Are there analog two-input one-output signal distributors? ?

First, the process requirements need to be clarified, and then ways to control and achieve them can be discussed.

I checked the product manuals for P+F and MTL, but there is no such two-in-one outlet distributor. If the PLC has available AI terminals, it is possible to connect from the DCS to the PLC; then an additional DO can be added from the DCS to serve as a switch for DCS/PLC control, which is connected to the PLC’s DI terminals, where the program assignments are handled internally by the PLC.

Safety barriers with one input and two outputs exist; those with two inputs and one output have not been heard of. Both PLCs and DCS can be used to control variable-frequency fans. Options include: 1. Using the control AO from the DCS to feed into the AI of the PLC; 2. Processing the two control signals within the PLC before sending out control commands; 3. The signal processing can be done manually through the PLC’s operation interface ； Based on the control requirements, high or low selection output of the two control signals is performed, etc.

This means that two signals (AO and DO) need to be sent from the DCS to the PLC: AO is used for the control signal, while DO is used for switching, which is equivalent to having a remote switch in the DCS. Make the appropriate configuration in the PLC. Is that what you mean?

Yes. If there’s no need to perform switching within the DCS, then check whether the PLC has a touchscreen or a host computer; switching can be done on the PLC side instead, which saves one signal.

The inverter is switched to remote mode; the DCS AO signal is then sent to the inverter. This is how it should be understood.

With two inputs and one output, a choice must be made from a control perspective; the logical relationship needs to be clarified from a technical standpoint. It is necessary to decide whether to choose one of the two options after making a comparison, to calculate an average value, or to perform some mathematical or logical operation before producing the output. In principle, such a circuit board can be designed, but it is essential to have a defined logical relationship for the output calculation. Send these two signals to the DCS or data processing unit; then, after programming it for operation or making a selection, simply output one of the signals.

The idea is good, but there’s a misunderstanding. It’s not about operating the PLC and DCS simultaneously, nor is it a matter of two outputs and one input. In my understanding, it should be control from the PLC side plus control from the DCS side. If that’s the case, the PLC device should have an AI interface, and by connecting that AI to the DCS’s AO ports, all outputs can be managed through the PLC’s output ports. I’m not sure if this is correct

The signal selector function is to choose one from two signals as the output signal; it comes in two types: high-value selector and low-value selector. The methods for controlling signal output include automatic high selection, automatic low selection, and control via external digital/level signals to select one of the input signals for output. The selected signal is then isolated from the input signals and power supply before being output. http://yunrun.com.cn/upload/201806/05/201806051604503283.png Features of the signal selector ◆ High precision, high reliability, high stability, and long service life ◆ There are four modes for signal output control: automatic high selection/low selection, digital control, and high/low level control ◆ Complete isolation between inputs, outputs, and power supply ◆ Low power consumption, minimal temperature drift, and strong resistance to interference Signal selector: yunrun.com.cn/product/1954.html Examples of signal selector models YR9033A-00-G-AH: Both input signals are 4–20mA; the signal output is controlled via automatic high selection, resulting in one 4–20mA output signal. YR9033A-00-G-AL: Both input signals are 4–20mA; the signal output is controlled via automatic low selection, resulting in one 4–20mA output signal. YR9033A-00-G-DI: Both input signals are 4–20mA; the signal output is controlled via digital control, resulting in one 4–20mA output signal. YR9033A-00-G-LS: Both input signals are 4–20mA; the signal output is controlled via high/low level control, resulting in one 4–20mA output signal. http://yunrun.com.cn/upload/201805/04/201805042220587856.png Technical specifications of the signal selector ◆ Product type: Two inputs / one output ◆ Control modes (specified by the customer): Automatic high selection, automatic low selection, digital control output, and high/low level control output ◆ Hysteresis: ≤0.5% ◆ Response time: 0.05s (0–90%, typical value) ◆ Ripple: <10mArms ◆ Accuracy: 0.2% ◆ Temperature coefficient: 50ppm/K measured at a 100Ω load ◆ Dielectric strength: 100MΩ at 500VDC; withstands 2000VAC for one minute ◆ Operating temperature: -25~+85°C ◆ Operating humidity: <90%RH (no condensation) ◆ Power supply: DC24V ±10% ◆ Power consumption: <2W ◆ Housing material: Nylon PA.66 ◆ Weight: Approximately 200 grams ◆ Dimensions: 16.5×106×112mm (width × height × depth, in mm)

The approach is wrong. It should be possible to connect the FM signal from the DCS directly to the AI interface of the skid-mounted equipment; a switch should be provided on either the skid-mounted equipment or the DCS to determine which signal – that from the DCS or that from the skid-mounted equipment – is valid