Chapter Six: Remote Operation Di
ital Lo
ic Control
91
The I/O board contains 6 type 74HC digital
outputs
, each having the capacity to drive one
standard TTL load. The approximate time constant of the outputs are 2.5 microseconds. Each
output includes a 240 ohms series resistor to protect it against line surges and spikes.
Additionally, there is a 0.01 µFD capacitor connected to the chassis.
µFD
From output port
(74HC377)
R
240 ohms
I/O Connector
0.01
µFD
0.01
Di
ital Ground
= Internal resistance
of output driver
25 ohms sinkin
/
50 ohms sourcin
)
int
R
int
Figure 7: I/O Board Digital Output Circuitry
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