Special I/Os
Inputs and outputs can also be used in special configurations. Common special applications are accumulating points, pulse width modulated (PWM) signals, multiplexed PWM signals and tri-state or floating points.

Accumulating Points
Accumulating points are typically associated with inputs and are special in that during each scan the controller adds the input point value to the accumulated value. Accumulating points may have either analog or digital input.

One of the most common applications of accumulating points is with turbine-type flow meters, which generate a pulse or change of input state with each rotation of the turbine rotor. The total number of pulses is proportional to the volume of fluid passing through the meter. The number of pulses per unit of time is proportional to the flow rate during that time interval. Accumulating points are also used to determine energy quantities, such as kilowatt-hours from a power sensor and MBtu from flow and temperature sensors.

Pulse Width Modulated (PWM)
Pulse width modulated signals are based on the amount of time a digital output circuit is closed over a fixed time base. This amount of time can range from 0 to 100 percent of the time base, providing an analog value for each time period that represents the time base of the signal. Common time bases are 2.85 seconds, 5.2 seconds, 12.85 seconds and 25.6 seconds.

Multiplexed PWM
A single pulse width modulated digital output is sometimes used to transmit analog values to multiple analog output devices. Many processes are possible. One scheme is to send an "attention" pulse, which is a pulse of longer duration than the time base. This pulse causes all of the analog devices to look for a selection signal to follow. A "select" pulse is then transmitted with duration less than the time base. Each analog device that is multiplexed looks for a fixed unique range of "select" pulse width. The device that receives the select pulse then looks for another pulse whose width corresponds to its updated analog value. When the pulse is received, the selected analog device updates its output to the new value and the process is repeated.

The time base of the PWM signal and the number of devices multiplexed on one signal limit the updating of multiplexed output values. Multiplexed outputs may not be suitable for control applications requiring rapid responses to system changes.

Tri-State or Floating Point
A Tri-State signal consists of two digital signals used together to provide three commands. This type of signal is commonly used to operate a damper or valve actuator in a modulating fashion, but may also be used with a transducer to generate an analog signal. If both digital outputs are "off", the actuator does not move. Output 1 "on" will cause movement in one direction; output 2 "on" will cause movement in the other direction. The fourth possible signal (both outputs "on") is not used in tri-state operation. The concept was initially developed to allow electric controls consisting of single pole, double throw switches with a center-off position to control actuators in a modulating fashion. Modulating operation is achieved by this action because the actuators being controlled drive slowly so the change in position is proportional to the amount of time the output remains energized.

Continue to Chapter 2