8-12 Limit Tests and Digital I/O Model 6487 Reference Manual
• External device control — Each digital output can be used as a control switch for
an external device (i.e. relay) circuit. Each output line can sink up to 500mA. Drive
voltage is provided by an external source (+5V to +33V).
• Logic Control — The four digital outputs can be used as inputs to logic devices.
The simplified schematic for the digital outputs is shown in Figure 8-9. Note that this
illustration shows the schematic for one digital output. All four digital output circuits are
identical.
Figure 8-9
Digital I/O port simplified schematic
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 8-9, each of the four digital, open-collector outputs includes a built-in
pull up resistor to +5V. The output transistor is capable of sinking 500mA 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 5 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 500mA.
+5V
1kΩ (Pull-up)
Pin 9 - Digital Ground
Digital Output
Pin 5 - External Voltage Flyback
connection (+5V to +33V)
Digital Output
Flyback Diode
Protection
Diode
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