Automation has become the backbone of almost all industries. PLC collects data from the field and processes it as pre-programmed and then produces one or more outputs. The data collected from the field is considered to be the input to the PLC, and the data passed from the PLC to the field is considered to be its output. The PLC has complex electronic blocks to collect this data and produce outputs.
Switching Signal
A switching signal is a digital signal with only two states, usually used to indicate “on” (1) and “off” (0). These signals are widely used in industrial control systems and automation. It can efficiently transmit information about the state of a device, such as when a machine starts or stops, when a valve opens or closes, and so on. Switching signals are characterized by a clear state that can be directly recognized by a PLC (Programmable Logic Controller) or DCS (Distributed Control System), and are an important means of achieving process control.
Characteristics:
Binary state: switching signals can only represent two states, simple and clear, easy to handle.
High reliability: Since there are only two states, the switching signal is not easy to be disturbed during transmission and processing, and has high reliability.
Easy logic operation: switching signals can be processed by logic operation (e.g. with, or, not, etc.), which is suitable to be used in decision-making logic of control system.
DI
The digital inputs (DI) to the controller include status switches and inputs from other control, monitoring and alarm systems.The DI will indicate the state of the device as ON or OFF, and can therefore indicate a fault or failure state. For the 24Vdc control module, 0V indicates OFF status and 24V indicates ON status. Digital inputs record the status of inputs from devices such as tank level switches or door close switches.
Use:
1. pressurization unit: commercial unvented heating systems will be equipped with a pressurization unit with high and low pressure limit switches that will indicate a fault condition if either state is detected
2. tank level switches: a limit switch is an electromechanical device that detects the presence or absence of an object when a physical force is applied to it. High level switches are used to indicate if the tank is overflowing due to a malfunctioning fill valve, while low level switches are used to indicate an interruption in the water supply to the tank.
DO
De digital outputs (DO) of a BMS substation are used to turn devices on or off (enable or disable devices), usually through relays or contactors. The digital output signal contains the output status of a device such as a motor starter or panel signal light. The digital output will change or maintain the state of a device by performing a momentary or continuous switch to start/stop a pump, fan, etc. DO can be used to operate two-position solenoid valves (on/off), relays, indicator lights, etc.
Use:
1. boiler enable: boilers equipped with manufacturer’s proprietary controls (available in multi-module installations to provide burner ignition modulation and cascade control) will receive activation signals from the BMS only.
2. Exhaust Fan Enablement: Toilet exhaust fans can be activated by motion sensors that provide an enable/disable signal.
Digital signals DI and DO are often used in industrial control, automated production and intelligent management, etc. DI and DO are used in monitoring and control for real-time monitoring of equipment status and execution of commands by means of precise input and output control. Reasonable application of DI and DO signals can completely improve the working efficiency of automation systems and also guarantee the stability of equipment operation.