9-8 Limits and Digital I/O Model 2701 User’s Manual
Sink mode — controlling external devices
Each output can be operated from an external supply (voltage range from +5V to +33V
applied through the external device being driven). The high current sink capacity of the
output driver allows direct control of relays, solenoids, and lamps (no additional circuitry
needed).
As shown in Figure 9-3, each of the digital, open-collector outputs includes a built-in pull
up resistor connected to +5V. The output transistor is capable of sinking 250mA from
voltages up to +33V. Each output channel contains a fly-back diode for protection when
switching inductive loads (such as a low power solenoid or relay coils). To use these fly-
back diodes, connect the external supply voltage to pin 7 of the digital I/O port. Make sure
the external supply voltage is between +5V and +33V and the current required by the
device does not exceed 250mA.
CAUTION On pin 7, do not exceed +33V. For the output lines, do not exceed the
maximum sink current. The maximum sink current for an output line
is 250mA. Exceeding these limits may cause damage to the instrument
that is not covered by the warranty.
An externally powered relay connected to the digital output port is shown in Figure 9-4.
Other externally powered devices can be similarly connected by replacing the relay with
the device. When the output line is pulled low (0V), the output transistor sinks current
through the external device. In the high state, the output transistor is off (transistor switch
open). This interrupts current flow through the external device.
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