7I76 14
OPERATION
FIELD OUTPUT CHARACTERISTICS
MAXIMUM PER CHIP CURRENT
Because of thermal limitations there is a maximum per driver chip total current of
1.4 amps continuous. Each driver chip connects to 8 sequential outputs. If this limit is
exceeded, the driver chip may go into thermal shutdown.
VOLTAGE CLAMPS
The output driver chips used on the 7I76 have built in Zener diode clamps to clamp
inductive turn-off (fly-back) spikes. This means that flyback diodes are not normally
required on small (less than 60 mA) inductive loads. If high current inductive loads are
switched or inductive loads are switched at high frequencies, they must have
flyback diodes to limit power dissipation in the 7I76's driver chips.
FIELD INPUT CHARACTERISTICS
The 7I76 field inputs have a nominal input resistance of 10K Ohms to field power
ground. 7I76 inputs sense positive input voltages above a preset threshold. For best
general purpose use, default input threshold is 50% of the field power supply voltage with
10% hysteresis. That is with a 24V field voltage an input must be brought to 60% of 24V
= 14.4V to be sensed as high and then brought to 40% of 24V = 9.6V to be sensed as low.
These accurate thresholds and hysteresis allow high speed field signal detection while
maintaining excellent noise immunity.
WHY SINKING INPUTS
7I76 field inputs are of the sinking type. That is, external power must be applied to
the input to register as activated. This mode was chosen so that accidental grounding of
an input will not register as an activated input.
It is suggested that inputs like limit switches use normally closed switches with one
switch leg connected to field power and the other to the 7I76 input pin, so the normal
machine state (not at limits) is to have the inputs activated. This way, a open switch wire
or wire shorted to ground will cause a detectable machine fault.
ANALOG INPUTS
All field input pins are capable of reading the input voltage. These are not highly
accurate or high resolution but can be useful for things like potentiometer inputs. Input
resolution is 8 bits and input full scale value is 36.3V. Accuracy is +-5%. Software process
data modes 1 and 2 allow reading the analog voltage on inputs 0 through 3, in addition to
the 32 digital bit inputs.
FIELD VOLTAGE MONITORING
The 7I76 monitors the field voltage and can send this information to the host in
some modes. If separate VIN is supplied to the 7I76, the 7I76 can report loss of field
voltage to the host.
Содержание 7I76
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