I am developing a program for the Siemens S7-300 PLC that involves an analog output ranging from 4 to 20 mA with a temperature range of 0 to 80°C. Can you provide guidance on how to achieve this?
Siemens offers a powerful UNSCALE function that you might want to explore. Have you located it? Discover more about using the UNSCALE feature, along with related scaling functions, by visiting these helpful resources: [How to Use FC105 Scale and FC106](https://program-plc.blogspot.be/2015/09/how-to-use-fc105-scale-and-fc106.html), [UNSCALE FC106 Support](https://support.industry.siemens.com/tf/WW/en/posts/unscale-fc106/101080?page=0&pageSize=10), and [Scale and UNSCALE Guide](https://support.industry.siemens.com/tf/WW/en/posts/scale-and-unscale/31698?page=0&pageSize=10).
Are you referring to an analog output that displays temperature readings? Or did you mean to inquire about an analog input?
I have several analog outputs designed to transmit "temperature" or "pressure" readings. Although it might seem inefficient in terms of processing resources and wiring, many individuals connect gauges directly to the PLC's analog outputs rather than using a serial loop. This approach can be particularly advantageous in hazardous environments, where it may not be necessary for gauges to possess intrinsic safety features. Instead of investing in intrinsically safe equipment for the gauge, you can simply connect any suitable device as a gauge and allow the PLC to control it effectively.
Cardosocea stated, "An effective approach to avoid wasting resources is when the instrument is situated in a hazardous zone, where the gauge does not necessarily need to comply with safety standards. Rather than investing in intrinsically safe equipment for the gauge, you can opt to connect any device as a gauge and allow the PLC to manage it." However, this claim is inaccurate. Instrumentation placed in hazardous areas must be specifically rated for those conditions and must be installed following the necessary hazardous area regulations. While 4-20mA signals are low power, this characteristic alone does not exempt the system from the requirement for hazardous area-rated instrumentation and proper installation procedures.
The user cardosocea explains that a potential benefit arises when an instrument is located in a hazardous zone, while the gauge itself is not. Instead of investing in intrinsically safe equipment for the gauge, one can simply connect a standard gauge to the system, allowing the PLC (Programmable Logic Controller) to operate it. I’ve highlighted the crucial part of this statement for clarity. This approach is logical, right? If the instrument is situated outside a hazardous area and the signal path does not traverse through one, the variable being displayed—even if it originates from a hazardous environment—doesn’t pose an issue. For instance, if an instrument is placed in a hazardous zone and is equipped with a compliant box and a properly configured intrinsically safe circuit, there may still be challenges. The loop’s power supply might lack sufficient capacity to support both the gauge and instrument on the same circuit. Alternatively, if the gauge is installed in a safe area, like the wall of an office, it doesn't need to meet explosion-proof (Ex rated) standards. In such scenarios, an independent PLC output can effectively drive the gauge, bypassing any hazardous area regulations. This method ensures safety while optimizing functionality and compliance with industry standards.
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