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The Apple 401 Color Plotter

Quick introduction and apology

I don't have access to the manual for the Apple 410, which describes all the commands in detail. Everything below (other than the dip switch settings) was sussed out by ripping the ROM, extracting the self-test script and the command lookup table, and trial and error. If you have access to better documentation, please update this document.

General concepts

The way the 401 interprets a coordinate at any given time depends on the current viewport and window settings. The "viewport" is a rectangle specified in hardware absolute coordinates and defines the physical area in which coordinates are resolved. The "window" defines the virtual coordinates of the drawing area, which are then mapped to the viewport.

All drawing operations are clipped to the current viewport and window; parts of arcs, circles, lines, etc. that would be rendered outside the viewport are not drawn.

If the size of the window has a different aspect ratio than the size of the viewport, the elements drawn will be stretched to fit. You can use this mechanism to draw ellipses or stretched text.

Commands sent to the 401 are terminated by the ASCII "end of text" delimiter, 0x03. A semicolon or newline may also be used in some situations; however it's safest to terminate a text-writing command (PL) with 0x03.


The base units for the viewport (as used in the VP command) are approximately 1 unit = 0.1mm. (In practice, this is slightly off, so I don't recommend drawing any rulers with this machine.)

Serial connection

The 8 DIP switches (SW1-SW8) on the back of the plotter are used to configure the serial interface.

SW1 selects the data bits. ON = 7 bits, OFF = 8 bits.

SW2 selects whether parity is used. ON = no parity bit, OFF = use a parity bit. If parity is selected, SW3 determines the type of parity bit. ON = odd parity, OFF = even parity.

SW4 and SW5 select the number of stop bits:

SW4 SW5 Stop bits
OFF OFF 2 stop bits
ON OFF 1.5 stop bits
OFF ON 1 stop bit
ON ON invalid

SW6, SW7, and SW8 select the baud rate:

SW6 SW7 SW8 Baud
OFF OFF OFF 75 baud
OFF OFF ON 150 baud
OFF ON OFF 300 baud
OFF ON ON 600 baud
ON OFF OFF 1200 baud
ON OFF ON 2400 baud
ON ON OFF 4800 baud
ON ON ON 9600 baud

The 401 uses hardware handshaking. It never returns data to the computer over the serial line and sets DSR when its internal buffer is full. Sending even one byte after the DSR line has been set will corrupt the 401's command buffer and trigger an error. Most USB to RS232 cables will attempt to bundle up several bytes before transmission, so unless you're sure that your cable can handle it I recommend flushing the serial connection after every byte and checking the DSR line yourself.

Command Reference

Move Absolute (MA)


Raise the pen and move the plotter head to the position specified by the x,y coordinates.

Params: 2

Move relative (MR)


Raise the pen and move the plotter head to a position offset x,y from its current position.

Params: 2

Draw absolute (DA)


Lower the pen and draw a line from the current position to the position specified by the x,y coordinates. If more than one coordinate pair is specified, continues to draw lines to the subsequent points as well.

Params: 2(+2*n)

Draw relative (DR)


Lower the pen and draw a line from the current position to the position offset x,y for its current position. If more than one coordinate pair is specified, continues to draw lines to the subsequent offset points as well.

Params: 2(+2*n)

Circle (CA)


Lower the pen and draw a circle of radius r centered at x,y. If x,y are not specified the circle will be centered at the current position.

Params: 1 or 3

Letter Initialize (LI)


Reset the text format.

Letter size (LS)


Sets the font size to s (?? in what units? relative to what system of coordinates? ??)

Params: 1

Letter rotation (LR)


Sets the angle to draw subsequent text at θ degrees clockwise.

Params: 1

Print letters (PL)


Lower the pen and draw the specified text at the current position, using the currently active letter size and rotation settings.

Params : 1 (string)

Pen select (PS)


Select pen i (where i is in the range 1-4).

Params: 1

Line type (LT)


Select the line style for subsequent draw operations. p is a value in the range 1-9:

n Line style
0 Custom pattern (see UL)
1 Solid
2 Dotted
3 Short dash
4 Medium dash
5 Long dash
6 Line w/ dots
7 Line w/ one short dash
8 Line w/ two dots
9 Line w/ two short dashes

k specifies the repeat length in mesurement units. The default is 100, but the value will persist.

Params: 1 or 2


Create a custom pattern for use with subsequent LT0 operations.

d specifies a draw length, m specifies a move length.

X-axis ticks (XT)


Draw an X axis with tick marks as specified. There are two "styles" s of input available, which determine how the distance d parameter is interpreted.

parameter function
s style
d distance
n number of tick marks
a length of tick mark above the axis
b length of tick mark below the axis

The s parameter determines how the d parameter is interpreted and whether the first tick mark (at the "0" point on the axis) is drawn or not:

style parameter description
0 d is distance between ticks
1 d is total length of axis
2 d is distance between ticks, first tick mark is not drawn
3 d is total length of axis, first tick mark is not drawn

Params: 5

Y-axis ticks (YT)


Draw a Y axis with tick marks as specified. The parameters are handled in the same manner as the XT command (see above).

Params: 5

Clear/Reset (CH)


Raise the pen and move the plotter head to its original position; usually used at the end of a plot.

Params: 0

Point mark (PM)


Draw a marker at the current position. The marker type is specified by the t parameter. The marker size is determined by the font size setting specified by the LS command; consider the markers as just special characters.

The various marker shapes are illustrated in the images below.

Image of PM1-5 Image of PM6-10 Image of PM11-15

Params: 1

Pen velocity (PV)


Sets the drawing velocity v, on a scale of 1-10, where 1 is slowest, 10 is fastest. The default speed is 10.

Params: 1

Slant lettering (SL)


Slants subsequent text by an angle specified by θ degrees.

Params: 1

Viewport (VP)


Specifies the viewport area, in absolute coordinates, to draw in. See the introduction for an explanation of how the viewport is used. x1,y1 specify the upper left hand corner of the viewport; x2,y2 specify the lower right hand of the viewport.

Params: 4

Window (WD)


The window into which to draw. See the introduction for an explanation of how the window is used. x1,y1 specify the upper left hand corner of the window; x2,y2 specify the lower right hand of the window.

Params: 4

Reset (RS)


Resets the error bit. If the error LED is lit, it should be extinguished.

RS will apparently accept some parameters, but it's unknown if this does anything.

Params: 0 or 1

Arc (AC)


Draws a circular arc of radius r from θ1 to θ2. If x,y are specified, they determined the center of the arc's circle. If they are not specified, the current position is considered to be the center of the arc's circle.

After the arc is drawn, the endpoint of the arc is considered to be the new position.

Params: 3 or 5

Error action (IM)


Selecting error action

a specifies the mode: 0 = turn on error light; 64 = reject commands ; 192 = write message.

a Mode
0 Turn on error light
64 Reject command
192 Write error message

Do note that "write message" does not mean to send a response over the serial port; it means that it will literally write the error message out on the plotter.

b specifies the errors to filter. Default is 31.

a Error type
1 Command not recognized
2 Numbers outside range
4 Not enough numbers for command
8 Bad syntax
16 Pen would go outside window

Size Paper (SP)


Sets the current paper size. This is essentially a preset "VP" command. The table below of drawing area sizes is approximate, based on measuring a rectangle drawn at the extent of the entire viewport.

n measurement (in mm)
0 241 x 175
1 178 x 240
2 394 x 240
3 282 x 280(?)
4 259 x 170
5 171 x 280(?)
6 326 x 217
7 218 x 142
8 143 x 217

Unknown commands

PK - ??? - no params/arb (Not in user reference)

LF - ??? - 1 param ? 0-9 Letter Font