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counter is not advanced and the terminal responds
with a NAK. The NAK response resets only the input
counter. Both records of the retransmission must
be received correctly for the second (error record)
to be outputted. Receipt of the first recorrl correctly
causes the input counter to advance to 1. Since the
output counter is also equal to 1, no transfer will
occur. With both records received correctly, the
input counter will have advanced to 2, a transfer will
occur, and normal operation follows.
Buffer Checking
Buffer checking depends on the transmission code:
• With the USASCII code, an odd vertical redun-
dancy check (VR
C)
is performed on all data
entering or leaving the buffers.
• With the Six-Bit Transcode, an odd vertical redun-
dancy check (VR C) bit is generated for each char-
acter that enters the line buffer or the I/O buffer.
As data is read from the buffers, it is checked
for parity.
• When the EBCDIC code is used, a modified longi-
tudinal redundancy check (LR C) is performed. As
a record enters a buffer, an odd/even bit count is
generated. Following the ETX, ETB, or US char-
acter, a character of all 0 (zero) bits is written
if
the number of bits in the record is even, or a
character of alII bits if the number of bits in the
record is odd. As the record is read from the
buffers, the odd/even bit count is generated again
and compared with the all-O or all-l character.
Failure to compare is indicated as a buffer parity
check.
In
addition to the above checking, the line
buffer is checked when transmitting and receiving
to determine that the buffer capacity has not been
exceeded. Also, a check is made to determine that
no more than two records per block are received on
a standard machine, or no more than seven records
on a machine equipped with the Multiple-Record
Transmission special feature. These conditions are
indicated as overrun. The I/O buffer is checked to
determine that address 170 has not been reached.
If address 170 has been used, it is indicated as an
overrun condition.
PRINTER
The 2780 print unit (see Figure 1) provides printed
output for the terminal when operating on-line, and
enables a card reader-to-printer listing operation
to be performed when operating off-line. The print
unit is similar in appearance and operation to the
20
IBM 1443 Printer used in data processing system
configurations. The maximum rated speed of the
printer is 300 lpm (lines per minute). However, the
actual speed depends on the communication facilities
used and the number of characters in the character
set being used:
39-character set--300 lpm
47/52-character set--240 lpm
63-character set--200 lpm
Figure 7 shows the formula for calculating the termi-
nal throughput rate in either lines per minute or cards
per minute. Figures 13 and 14 show the approximate
printer throughput rate for both dial and leased lines
by code.
The basic 2780 terminal has a maximum print
speed of 240 lpm and a print cycle of 250 ms. The
actual print time required is 198 ms--the remaining
52 ms being used to move the paper forms and
restore the print hammers. Skips or line spaces
greater than two single spaces take an additional
10 ms per line. Normal forms movement takes place
during the last 24 ms of each printing cycle, and two
lines can be spaced during this period of time. Each
additional line requires another 10 ms. This speed
is equivalent to approximately 15 inches per second.
The standard printer can print 80 positions on one
line. Both 120 and 144 positions are also available
as special features (see "Special Features" section
in this publication). All print positions will print all
characters of the character set being used. Char-
acter density is 10 per inch, thereby providing a
printing line of 8, 12, or 14.4 inches.
NOTE: All data to be printed that exceeds the capacity of the
printer (80, 120, or 144 print positions) will be lost with no
indication unless the record contains more than 170 characters,
in which case the terminal will give an overrun error indication.
If the Horizontal Format special feature is used, the 80, 120,
or 144 print positions include the print positions involved in the
tab functions.
All characters of the character set are mounted
on a type bar that travels horizontally on the paper.
The type bar ensures that each character of the
character set successively passes each print pOSition.
To print, a magnet releases a spring-loaded hammer
at the proper time so that the desired character is
pressed against the ribbon and paper. Characters to
be printed are checked for parity while in the buffer.
A parity-check error at the receiving terminal
results in an EaT (end-of-transmission) character
being encoded in place of the normal block-checking
response. This may result in a partially-printed
line depending upon when the error was detected.
Character Sets
The character sets available for the print unit depend
on the type of transmission code used with the system.
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