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Since page size is defined in inches, and not in lines, it is
possible to have more or less than 126 lines per page. If
there are more than 126 lines per page, it is possible to tab
to only the first 126 lines; if there are less than 126 lines
per page, it is possible to tab beyond the end of the page.
For example, if there are 66 lines per page and a tab to line
76 is executed, the paper will be positioned to line 10 of
the next page. Once a page boundary is crossed, all
subsequent paper movement is in relation to line 1 of the
new page.
When a forms tractor or bottom-feed printer is used,
absolute vertical tabs of more than two or three lines in the
negative direction should not be attempted, because the
forms tractor cannot push the paper back down. Uneven
print lines or paper jams will result.
3.6.4
Other Control Functions
The remain ing control functions have to do with
sounding the audible alarm and preventing print buffer
overrun.
3.6.4.1
SOUND AUDIBLE ALARM (BEL)
Each time the BEL character is rece ived it sounds the
audible alarm for 1/4 second. This is normally used by the
host computer to get the operator's attention for some
reason or other. This alarm indication is the same as that
given for error cond itions (see 2.3.4), except that the
ERROR lamp is not illuminated.
3.6.4.2
PREVENTING
BUFFER
OVERRUN
(ACK,
ETX, NUL, DEL)
All data to be printed, whether from the keyboard or
the data link, is stored in a print buffer (memory) until the
printer is able to accept it. This buffer can store up to 158
characters, but if data is received faster than it can be
printed, a buffer "OVERRUN" can still occur. This
normally does not occur at 10 or 15 characters per second,
but it can occur at 30 or 120 cps if suitable precautions are
not observed. These precautions traditionally include NUL
or DEL codes transmitted as "pad" or timefill characters
after carriage returns, vertical tabs, form feeds, etc. (NU L
and DEL are always discarded when received over the data
link: they are not loaded into the print buffer.) Backward
Printing can also be utilized to eliminate carriage returns.
However, when timefill characters are used, a sufficient
number of them must always be used in order to fully
insure correct operation in worst case situations, and this
results in lower overall throughput.
3-8
In order to speed up throughput and still guarantee
against buffer overrun, the ETX/ACK sequence can be used
in a variety of different configurations. The HyTerm
transmits an ACK character each time an ETX is removed
from the print buffer. A typical example is as follows:
(1)
Host computer transmits fixed-length, 79-character
messages. Each message is ended with an ETX.
(2)
As the HyTerm is printing out the first message, it
may already be receiving the second. As it withdraws
the ETX (end of the fi rst message) from its pri nt
buffer, it transmits an ACK to the host computer,
which interprets this as a signal to begin sending the
third message.
(3)
So long as no more than two messages (158 char-
acters, enough to fill the print buffer) are outstanding
at anyone time, data transfer can continue un inter-
rupted, resulting in highly efficient use of the data
link.
(4)
If two messages are outstanding, the host computer
must delay transmission of the next message until an
ACK is received from the HyTerm.
3.7
MOTION MINIMIZATION
The motion min imization feature resolves a sequence of
carriage- and paper-movement commands to one single
operation which moves the print head and paper via the
shortest path to the position which would result from
perform ing each individual operation. The purpose of th is
feature is to reduce the time taken by the printer in
executing these commands. Therefore, any motion com-
mand received while the print buffer is empty will be
executed immediately, and will not be accumulated;
motion is accumulated only while the printer is busy
performing other functions. Likewise, if motion has been
accumulated and no data has arrived for a period of time,
allowing the print buffer to become empty, the accumula-
tion process will terminate and all motion accumulated to
that point will be executed. The net result is faster
throughput.
Reception of any printing character terminates the
motion accumulation and causes all motion accumulated to
be executed before the character is printed. Reception of
any command that would cause paper movement across a
page boundary terminates vertical accumulation and causes
any vertical motion accumulated to be executed; it does
not affect horizontal motion accumulation.
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