Practical SCADA for Industry David Bailey - FER-a

Figure 2.13
Configuring the input module as a sink or source
SCADA systems, hardware and firmware 29
2.3.5.1 Typical digital input module
Typically the following would be expected of a digital input module:
• 16 digital inputs per module
• Associated LED indicator for each input to indicate current states
• Digital input voltages vary from 110/240 VAC and 12/24/48 VDC
• Optical isolation provided for each digital input
2.3.6 Counter or accumulator digital inputs
There are many applications where a pulse-input module is required – for example from a
metering panel. This can be a contact closure signal or if the pulse frequency is high
enough, solid state relay signals.
Pulse input signals are normally ‘dry contacts’ (i.e. the power is provided from the
RTU power supply rather than the actual pulse source).
The figure below gives the diagram of the counter digital input system. Optical
isolation is useful to minimize the effect of externally generated noise. The size of the
accumulator is important when considering the number of pulses that will be counted,
before transferring the data to another memory location. For example, a 12-bit register
has the capacity for 4096 counts. 16-bit gives 65536 pulses, which could represent 48
minutes @ 20 000 barrels/hour, for example. If these limits are ignored, the classical
problem of the accumulator cycling through zero when full could occur.