Cascading of Input Serializers Boosts Channel Density for Digital Inputs
Frank Dehmelt · 2008
The traditional approach There are several types of digital inputs; the IEC-61131-2-standard defines those most commonly used. Traditionally, digital inputs used discrete components and required a parallel processor interface. Current limitation was achieved by a series of high-power resistors. Resistor-capacitor (RC) filters reduced bouncing of mechanical switches, while a per-channel optocoupler connected to the parallel processor interface. This design, however, requires bulky components, many isolation channels, and a large footprint host controller to allow for the parallel inputs. It also creates significant power dissipation. With a typical resistor chain providing about 2.2 kΩ, the current at the nominal 24 V rises to 11 mA and results in power consumptions of 260 mW or 400 mW at 30 V. Considering that this dissipation may occur simultaneously for all input channels — along with the bulky components and the processor interface — it severely limits channel density. By Frank Dehmelt, Systems Engineer Analog Products