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4.8
TIMING
The timing between characters can vary greatly, as noted
above. However, once a character has started, it must be
completed at a definite rate, specified by the baud rate
selected. Table 4-3 lists bit times and character times for
the various baud rates.
Table 4-3.
Interface Data Timing
Baud
Time in Milliseconds
Rate
Bit
Character
110
9.09
150
6.67
300
3.33
1200
.83
LOGIC SYMBOL
IT
3
IA~IV
2T
2
2A~2V
3T
I
3A~3V
4T 14
100
66.7
33.3
8.3
4.9
CIRCUIT ELEMENTS
The three main circuit elements are the input level
converter, the USART, and the output level converter.
Information concerning the USART can be found in
Section 4 of the Maintenance Manual.
4.9.1
Input Level Converter
The input level converter is used to change the ±12V
modem signals into +5V or OV for use by the TTL logic
inside the HyTerm. Type 75154 quad line receiver integra-
ted circuits are used. The logic symbol, truth table, and
representative waveforms are shown in Figure 4-5. "Nor-
mal" operation is provided for data inputs, and "fail-safe"
operation is provided for control inputs. Input resistance is
from 3K to 7K ohms, with 5K typical.
A
=
input
Y
=
output
T
=
threshold
control
VCC1
=
+5V (pin 15)
VCC2 not used (pin 16)
GND (pin
8)
TRUTH TABLE
(each receiver)
INPUT A
OUTPUT Y
-12
H(+5)
+12
L(O)
4A~4V
WAVEFORMS
OFF
ON
OFF
+12V
OV
-12V
+5V----
I
OV
---4{~
FAIL-SAFE
OPERATION
(CONTROL
SIGNALS)
MARK
SPACE
NORMAL
OPERATION
(RECEIVED
DATA)
Figure 4-5. Input Converter IC
4-5
MARK
SPACE
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