COMMODORE  PLUS/4 INFORMATION     1-15-05

I own a Commodore Plus/4 Computer, and was fortunate enough to get it
with a working power supply and two books: The Users' Manual and the
Integrated Software Manual. Most people who own one of those "orphan"
computers inherit the keyboard only, and are hard pressed to make it work
with little or no information and usually no peripherals or software. The
Plus/4 came after the VIC-20 and the C-64 (about 1984), and is not
software compatible with either of those computers, although it does share
some hardware comparability. I decided to do a little digging inside my
Plus/4 and share whatever information I could. As a repair tech, I'm more
into hardware than software or programming and I have no software, but I
hope this info is helpful to other owners of Plus/4s. Here we go.... 

     The Plus/4 comes up in BASIC V3.5 with usable memory a little over
"60K bytes free" (The C-64 comes up with about 40K). Commands are similar
to the C-128 (DIRECTORY, HEADER, DLOAD, and COPY, for example) and such
things as reprogramming Function keys is possible. Obviously the Plus/4
requires software written for its version of BASIC, and memory locations
are specific to this model. For example, screen memory goes from location
3072 to 4071 (40X25 screen) and color memory from 2048 to 3047. BASIC
starts at $8000. 
     The Plus/4 has several built-in programs stored on internal ROMs.
There is a Word Processor, File Manager, Spreadsheet, Graphics, and a ML
Monitor called TEDMON. The internal programs are initiated from the BASIC
startup screen by pressing the F1 key. A SYS number is displayed... hit
. This starts the word processor. Hold down the C= key and press C
to activate the Command mode, indicated by the "W>" prompt at the bottom
of the screen. Type TF (To the File manager) and  to go to the " 
database". Similarly, typing TC (To the spreadsheet Command) and TW (To
the Word processor) will move to those ROM programs. Most commands are
two-letter combinations such as: LF (Load File) followed by .
TEDMON is started from BASIC by typing MONITOR & , and exited by
typing X . GRAPHICS is part of the spreadsheet and allows
manipulation of charts and graphs. The GRAPHIC command entered from BASIC
allows direct entry of HiRes graphics characters, drawing shapes and
controlling color. Because they are integrated, data can be moved across
the internal ROM programs. Obviously this is a simplified overview and is
intended only to get you started if you don't have the books. To properly
run these programs, you really need the Integrated Software Manual. Try for more information and schematics.
     The Plus/4 has three voices (or rather two programmable tone
generators and a noise generator). The first two output square waveforms
and # three outputs noise for special effects. As an example, to enable 
and play voice #2... 
   10 VOL 7: REM sets volume level (0 is off) 
   20 SOUND2,800,360: REM turns on #2, sets freq (about
      500Hz), sets the on-time (about 2 seconds).

     The RF (TV) output is the same as on the C-64. An RCA jack on the
right side of the Plus/4 will connect, through an adaptor or switchbox, to
a standard Color TV set. There is a switch near the connector to select
channel 3 or 4. The color screen shows a 40 column display with border. 
     The Plus/4 serial port is identical to the C-64 and will work with
drives such as the 1541 and 1551 as well as CBM printers. 
     The User port (RS-232) appears the same as the C-64 and uses 5 volt 
TTL level input/output signals. It will work with CBM modems such as the 
1660 with no modifications, but like the C-64, requires an adaptor to
interface IBM type (Hayes compatable) modems.
     The Memory Expansion Port (Cartridge port) is different from a C-64.
The connector looks similar, but has a total of 50 pins compared to the
C-64 which has 44. The SFS-481 Disk Drive and specific Plus/4 cartridges
were made to interface this parallel port. I was unable to find the 
pinout, but I believe the Programmers Manual has that information.
     The cassette port is a 7 pin Mini-DIN type similar to the joystick
ports but with a slightly different pin spacing. It works with the 1531
Datasette, and the pinout is as follows It is a mini-DIN, unlike the 
inline edge type connector of the C64 cassette interface. The pinout is
shown facing the rear of the computer. It is important to get these pins 
correct if you are going to make an interface cable for your computer. A 
"mirror image" error in wiring will reverse power and ground connections 
and will likely damage your datassette and/or computer. Note the blank 
(unconnected) pin in the center of the connector. 

                       _____ sense
      ground-----7   6   5-----write
      read -----4   [ ]   3----- motor drive

        +5VDC -----2    1----- ground

     The power supply for the Plus/4 is the same as the C-64, namely the
non-repairable "black brick" which outputs two voltages: 5 volts DC at
about 1 Amp, and 9 volts AC. Although some Plus/4s were built with the
round DIN type connector, most have a square 4 pin type, similar in
appearance to the C-128 but not compatible. The pinout is as follows 
(viewed facing computer): 
     9VAC ------|-0     0-|------ 9VAC
                |         |
    +5VDC ------|-0     0-|------ Ground

Note: you can use a standard power connector jack removed from a spare
C64 motherboard to replace this unavailable square DIN type. It fits in 
the same place on the Plus/4 board without modification. Then you can use 
a standard C64 power supply, but as rare as the Plus/4 is, I would 
recommend a more reliable power pack than the old Commodore black brick. 

     The video port (8 pin DIN) is nearly the same as the later models of
C-64, and monitors such as the 1702 (separated Y and C) will work with no
modifications. As with the C-64, a standard 5 pin DIN plug will also work
for monitors that accept only composite video and audio. The only
difference in the Plus/4 is -two- audio outputs, whereas the C-64 has an
audio -input- on one of those pins. The unloaded level on the second audio
output line is over 1 volt (PP). The lower level output connects 
internally to the RF modulator as in the C-64. The pinout is as follows 
(facing rear of computer): 

                          ________ Chroma Out
         NC ------8      |      7------ NC
  Audio Out ------5      6      1--- Luminance & Sync Out 
             ------3           4------ Comp Video Out
  2nd Audio Out          2
     (.5VPP)              \________ Ground

The joystick connectors are a departure from the "standard" 9 pin "D"
types that are used on the C-64 and the C-128. The 8 pin joy port
connectors on the Plus/4 are called Mini-DIN and are the same as the
joystick ports on the Commodore C16 and C116. Similar DIN plugs were 
used for Apple (MAC) computers on their printer interface cables. 

The following is a program to test the joystick ports of the Plus/4. 

    10 PRINT JOY (1)
    20 PRINT JOY (2)
    30 GOTO10

This BASIC program scrolls a number on the screen. The numeric value
depends on which direction the joystick is moved. Each position of the
joystick (or a pressed Fire button) represents a contact closure
(grounding one or more pins of the joy port). For example, when the
joystick is moved to the left, the number on the screen will go to 7. 
If no switches are closed, the output number will be zero... 

Output   Function
------   ------------------
  1      UP 
  2      UP AND RIGHT
  3      RIGHT
  5      DOWN
  6      DOWN AND LEFT
  7      LEFT
  8      LEFT AND UP
 128     FIRE (JOYSTICK #1)
 135     FIRE (JOYSTICK #2) 

Here is the pinout for the mini-DIN Joystick ports. It is shown facing 
the computer. Note the offset center pin! 
                                _____ Ground
                               |  ______ Right
                               | |
            not used ------8   7 | 6------ Fire
              +5VDC ------5     4   3------ Left   
                  Down ------2   1------ Up   

     Since Plus/4 joysticks are so rare, standard CBM and Atari types
could be modified by cutting off the 9 pin "D" connector and installing
the 8 pin Mini-DIN male plug, or just changing the cable for one with the
Mini-DIN. Be careful with the wiring! There is a +5 volt source on those
joyports. You could damage ICs in the computer if you accidently short the
5 volts to ground. I would put a 100 ohm resistor on the ground-return
side of the switches just to be safe. 
     The larger electronic parts houses sell Mini-DIN plugs and cables. 
One source here in Seattle is Westlake Electronic Supply 1-800-523-8677
FAX: (206) 628-0508. They carry the GC (General Cement) Electronics line.
The 8 pin joystick mini-DIN plug is GC #33-938BU and the 7 pin cassette 
port plug is GC #33-937BU. A less expensive 8 pin is GC #45-1160, and 7 
pin is GC #45-1159. Two other sources for DIN plugs are: Jameco 
Electronics 1-800-831-4242 ( and JDR Microdevices 
1-800-538-5000. Jameco sells a printer cable (Apple MAC) with 8 pin 
mini-DINs on each end: part #10604. That 6ft. cable could be cut in half 
to make two 3 ft. joystick cables.

The proprietary IC chips used in the Plus/4 are apparently not used in
any other CBM equipment, and I know of no sources for these parts. The
rest of the chips are mostly LS TTL devices available anywhere. Here is
a list of the the major ICs:

U1         7360/8360   40 pin     TED
U2         7501/8501   40 pin     CPU  (microprocessor)
U3         8551        28 pin     ACIA (RS-232 interface)
U5 & U27   6529B       20 pin     I/O  (single port interface)
U19        251641-02   28 pin     PLA 
U23        318006-01   28 pin     BASIC ROM 
U24        318005-05   28 pin     Kernal ROM 
U25        317055-01   28 pin     function low ROM  \___ 3+1
U26        317054-01   28 pin     function high ROM /    software

Ray Carlsen
CARLSEN ELECTRONICS... A leader in trailing-edge technology. 


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