The 7-Bit Limitation - Epson FX-80 User Manual

Note: The FX-80 will accept numbers larger than 7 for n
2 ,
but it is
pointless. If you reserve room for more graphics data than the
printer can use in a single print line, the printer will print to the
end of the line, then wait patiently for the rest of the data. If you
don't send enough, it will hang. If you do send the amount that
you specified, the printer graciously accepts the data, tosses it
into the circular file, then moves on to print the next line as if
nothing strange had happened.
The maximum number of dots that can be reserved, then, is:
CHR$(27)"K"CHR$(255)CHR$(7)j
which is 255 dots plus 7 times 256 dots . . . for a grand total of 2047!
But the total width of a line in normal density is only 480 dots. For
now, we'll stick with that density, which means that we won't use
numbers over 480. Later we'll see that the FX-80 does have Graphics
Modes of greater density, and that these modes can require numbers
as large as 1920.
The 7 -Bit Limitation
There always seems to be a catch. In Chapter 6, we mentioned that
some computers have problems sending the upper half of the ASCII
codes (128 to 255). And we said that these computers would have
problems with dot graphics . . . well, this is probably a good time to
explain why. (If you have an Apple II, you might want to sit down and
take a very deep breath. If you are not sure if your computer fits into
this category, see Appendix H.)
It all starts with the computer/printer interface (that figures!). You
see, to send numbers greater than 127 requires 8 active data lines be-
tween the computer and the printer. That's OK except that some com-
puters (Apples in particular) snuff out that eighth bit and allow only 7
lines of data to pass through to the printer. Such a computer automati-
cally subtracts 128 from all numbers in the range 128 to 255, making
the effective range 0 to 127. The number 227, for example, would
arrive at the printer as a 99. Arrrgh!
This means that a whole lot of information cannot be transferred to
the printer even though the printer is ready and willing to accept it.
(Computers sure do have a hard time keeping up with printers, don't
they?) You users with this problem will have to find ways around it.
We'll help whenever possible.
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