Teletype SA110 Description And Operation page 2

SECTION 578-200-100
Figure 1 - SAll0 Parity Failure Detector (Cover Removed)
parity bit is the only bit in error. With par-
ity checking alone, 10 percent of all error
indications can be expected to be false.
(b) The significant distortion check detects
characters with one or more bits having
50 percent or greater distortion (a bit half
as long as it should be• or shorter). Some
erroneous charade rs with correct parity will
be detected by this check, but its effective-
ness depends on the noise characteristics of
the transmission channel used. In one case,
90 percent of all transmission errors were
detected by the SAl 10 when this check was
combined with parity checking. However, the
number of false error indications increased
considerably; in the same case, 30 to 35 per-
cent of the error alarms were false. The
increase in unnecessary retransmissions
resulting from false error alarms is the price
of more effective error detection using com-
bined parity checking and significant distor-
tion checking .
Page 2
(c) Error rates, error detection percent -
ages, and false error indication percent-
ages are a function of the noise characteristics
of a channel during the time of transmission.
These characteristics can vary significantly
from channel to channel, and from time to
time on the same channel.
1. 06 Errors may be indicated in the following
ways:
(1)
By
generating a ''break" signal (spacing
line) on a half-duplex circuit (33 and 35
type receivers) or a break on the reverse
channel lead (Type 2 DATASPEED receivers).
(2)
By
lighting an indicator lamp which
flashes on each error or error burst.
(3)
By
lighting an indicator lamp which
lights on the first error and blinks off
briefly on following errors until reset.
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