Understand the basic composition of line numbers
PLC control cabinet line numbers are usually composed of letters and numbers. The letter part may represent the type of signal, functional area or device category, and the number part is used to distinguish different lines. For example, in “DI1”, “DI” may stand for digital input, and “1” represents the first digital input line.
Understanding line numbers by signal type
Digital input (DI) line numbers
These lines are mainly used to connect external input devices such as switches and sensors. The line number usually starts with “DI” and is followed by a number. The number is usually arranged according to the physical location of the input point on the PLC module or the order in the control logic. For example, a proximity switch connected to the digital input point of the PLC may have a line number of “DI3”, indicating that it is connected to the third input point of the PLC digital input module.
Digital output (DO) line number
Used to connect actuators such as relays, contactors, indicator lights, etc. The line number often starts with “DO” and is also numbered. For example, the line number of a contactor coil that controls the start of a motor may be “DO7”, indicating that it is controlled by the seventh output point of the PLC digital output module.
Analog Input (AI) Line Number
Used to connect analog signals, such as continuously changing signals output by temperature sensors, pressure sensors, etc. The line number generally starts with “AI” and is followed by a number. These numbers may be related to the order of the analog input channels. For example, the signal of a temperature sensor is connected to the analog input channel of the PLC, and its line number may be “AI2”, indicating that it is connected to the second analog input channel.
Analog Output (AO) Line Number
Used to output analog signals to control devices, such as the frequency setting of the frequency converter, the opening control of the regulating valve, etc. The line number starts with “AO” and is numbered, and the number corresponds to the analog output channel. For example, the line number of the analog output signal used to control the frequency of the frequency converter may be “AO1”, which is output from the first analog output channel of the PLC.
Recognize line numbers based on the functional area of the equipment
In large PLC control cabinets, line numbers may be divided according to different functional areas of the equipment for easy maintenance and management. For example, in a control cabinet of an automated production line, for the motor control line on the conveyor belt, the line number may start with “CONVEYOR_MOTOR”, and then add specific numbers or letters to distinguish different motor control lines, such as “CONVEYOR_MOTOR1_DO3” represents the control line of the first motor on the conveyor belt, connected to the digital output point of the PLC.
Refer to the control cabinet wiring diagram and PLC program comments
The wiring diagram is an important tool for recognizing line numbers. It clearly marks the line number, the starting and ending points of the line, and the connected devices. At the same time, the comments in the PLC program may also mention the devices and functions corresponding to the line numbers. By comparing the wiring diagram and program comments, you can accurately understand the meaning of the line number. For example, in the comments of the PLC program, there may be “//DO5 controls the opening of the unloading valve”, so you can know that the line with the line number “DO5” is used to control the unloading valve.
