Learn the DCS system point calculation method easily

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The number of points in a DCS control system is usually provided by the design institute; it represents the sum of the I/O points for the instrumentation field, those for the electrical field, and the communication points between the DCS system and other systems. Accurately counting the electrical and instrumentation I/O points can prevent coordination issues within the electrical and instrumentation teams, while an accurate count of DCS points provides a basis for the user entity to decide on the brand and cost of the DCS system. DCS systems typically involve automated control functions such as thermal monitoring, analog control, sequential control, and logical control. The number of points in a DCS system is determined by counting the AI analog input points, AO analog output points, DI digital input points, DO digital output points, as well as the points used for communication between the DCS and other systems. Below are the methods for calculating the number of points in DCS systems for the instrumentation and electrical engineering fields: 1. How to calculate the AI input points in a DCS system: AI refers to the analog input signals that enter the DCS system or PLC. The AI input signals that can be directly entered into the DCS system from the field include thermocouples (thermocouples with J, K, T, N, E, R, S, and B calibration scales), thermal resistance signals (with Cu50, Cu100, Pt100, and Pt50 calibration scales), standard current signals (4-20mA, 0-20mA), standard voltage signals (1-5V, 0-5V, and 0-10V), and pulse signals ； If other forms of signals need to be fed into the DCS system, signal conversion devices such as signal isolators, current transmitters, and voltage transmitters are used to convert those signals into 4-20mA or 1-5V before feeding them into the DCS system. (1) Count of thermocouple AI input points: Each individual assembled thermocouple or each individual armored thermocouple is counted as 1 AI point ； For dual-element assembled thermocouples or dual-element armored thermocouples, the temperatures of the two sensors at the same measurement point need to be calculated as 2 AI points when displayed in the DCS system; if only one temperature of that measurement point is shown, it is counted as 1 AI point ； Single-multipoint thermocouples or multipoint thermocouples are often used to monitor the temperature at different locations of the same measurement point; the number of measurement points on the thermocouple determines the number of AI inputs required. (2) Statistics on the number of thermistor AI input points: The method for counting the number of thermistor AI input points is the same as that used for counting the number of thermocouple AI input points. (3) Statistics on standard current and voltage AI input points: For each 4-20mA, 0-2mA, 0-5V, 1-5V, or 0-10V signal sent to the DCS system, one AI point is calculated; simultaneously, the range of that input signal is also recorded. For two-wire transmitters (including temperature transmitters, pressure transmitters, level transmitters, flow transmitters, etc.), since they rely on DC24V power supply, it is advisable to count the AI points separately to facilitate wiring in the DCS system. Special note: On-site instruments such as pressure gauges, bimetallic thermometers, and glass rotor flowmeters displayed at the site are not included in the DCS system’s point count. 2. How to calculate the number of AO output points in a DCS system? AO refers to the analog output signals sent by a DCS system or PLC to control devices on-site. AO outputs generally come in five types: 4-20mA, 0-20mA, 0-5V, 1-5V, and 0-10V. 4-20mA is the most commonly used AO output in DCS systems. AO outputs are typically connected to devices such as electric actuators, pneumatic actuators, frequency converters, power regulators, and industrial control modules. Usually, each controlled device corresponds to one AO output, and the number of AO output points is equal to the number of controlled devices. 3. How to calculate the number of DI input points in a DCS system? DI refers to digital input signals that enter the DCS system or PLC. DI inputs must be passive contacts, or TTL or CMOS level signals. Once these DI signals enter the DCS system or PLC, they are usually connected to a supply voltage of DC24V or DC48V for reading purposes. In the instrumentation field, DI inputs typically come from the alarm contacts of instruments such as electrical contact pressure gauges, electrical contact bimetallic thermometers, electrical contact level gauges, level switches, flow switches, flame detectors, and other similar devices; each alarm contact counts as one DI input when connected to the DCS system. 4. How to calculate the number of DO output points in a DCS system? DO refers to the digital output signals sent by a DCS system or PLC to control field devices; these signals are usually transmitted to electrical devices with different voltage levels through intermediate relays. The DO outputs in the instrumentation field are commonly used to control external indicator lights, solenoid valves, audio-visual alarms, electrical control systems, multi-turn electric actuators, contactors, and other devices. The DCS system requires different numbers of DO output points to control various devices. Below are the typical I/O point counts for the controlled devices: (1) On-off electric actuators: 4–20mA feedback for valve position accounts for 1 AI input point; control for forward/reverse valve operation accounts for 2 DO output points; signals indicating when the valve is fully open or fully closed account for 2 DI input points; and signals indicating faults related to excessive opening/closing torque account for 2 DI input points. (2) Switch-type multi-turn electric actuator (AC380V power supply): 4–20mA feedback from each actuator’s valve position is used to generate 1 AI input point (no AI point is generated if there is no feedback signal); control for the valve to rotate forward/reverse results in 2 DO output points; detection of when the valve reaches the open/closed position (via limit switches) generates 2 DI input points; fault signals related to excessive torque during opening/closing of the actuator also result in 2 DI input points. (3) Adjustable electric actuator: 1 AI input point is generated for the valve position feedback of each actuator, 1 AO output point is generated for the valve control signal, and 1 AI input point is generated for the actuator fault alarm signal (fault alarms are common in intelligent electric actuators; if there is no fault alarm signal, no AI point is calculated). (4) Adjustable multi-turn electric actuator: 1 AI input point is generated for the valve position feedback of each actuator, 1 AO output point is generated for the 4-20mA control signal of the actuator, 1 DO output point is generated for the ESD emergency control signal (ESD emergency control signals are common in intelligent multi-turn electric actuators; if this function is not available, no DO point is calculated), and 2 DI input points are generated for the over-opening torque/over-closing torque alarm signals. (5) Inverter: 1 AI input point for frequency feedback calculation per inverter; 1 AO output point for frequency setpoint signal calculation; 1 DO output point for start/stop command calculation; 1 DI input point for inverter fault alarm calculation; 1 DO output point for fault reset calculation; 1 DI input point for inverter operating status calculation. If the frequency converter is connected to the DCS system via a communication method, only 1 communication point needs to be calculated; no other points are required. (6) If devices such as solenoids, indicator lights, and contactors are connected to the DCS system, 1 DO output is calculated for each device (if multiple devices share the same control signal, this is usually achieved by using additional relay contacts, and only 1 DO output needs to be calculated). 5. How to calculate the number of points in a DCS system for electrical applications? (1) Number of points in a DCS system for conventional electrical control: The simplest motor control circuit requires 2 DI inputs and 1 DO output. Each circuit’s operating status (from the contactor’s auxiliary contacts) generates 1 DI input point; the start/stop control signal (connected to the contactor coil) generates 1 DO output point; and the fault signal (from the thermal relay or motor protector overload signal) generates 1 DI input point. For motor circuits that require current display and local/remote control, in addition to 2 DI and 1 DO, the current signal (from the current transmitter) requires 0 to 3 AI input points; low-power motors usually do not need current monitoring, so no such AI input points are required ； For high-power three-phase motors, the number of AI input points required is equal to the number of phase currents that need to be displayed on the DCS. Each 0-5A current signal must be converted into a 4-20mA signal using a current transmitter before being sent to the DCS; a maximum of 3 such points are allowed ； If the motor needs to be controlled from multiple locations, one DI input is required for the location selection switch. For easier understanding, the diagram below shows the electrical secondary control schematic for control from the GGD electrical cabinet, the on-site operation panel, and the DCS system. Function description of the motor control secondary circuit: The stop buttons on the electrical cabinet and the on-site operation panel can stop the motor from operating in any situation ； The control location selection switch allows selection between \"cabinet control\", \"on-site control\", and \"DCS control\"; the start button located at the corresponding position of the switch can be used to start the motor ； When the selector switch is set to “DCS control”, motor start/stop operations can only be performed on the DCS system. Electrical component description: In the secondary schematic diagram, 1SS refers to the stop button on the electrical cabinet, while 1SS1 refers to the stop button on the field operation panel ； 1SB is the start button on the electrical cabinet, and 1SB1 is the start button on the field operation panel ； DO is the start/stop control output contact for the DCS system ； 1HR5 is the power indicator light ； 1HR is the operation indicator light on the electrical cabinet, while 1HR1 is the operation indicator light on the field control panel ； 1HG is the stop indicator light on the electrical cabinet, and 1HR1 is the stop indicator light on the field operation panel ； 1KK is the operational toggle switch ； 1KH is a thermal relay ； 1KM is the contactor ； 1KA is an intermediate relay ； 1FU is the secondary circuit fuse. (2) Number of DCS system points for electrical control of reduced-voltage starting: One DI input point is generated for the signal indicating that the motor in each reduced-voltage starting circuit is operating at full voltage (coming from the auxiliary contact of the main contactor 1KM1); one DO output point is generated for the DCS start/stop control signal (connected to the contactor coil); one DI input point is generated for the electrical fault signal (coming from the thermal relay or motor protector overload signal); and three AI input points are generated for the motor current signals (from the three-phase current transmitters for motors A, B, and C) ； If the motor needs to be controlled from multiple locations, the status of the selection switch at the control location is used (when selecting DCS system control), resulting in 1 DI input point. For easier understanding, the following diagram shows the control schematic for the reduced-voltage starting circuit, which is controlled from three locations: an electrical cabinet, a field operation box, and the DCS system. (3) Number of points in the DCS system controlled by frequency converters: Each operating status signal from the frequency converter (coming from the contacts of intermediate relays) generates 1 DI input point; the start/stop control signals from the DCS system (connected to the coils of intermediate relays) generate 1 DO output point; the frequency converter fault signals (coming from the frequency converter) generate 1 DI input point; the fault reset signal generates 1 DO output point; and the frequency feedback signal generates 1 AI input point ； The variable frequency setpoint signal calculates 1 point for the AO output. (4) Number of DCS points for motor forward and reverse control: 2 points for the motor’s forward operation state/reverse operation state (from the contactor auxiliary contacts) to calculate D1 inputs; 2 points for forward fault/reverse fault signals (from the thermal relay) to calculate DI inputs; 2 points for forward control/reverse control (connected to the contactor coil) to calculate DO outputs; up to 3 points for motor current feedback signals as AI inputs (this point is not calculated if there is no current feedback). (5) The DCS system is compatible with PLC functions; for complex logical controls, the number of I/O points in the DCS system is calculated based on the requirements of the actual project (the calculation method is the same as that used for PLC points), and no detailed explanation is provided here. Through the above calculation method for DCS system points, it is possible to quickly determine the actual number of hardware points required for the DCS system. When configuring the DCS system, system redundancy also needs to be taken into account; typically, 20% redundancy is added to the actual number of I/O points required by the user. Related readings: Grounding principles and methods for DCS systems; Guidelines for selecting DCS systems; Technical documents | Selection of DCS distributed control systems as well as measures for troubleshooting startup and shutdown issues; Basic principles of DCS control systems; Instrument Control Expert provides step-by-step guidance on how to install and use instruments in DCS systems