Margin Set as a normal function key. 

This means that the effect of these keys is application specific.  Eg in CP/M the Spec Shift key is used to create control keys eg Ctrl-C to do a soft boot.  In the word processing program it has no effect on the letter c.  Spec Shift does not beep.

Shift really is different.  No key code is sent with just the shift key, and the character that is sent is modified if a shift key is pressed.  When shift is pressed the most significant bit is set.  This applies to all keys including Spec Shift and Margin Set.

The various reads and writes change a little with CP/M.  CP/M still reads the front panel switches but I have no idea what it does with the state. Similarly, the LEDs have no apparent function. 

Under CP/M Select behaves like a caps lock.  The shift is done in software.

Observations

Key hit

Close up of subsequent reads and write.

With the autorepeat:

The regular status read (word processor software):

Revised Keyboard Code

The keyboard code has been substantially reworked based on the observations that were possible using the logic analyser function of the Analogue Discovery 2.

• Autorepeat has been removed except to the extent required by the word processing program.
• Each key press and release is transferred to the 8085.
• The timings have been changed to better match the real keyboard.
• Only one key is checked and processed on each pass of the main loop so that commands are less likely to be missed.
• All sounds are generated only at the request of the 8085.  There are no automatically generated sounds.
• An additional signal was discovered that is set when a command is written and is cleared when the command has been read.
• A few missing key codes were found.

Things like Spec Shift, Set Margin, and Select now work as advertised (as far as I can tell).

The code works on the VT1303 and the VT202.  It works with the word processing programs and with both the VT1303 CP/M and the VT202 CP/M.

Mapping

The transmitted codes can be read straight off the data lines (see waveform pics above).

Circuitry

https://www.pjrc.com/store/teensypp.html

The mapping from the microcontroller to the teensy are shown in this table:

uC Pin Nr	uC Pin Name	Teensy Pin Nr	Signal Name	Comment
1	T0
2	XTAL1
3	XTAL2
4	RESET_L
5	SS
6	INT_L	D4	uC_New_Command
7	EA
8	RD_L
9	PSEN_L
10	WR_L
11	ALE
12	DB0	C0	DB0	Data bus for the keyboard.
13	DB1	C1	DB1
14	DB2	C2	DB2
15	DB3	C3	DB3
16	DB4	C4	DB4
17	DB5	C5	DB5
18	DB6	C6	DB6
19	DB7	C7	DB7
20	VSS	GND
21	P20	F6	LED_TypeThru
22	P21	F5	LED_Unused
23	P22	F4	LED_Select
24	P23
25	PROG
26	VDD
27	P10	B0	Column0	Keyboard Column
28	P11	B1	Column1
29	P12	B2	Column2	Also drives a status register input to indicate that the current key has been pressed.
30	P13	B3	Column3	Also indicates when an autorepeat could be started.
31	P14	B4	Row0	Keyboard Row
32	P15	B5	Row1
33	P16	B6	Row2
34	P17	B7	Pulser	This creates an opportunity for a pulse at E7
35	P24	F3	LED_ContType
36	P25	D3	uC_Rd_L	Enables command register so it can be read via Port C.
37	P26	D2	uC_GotOne_Loud_L	Loud Beep for Alarms
38	P27	D1	uC_GotOne_Adj_L	Adjustable beep for key presses
39	T1	E7	uC_Sense	Senses when a key is down
40	VCC	+5V