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The Logic Box provides up to 20 logic outputs. Logic outputs can be used to control relays and indicators
or to drive logic inputs of other devices. The Logic Box logic outputs are ‘open collector’ with an internal
pull-up. Each logic output is an NPN transistor with the collector being the output and the emitter being
ground (see diagram below). When a logic output is turned on, the transistor provides a path for DC current
to flow, lowering the output voltage to below 0.8V. When a logic output is turned off, the internal pull-up
resistor raises the output voltage to near 5V, depending on the output current. To activate external relays
or indicators, an external power supply must be used (see diagram below). The logic output transistors are
rated up to a maximum of 40 VDC and 500 mA per output (24 volt relay coils maximum). However, +12
Volts DC is sufficient power for most applications. When using the logic outputs to control relays, protection
diodes must be used to suppress high voltage transients that are generated when the relays turn off (see
diagram below).
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