Tynemouth MINI PET 40/80 User manual
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TYNEMOUTH MINI PET 40/80 KIT USER MANUAL
PARTS LIST
CAPACITORS –ALL RATED 16V OR HIGHER
2 x 22pF axial (usually marked 220 or 22J)
30 x 100nF axial (usually marked 100n or 104)
4 x 1µF axial (usually marked 1u0 or 105)
1 x 100µF (axial electrolytic rated 25V or higher)
RESISTORS –ALL ¼W 5% OR BETTER (4 BAND RESISTOR COLOUR CODES SHOWN)
2 x 75Ω
2 x 470Ω
1 x 1KΩ
3 x 2.2KΩ
2 x 4.7KΩ
6 x 10KΩ
5 x 100KΩ
3 x 390KΩ
2 x 4K7 7 bussed resistor array, 8 pin (usually marked 8X-1-472) - pin 1 marked by dots on package and PCB
5 x 10K 8 bussed resistor array, 9 pin (usually marked 9X-1-103) - pin 1 marked by dots on package and PCB
SEMICONDUCTORS - NEW TEXAS INSTRUMENTS ICS RECOMMENDED.
3 x 1N4148 diodes (no part number marked on the PCB)
1 x 1N4001N diode (marked on the PCB as 1N4001)
4 x 5mm LED (optional)
1 x BC548B
3 x 74LS07
1 x 74HC00
1 x 74HC86
3 x 74HC138
1 x 74LS145
1 x 74HC166
1 x 74HC257
1 x 74HC393
1 x CD4013
1 x CD40106
1 x WDC W65C02S (do not fit anything else into this position other than a W65C02S, it will not work, the pinout is different)
2 x WDC W65C21N (not W65C21S)
1 x WDC W65C22N (not W65C22S)
1 x 62256 32K SRAM 600mil wide (e.g. Alliance AS6C62256 or Cyprus CY62256)
1 x IDT7132 (2K Dual Port RAM)
1 x 27C256 EPROM (font ROM)
1 x 27C020 EPROM (OS/BASIC ROM)
1 x ATmega164P (pre-programmed CRT controller)
4 x MIC2951-03YN (voltage regulator)
1 x 16 MHz Crystal (HC-49/U package)
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CONNECTORS / SWITCHES / SOUNDER
4 x standard tactile switch 12x12mm + keycaps (e.g. Omron B3F-4055)
73 x light touch tactile switch 12x12mm + keycaps (e.g. Omron B3F-4050)
1 x Piezo AC transducer (not a buzzer or any sounder that has internal circuitry)
1 x 48 pin, 5 x 40 pin, 1 x 32 pin, 2 x 28, 6 x 16 pin, 8 x 14 pin, 4 x 8 pin IC sockets (turned pin recommended)
1 x 10 way DIP switch
1 x Phono jack (e.g. CUI RCJ-011 –Digi-Key CP-1400-ND)
1 x 2.1 mm DC Jack
OPTIONAL CONNECTORS
1 x 7 way 0.1" connector for video (pin 2 removed) (Optional if using monitor connection)
2 x 7 way 0.1" connector for video (pin 3 removed) (Optional if using datasette header connection)
1 x 20 way 0.1" connector for keyboard (pin 2 removed) (Optional if using original keyboard)
1 x 9 way D right angled PCB socket (Optional if using RGBI monitor)
NOTES ON IC SELECTION
This board has been designed to work with 74HC series logic and modern WDC 65Cxx series chips. The W65C02S is not pin
compatible with the original NMOS 6502, and so nothing designed to plug into a 6502 socket should be used with the Mini
PET. No PET ROM/RAM, PET Diagnostics, 64K RAM boards or Super PET boards or anything not specifically designed for the
Mini PET will work in that socket.
The chips that act as external driver chips, the 74LS07s used to drive the IEEE-488 bus and the 74LS145 used to drive the
keyboard are not available in 74HC series, but this is not an issue as they all have open collector outputs and are driven
from NMOS outputs on the W65C21N / W65C22N.
ASSEMBLY
Assembly should follow standard procedures for this sort of kit. The following is offered as a guide to the order parts
should be assembled:
1. Single resistors, small capacitors and diodes (support board in a frame and solder from above can be easier than
soldering from below and having to keep flipping the board over)
2. Crystal and Logic ICs (if not fitting sockets, noting orientation of pin 1 circles or indents, all pin 1's are to the left)
3. IC sockets for remaining chips (noting orientation of pin 1 indents all pin 1's to the left hand side of the board)
4. Resistor arrays (noting the dot for pin 1 on the package and the board)
5. Large capacitor, LEDs, buzzer, switches, transistor, connectors etc.
Clean the board after assembly and check for any shorts or dry joints before inserting the ICs. Check jumpers and DIP
switches are set appropriately before powering on.
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DIP SWITCHES
The recommended settings for the DIP switches are highlighted in green, all switches off.
ROM SETTINGS
The Mini PET can run a selection of ROM sets. Commodore ROMs used under license from Amiga Corporation.
Switch 1
Switch 2
40 Column
80 Column
OFF
OFF
MINI PET BASIC 4.1 *
MINI PET BASIC 4.1 *
ON
OFF
BASIC 4.0 #
BASIC 4.0 ǂ
OFF
ON
BASIC 2 #
Simple RAM Test
ON
ON
BASIC 1 #
Full RAM Test
# Original Commodore ROM set
ǂNon-original editor ROM with modified keyboard table
*Custom ROM set
*** COMMODORE BASIC ***
BASIC version 1 was supplied on the first 2001 PETs, and only supported the normal / graphics keyboard. It is
provided for completeness, and would not be recommended for general use due to a number of bugs,
particularly in the IEEE-488 support. The version here is usually referred to a 1r, which patched a few of those
bugs.
### COMMODORE BASIC ###
BASIC 2 was the standard version on the 2001N or 30xx machines, and has support for normal and business
keyboards. (You sometimes see version 1r referred to version 2 and this as version 3. None of these versions are
numbered until 4.0, so use whichever numbers you prefer).
*** COMMODORE BASIC 4.0 ***
BASIC 4 was supplied as an upgrade to 2001N and 30xx machines and also on the 40xx machines. It has
additional disk commands over BASIC 2, and came in 40 or 80 column versions.
*** MINI PET BASIC 4.1 ***
Mini PET BASIC is a custom version of BASIC 4, specifically for the Mini PET. This version has added support for
the sounder and several other improvements normally only found in later CRTC based PETs. There is a built in
automatically activated DOS Wedge (see later). This is the recommended ROM to use. It terms of hardware,
the Mini PET is closest to the original 4032 with the 2001N-32 board, but with many of the tweaks from the
later machines. There is also a built in file browser that can be activated using the NMI / Menu button.
PET TESTER –SIMPLE RAM TEST
PET Tester is a simple test ROM which tests the first 200 bytes of RAM and for each byte, display bfor bad or
gfor good. It alternates this with a screen showing the full character set. This is useful for testing monitors as
it gives you an easy way to get a screen full or characters. This version has been specially modified to alternate
between uppercase / graphics and lowercase / uppercase fonts.
SYSTEM TEST –FULL RAM TEST
This is a special build of the self diagnostics built into the Mini PET ROM sets. This tests system RAM and IO
chips, but will not test the ROM as it is running from its own ROM set. See later for further information on the
Self Test in each ROM set.
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FONT SETTINGS
The character ROM can be changed using switches 3 and 4. The default character ROM font is that found on
BASIC 2 and BASIC 4 PETs. The older BASIC 1 set, and two international sets are provided as alternatives. The
BASIC 1 set can be useful if when running software written to use lowercase mode on the PET 2001, and you
see tHE iNCORRECT cASE. (the BASIC 1 set seems more logical, not sure why they changed it?, but the BASIC
2/4 set should be used in most cases for the compatibility)
Switch 3
Switch 4
Character Set
Charset 1
Charset 2
OFF
OFF
901447-10 (BASIC 2/4)
Uppercase / Graphics
Lowercase / Uppercase
ON
OFF
901447-08 (BASIC 1)
Uppercase / Graphics
Uppercase / Lowercase
OFF
ON
German DIN
Uppercase / Graphics
Lowercase / Uppercase
ON
ON
Japanese
Uppercase / Graphics
Uppercase / Symbols
40/80 COLUMNS
The Mini PET 40/80 can generate a 40 column or an 80 column screen. This is supported on any of its video
outputs, although readability of 80 columns may be limited on smaller screens.
The Mini PET starts in 40 column mode, and the 40/80 button on the keyboard can be used to change mode.
Changing mode also changes the ROM selection, and resets the Mini PET. DIP switch 5 controls the mode that
is selected at power on.
Switch 5
Power on mode
OFF
40 column
ON
80 column
VIDEO OUTPUT
The Mini PET can generate a number of different video formats, selected with switches 6, 7 and 8.
Switch 6
Switch 7
Switch 8
Video output
OFF
OFF
OFF
NTSC Composite Video
ON
OFF
OFF
PAL Composite Video
OFF
ON
OFF
NTSC RGBI / CGA*
ON
ON
OFF
PAL RGBI*
OFF
OFF
ON
9” PET*
ON
OFF
ON
MDA / Hercules
OFF
ON
ON
12” PET/CBM 60 Hz
ON
ON
ON
12” PET/CBM 50 Hz
* Composite video is also generated in these modes (with less appropriate timing and positioning). If you have problems
with the main composite video modes on older monitors, try these modes instead.
TEXT MODE LINE GAPS
Switch 9 is used to enable a two pixel gap between each line of characters, to match 80 column PETs in text
mode. This is not available for NTSC / 60Hz modes of composite, RGBI and the 9” PET monitor (there are no
enough lines available). When selected, there will be a 2 pixel gap between each line in text mode. Graphics
mode remains without gaps in all cases.
Switch 9
Graphics mode
Text mode
OFF
8 Lines
8 Lines
ON
8 Lines
8 Lines + 2 Line Gap
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RGBI COLOUR
Although the Mini PET is a monochrome machine, the RGBI output can be configured as white on black or
green on black using switch 10. Switch 10 has no effect on other modes.
Switch 10
Colour
OFF
Green on black
ON
White on black
BUILT IN SELF TEST
The Mini PET BASIC ROM sets and BASIC 4 ROM sets include a built in self test in ROM at 0xA000.
To activate this, type SYS 40960.
This tests system RAM, ROM and some functionality of the IO chips. The tests will loop after the first pass is
completed, a counter at the bottom of the screen keeps count of the number of cycles and any errors
detected.
RESET BUTTON
The Mini PET reset button controls the video microcontroller. When reset, this provides a clean reset pulse to
the main CPU. The READY LED is off during reset to indicate this.
40/80 BUTTON
When the 40/80 button is pressed, the video mode is toggled between 40 and 80 column vide output. Since
these require different editor ROMs, when the button is pressed, the ROM is changed and the PET is reset to
boot up into the new mode.
NMI BUTTON
The Mini PET has a button wired to the NMI line of the 65C02S processor. On the Mini PET BASIC 4.1 ROM sets,
this is configured to activate the built in file browser. On other versions of BASIC, it will trigger a BASIC warm
start, and bring up the READY prompt.
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DOS WEDGE
The DOS Wedge gives a number of additional shortcut commands to access a disk drive, and is particularly
suited to use with the SD2PET SD card disk drive.
THIS IS BUILT IN TO THE MINI PET BASIC 4.1 ROMS, AND IS AUTOMATICALLY ACTIVATED AT POWER ON,
THERE IS NO NEED TO LOAD FROM DISK OR USE A SYS COMMAND.
If you are not using Mini PET BASIC 4.1, the standard BASIC 2 and BASIC 4 ROM sets also include the wedge as
an option ROM, starting at address 0xE900. To activate those versions, type SYS 59648
COMMANDS:
/filename –load the program filename
↑filename –load and run the program filename
@- Display drive status (at power on, this will show drive version information)
@$ - Display a directory of files (does not overwrite the current program like LOAD”$”,8 does)
@CD:folder –change into a subdirectory on the SD card
@CD:name.d64 –mount a disk image
@CD←–unmount a disk image (if mounted) or change to the parent folder
@C:new=source –create a copy of ‘source’ called ‘new’in the current folder
@R:new=old –rename a file from ‘old’ to ‘new’ in the current folder
@S:filename –scratch (delete) the file filename (you will not be asked for confirmation)
=- toggle font uppercase / lowercase and uppercase / graphics (BASIC 4.1 only)
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FILE BROWSER
A file browser is built into the MINI PET BASIC 4.1 ROM images. This is particularly suited to use with the
SD2PET SD card disk drive.
This can be activated by pressing the NMI / MENU button on the bottom right of the keyboard.
The standard BASIC 4 ROM sets also contain the file browser as an option ROM, at 0x9000. To activate those
versions, type SYS 36864 (or it can be easier to remember to type SYS 9*4096 ).
CONTROLS:
Up / down arrow keys –scroll through the list of files
Left /right arrow keys –scroll up or down one page of files
Return –load the select program change into the selected directory or mount the selected disk image
Clr / Home –Move up one directory, out of a disk image
S –Toggle sorting the list of files alphabetically
D –Cycle through the available drives 8,9,10,11 (if present)
Q –Quit and exit to BASIC
= –toggle font uppercase / lowercase and uppercase / graphics
When a program is selected, the Mini PET will be reset and the program automatically loaded and run.
Alternatively, you can browser to the appropriate folder and / or disk image and then exit to BASIC and load
the program as normal.
When you return to the file browser, the previously selected folder will be preserved or disk image will still be
mounted, to save having to browser back there again.
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MACHINE LANGUAGE MONITOR
All BASIC versions apart from 1 have a built in machine language monitor. This is activated by the break
handler, so is called at any point the BRK instruction is executed. This is code 00, so the monitor can be
launched by jumping to anywhere there is a 00 stored in memory. There is usually a 00 at address 4, so
you can launch the monitor by typing SYS 4.
The monitor is fairly simple, but allows you to view, modify, load or save memory and also to run code.
All addresses must be entered as four hex digits, including any leading zeroes (e.g. M 0094).
All drive IDs must be two hex digits (e.g. L "TEST",08)
All data must be two hex digits (e.g. : 0094 78 D4 00 0D 00 D3 01 FF)
Any errors will either do nothing, or a ?will appear in the line you have typed to indicate the error.
COMMANDS:
M start - display 8 bytes of memory from this address onwards (BASIC 4.1 only)
M start,end - display memory between these addresses, 8 bytes per row
: start data –write 8 bytes to memory, all 8 bytes must be present (hint: cursor up to the output
from a previous Mcommand and overwrite any values you want to change)
R- display register status
; pc irq sr ac xr yr sp –pre set register status at exit (hint: cursor up to output of previous R
command and overwrite any values you want to change)
L "filename",deviceID –load filename from the device into memory at it's saved load address
S "filename",deviceID,start,end+1 - Save memory between addresses to disk
G address –jump to address
X–exit to BASIC
=- toggle font uppercase / lowercase and uppercase / graphics (BASIC 4.1 only)
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SCHEMATICS AND THEORY OF OPERATION
6502 CPU
The heart of the Mini PET is the 6502 processor. Here a WDC W65C02S is used. This is a modern version of the
NMOS 6502. It is not pin compatible and the read / write timing differs, so an original chip cannot be used with
the Mini PET (nor can anything that would plug into the CPU socket).
The main changes here from the PET schematic are the removal of a number of buffer chips. The minimised
chip count within the Mini PET reduces the loading on the address and data bus pins so they are no longer
necessary.
RAM AND ROM
The Mini PET has the full 32K of RAM a PET can support (without paging). This is provided by a single 32K chip,
which is decoded simply using the A15 line of the CPU to occupy 0000-7FFF. The ROM is mapped in 32K chunks
with 1000-7FFF in the ROM being mapped into the address space as 9000-FFFF. Up to 8 sets of 32K ROM
images can be stored in a 27C020 ROM chip and selected using the ROM switches. A17 is controlled by the
40/80 line, as the ROMs need to be different to support 40 or 80 column modes, this avoids ending up with the
wrong ROM for the currently selected screen mode.
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ADDRESS DECODING
The ROM chip provides ROM in two ranges, from 9000 through to E7FF and E900 through to FFFF. This is
implemented here by decoding the video RAM (8000-8FFF) and the IO cut out (E800-E8FF) and selecting the
ROM anywhere that is not RAM, video RAM or IO. (Thinking of it that way made the decoding much simpler).
The timing requirements of the W65C02S are a little tighter than the NMOS 6502, so the read and write signals
are gated with the 1MHz clock.
MEMORY MAP
The 64K address space is split as shown in the following table. The size of the video RAM and its mirrors
changes between 40 and 80 column mode.
Address Range
40 Column Mode
80 Column Mode
Function
0000 - 7FFF
32K
Main RAM
8000 - 83FF
1K
2K
Video RAM
8400 - 8FFF
3K
2K
Mirror of video RAM
9000 - AFFF
8K
Options ROMs
B000 - DFFF
12K
BASIC ROMs
E000 - E7FF
2K
Editor ROM
E800 - E8FF
256 bytes
IO Range
E900 - EFFF
1792 bytes
Editor Extension ROM
F000 - FFFF
4K
Kernal ROM
IO MEMORY MAP
The IO space from E800-E8FF is decoded in the same way as the PET, using address lines A4, A5 and A6 as
select lines. That means that chips will be active anywhere in the E800-E8FF range that their address line is
low, so there are multiple duplicates of each of the IO chips in the range, and addresses that will select
multiple chips. These are the recommended addresses:
Address Range
Chip
Main Function
E810 - E813
PIA #2
Keyboard
E820 - E823
PIA #1
IEEE-488
E840 - E84F
VIA
Userport
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INPUT AND OUTPUT
USERPORT
The userport is provided by a W65C22N VIA (a modern version of the NMOS 6522). This chip also provides
further datasette and IEEE-488 signals. The PET userport is not compatible with Commodore 64 or VIC20
Userport peripherals.
The Jiffy Clock signal is a 50 or 60Hz pulse generated by the video circuitry. This is used to detect when the
screen was not being drawn to allow access to the video RAM without generating 'snow' on the screen, or to
synchronise screen updates to avoid tearing. It is also used to update the real time clock.
Some early PET software achieved a speed increase by changing the input on the VIA chip to an output and
driving that signal low so it would appear it was always safe to draw the screen, at the expense of visible
'snow'. This also has the effect of blocking the clock signal, so a highly complex 'anti killer poke' device (a 470Ω
resistor) is used to stop the timing pulse being blocked if the poke is used.
PIEZO
The PET 2001 had a very good version of Space Invaders, which was not only very playable, but added sound,
with in game instructions to wire an audio amplifier to the CB2 pin on the userport.
Later PET models had a piezo speaker also driven by CB2, but NANDed with PA7 on PIA#2. PA7 (the Diag input)
is read at startup and if held low before booting up, will start the machine code monitor. It can also be used to
disable the onboard piezo, if grounded after boot up.
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IEEE-488 PORT
The IEEE-488 port signals are mainly provided by A W65C21N chip as the PIA#1 (the W65C21N being a modern
version of the NMOS 6520). The remaining signals are provided by PIA#2 and the VIA.
The IEEE-488 bus needs to be driven via open collector outputs. Between the IO chips and the edge connector
there are the bus drivers. On the PET these were mainly MC3446 chips, with a few functions being
implemented using a resistor divider and open collector buffer. On the Mini PET, all the pins have been
implemented using the second option as the original driver chips are no longer in production. Board space is
saved by using four resistor arrays in place of individual resistors. Two of these pull up to 5V, and two pull
down to 0V. The bus then floats high unless driven low by the 74LS07 buffers or an external device. Nothing
should ever drive the bus pins high.
Note do not use the W65C21S or W65C22S versions as they have different
electrical characteristics and totem pole outputs on the IRQ pins rather than the
open collector versions on the NMOS compatible W65C21N and W65C22N.
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DATASETTES
The Mini PET provides two datasette connections, driven by identical circuitry. Previous designs used a simple
voltage regulator based on a Zener diode and a pass transistor. In the PET 2001, a 1.5K resistor and 7.2V Zener
diode form a simple voltage regulator, buffered by a Darlington transistor pair. The two transistor base
voltages mean the Motor Power ends up around 6V. When the /Control line is high, the 2N3904 transistor
turns on and shorts out the Zener diode and disables the output.
This was revised over the years, and the final versions seen in the TED series and the Commodore 128 was
simplified. Gone was the Darlington drive transistor, so the Zener circuit would draw more power. The output
here will be about 6.2V. The control transistor was replaced by a 7406 open collector buffer. This would do
nothing when /Control is low and will short out the Zener when high, disabling the output.
These are not ideal circuits, as it leaves the 470Ω resistors across the 9V rail and drawing close to 20mA when
the datasette is off, and 5mA when it is on. To avoid the power drain when the board is powered down, this
circuit has been replaced.
The new design uses a Micrel MIC2951 voltage regulator IC. This has a shutdown pin, which is pulled high
normally, disabling the supply. When the motor should be running, the 74LS07 gate pulls the shutdown pin
low and enables the 6V motor supply. An LED has been added for each port to show the state of the motor
supply to that port. There are two copies of the above schematic, each port has its own dedicated voltage
regulator and activity LED.
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The output voltage can be set using the ratio of the two feedback resistors, according to the following
equation:
Vref is an internal 1.235V reference, and R2is using the recommended value of 100KΩ. The nearest standard
resistor value to give the output voltage we need is 390KΩ.
That gives an output voltage of around 6V. The value depends on the resistors which have a 5% tolerance
(worst case 5.59V-6.56V). The exact value is not important, as long as the voltage is within range and is
constant during playback. Speed was expect to vary over the life of the drive due to belt stretching, so is
accounted for in software (by timing the header tone at the start). The output is current limited to 150mA, to
avoid motor burnout when rewinding or fast forward is left pressed and the end of the tape is reached.
KEYBOARD
The final IO chip is another W65C21N, as PIA#2. This is mainly involved with the keyboard, but also provides
the remaining datasette and IEEE-488 signals.
The keyboard is a 10x8 matrix, the rows drive the outputs of a 74LS145 4 to 10 line decoder. The columns are
read into the PIA port B, pulled high via a 10K resistor array (although there are internal pullups in the PIA).
The PET had two main keyboard types. The PET 2001 chiclet keyboard and the normal / graphics keyboard
from later machine are electrically compatible, using the same matrix of keys. The business keyboard used
mainly (but not exclusively) on 80 column PETs had a different arrangement of keys with a different matrix.
Even when the same key was present on both keyboards, they were not in the same matrix positions. To
accommodate that, business machines had a different version of the editor ROM which had a different
keyboard lookup tables to cope with the alternate keyboard matrix.
The Mini PET has the normal / graphics keyboard, which is the most commonly used version, and the one most
software was likely to be written for.
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KEYBOARD MATRIX
The keyboard matrix matches that of the PET Chiclet keyboard, with 10 rows and 8 columns
There are four additional buttons on the Mini PET 4080 keyboard which are not part of the normal keyboard
matrix.
NMI BUTTON
The NMI button provides a “None Maskable Interrupt”to the 6502 CPU. This is debounced by the circuit
above. The Schmitt Trigger inverters used are also used for the soft power on circuit, so run at 9V. In normal
operation, the 100nF capacitor will charge up to 9V. The output of the second inverter will be 9V. The diode
stops this being passed on and the NMI pin is pulled to 5V via one of the pullup resistors in the resistor array
by the CPU.
When the NMI button is pressed, the 100nF capacitor is discharged via the 100K resistor. Once the voltage
across this goes past the lower threshold, the first Schmitt trigger output goes high, and the second Schmitt
trigger inverts this and current flows to ground via D3.
When the button is released, the 100nF capacitor starts charging again via the 10K and 100K. The output will
not immediately change because it is a Schmitt trigger; it will not change state until it has charged past a
higher threshold. This should mean a single clean negative pulse is generated each time the button is pressed.
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40/80 BUTTON
The 40/80 button is used to switch between 40 column and 80 column video modes. This has the same
debouncing circuit as the NMI button, but without the second stage of inversion. This gives a clean positive
pulse when the 40/80 button is pressed. This clocks a 4013 flip flop, which is wired as a toggle switch. /Q is
connected to D, so that each time a pulse is received, the output goes to the inverse of the state it was
previously in.
The reset pin of the flip flop is fed by the /5V_On signal generated as part of the soft power on (see later). This
will be high when the Mini PET is in standby, so this will keep the flip flop in reset until the Mini PET is switched
on. When it comes out of reset the Q output will always be low, so the Mini PET will always start in the same
mode.
The diode prevents current flow when the output is high, the signal being pulled high by the 10K resistor. This
signal is optionally inverted again by the 74HC86 XOR gate, depending on a DIP switch setting. This allows the
Mini PET to start in 80 column mode if desired.
RESET BUTTON
The reset button is not debounced like the others as this is handled by the CRTC microcontroller, so a pullup to
5V is all that is required. This uses one resistor which is part of the resistor array pulling other CPU pins high.
The microcontroller generates a Ready signal once it has booted. This is initially held low by a 10K resistor (part
of one of the IEEE-488 resistor arrays) as the microcontroller does not drive any outputs during its reset
period. It is also held low for a further 100mS to generate a reset pulse long enough for the 6502 and IO chips.
The pulse feeds an open collector 74LS07 gate which will pull the 6502 reset line low, but cannot drive it high.
It is pulled high by a 10K resistor (also part of one of the resistor arrays).
The Ready signal also drives an LED which will illuminate after the reset pulse, showing the system is ready.
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POWER BUTTON
The power button again uses the same 40106 debouncing circuit and 4013 flip flop as the 40/80 switch above.
In addition, a second RC circuit is used to generate a power on positive pulse to the set pin. This ensures the 5V
regulators are always in standby when power is applied. The Schmitt trigger is not really necessary here, but
there are six of them in the 40106, so may as well have a clean pulse here as well.
The 4013 and 40106 are CMOS circuits, which are powered from the 9V input to the board. The connector is
wired for centre negative supplies, and there is a 1N4001 diode to block reverse polarity.
The 9V rail only powers these two CMOS ICs, the four 2561 voltage regulators and a power LED, a total of
around 2mA (mainly the LED). Everything else is powered from 5V, which is switched on or off as required.
There are two MIC2951 regulators used to supply the 5V rails. These are the same type as those used for the
datasette motor power, but in this case, they are wired to use an internal resistor divider to give a 5V output.
The first 5V regulator provides the main 5V supply to the Mini PET. The second provides an auxiliary 5V supply.
This powers the composite video output which can draw up to 30mA as it is driving a 75Ω load. The 5V Aux
also provides 5V to the datasette ports. This powers the inverters and/or op amps in the datasette drives
(there were a surprising array of completely different designs over the era of datasette drives), and also
provides power for an SD2PET drive if one is connected.
Yes, I know I could just have used a simple rocker switch to turn the power on
and off, but where’s the fun in that?
