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Graphics
Note that if a parameter byte is lost or garbled in transmission, all following end points will be
improperly read. To minimize data errors caused by the loss ofa data byte, any command can be
used
to
reset the parameter count and restore synchronization. Nops (z), redundant format, or
pen down commands can also be inserted to insure synchronization if necessary.
Graphics sequences can extend indefinitely. In general, longer sequences are preferred as they
minimize the overhead necessary for a plot sequence. Ec • p <for ma t >must be sent for each
series of vectors. As the sequence length decreases, the percentage of preamble characters
increases, and the vector drawing rate goes down. The worst possible case would be to send
Ec • p <f ormat >for each vector; approximately 50/ of the characters sent would be overhead,
reducing vector speed by a factor of 2.
The general format for an absolute plotting sequence is:
Ec • pia <byte1) <byte2> <byte3> <byte4> (z)
<byte1> <byte2> <byte3> <byte4> . . .
. . . <byte1> <byte2> <byte3> <byte4>Z (or any upper case command)
Each block of
4
bytes specifies a single point. The
«
i " indicates that absolute format is to be
used. The «a" raises the pen before it is moved to the point specified by the next four bytes and
lowered. A
HOP
(z)
can be added to insure synchronization, ifnecessary. The lowered pen draws
a vector as it moves to the next point, and so on. The upper case «2" terminates the plotting
sequence.
The vector end point formats allow the pen to be moved completely off the screen (an absolute
coordinate of 1000, for example). The actual range of the pen position can be from -16384 to
16383. Vectors that extend beyond the screen are clipped so that they will not wrap around.
Pen Control
The terminal uses the concept ofa «pen" in drawing vector data. The pen can be lifted or lowered
as well as be positioned using absolute or relative coordinates. For example, the pen is lifted,
moved to a starting coordinate, lowered and moved to an endpoint to draw a line. The pen is
initially down and positioned at absolute coordinates 0,0 following power up or a full reset. If
the pen is raised and coordinates given, the pen is moved to the coordinates and then lowered.
The pen is normally left in the down position.
Raise Pen:
Lower Pen:
Ec •
p a
Ec •
p
b
This command lowers the imaginary plotting pen to the drawing surface. The pen draws a line
as it moves from its current position.
Use Cursor As Next Data Point
Ec • pc
This command causes the position of the graphics cursor to be used as the next data point.
D-27
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