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Q23 Expansion Module Page 33
TECHNICAL NOTES
This section describes the technical aspects of interfacing various devices and user designed interfaces to the Q23
Module.
Digital Inputs
The Q23 Module has 8 digital input channels. These are available at a 9 way connector labelled
‘DIGITAL IN’
on
the front panel of the Q23 Module. The digital input channels are numbered 1 to 8. The terminal labelled
‘G’
is a
system ground connection. Each digital input channel has an associated green channel status LED that turns on
when that channel is in the ON state.
❖
A digital input channel is in the ON state when the corresponding input terminal on the digital inputs
connector is grounded, i.e. connected to the
‘G’
terminal on the digital inputs connector. The digital input
channel’s associated green status LED will be ON.
❖
A digital input channel is in the OFF state when the corresponding input terminal on the digital inputs
connector is open circuit or at the supply voltage potential. The digital inp
ut channel’s associated green
status LED will be OFF.
Circuit details of a Digital Input Channel.
The above circuit diagram shows the input circuit configuration of a digital input channel on the Q23 Module. The
digital input circuit is opto isolated from the main processor circuitry to prevent noise and voltage spikes entering.
The 4K7 resistor limits the current through the opto isolator while the 10nF capacitor filters noise from the input line.
The BAV99 prevents input voltages spikes above the supply voltage from exceeding the reverse breakdown
voltage of the opto isolator’s LED.
When the module’s supply voltage is 12 volts the digital input’s ON state current to ground will be approximately
2.5mA, and with a module supply voltage of 24 volts the d
igital input’s ON current to ground will be approximately
5mA.
The digital input can be switched with a variety of devices. These can include mechanical switches, solid state
switches, transistors, transducers, etc.
The current transfer ratio of the H11A817D opto isolators is greater than 100% therefore allowing for a 50% safety
margin a minimum of 1mA must flow through the opto isolator’s LED to ensure reliable switching. This represents
a maximum external switching device resistance of 5K with a module supply voltage of 12 volts and 10k with a
module supply voltage of 24 volts.
Note
The input voltage to any digital input should not be allowed to go above the module’s power supply voltage.
The digital input channels are sampled every 100mS by the processor therefore a given channel input state must
be maintained for greater than 100mS for the state to be recognized.
Digital input channels No’s. 1, 2 and 3 can be configured for use as a high speed pulse counting and accumulation
channels. Please contact
QTech Data Systems Limited
for application specific details.
The 8 green status LED’s associated with each of the digital input channels are driven by the central processor.
Each individual status LED will turn on to indicate that an ON state has been recognized by the central processor
for the associated digital input channel.