![]() But if you have 4-wire magmeters, or other line/mains-powered transmitters, both sides of the loop are trying to source the 4-20 mA. This is not a problem when all inputs are from loop-powered transmitters. However, there is a significant trade-off - the card must power all the loops. Some DCS manufacturers offer lower cost 4-20 mA input cards. Some of the popular splitter applications (other than area isolation) include custody transfer and isolating validated systems from non-validated systems in the biopharmaceutical market.įigure 2: A signal splitter takes one signal input and provides two or more identical outputs proportional to the input. The advantage of the splitter approach (versus area isolation) is that you can disconnect either control system for maintenance without affecting the signal going to the other system. In Figure 2, the transmitter is provided a 24 V power supply, called ‘transmitter excitation’, by the splitter. All inputs and outputs, and power, are isolated from one another. Fortunately, there is an alternative solution using a ‘splitter’.Ī signal splitter is a 4-wire signal isolator/converter that takes one signal input and provides two or more identical outputs proportional to the input. Disconnecting the ESD input removes power from the transmitter. This architecture is very common, but there is a weakness to this design: if you had to disconnect the input to the ESD, then the DCS also loses the signal. ![]() Maintenance can disconnect the input to the DCS without impacting the signal going to the ESD. The output loop-powered isolator (with a passive input) isolates the primary loop from the secondary loop. In Figure 1, the primary system is an ESD. One system is declared the primary system and it powers the transmitter.įigure 1: A signal isolator can be used to share a process variable with two different systems. One solution to sharing a variable with two systems is to use a single signal isolator (Figure 1). Why? You would not want a series loop if you had to disconnect the input at one system for maintenance purposes, because both systems would lose the signal. Generally it is unacceptable to create one series loop between the transmitter and two systems. It could be two control systems, one emergency shutdown system (ESD) and one control system, one DCS and a data acquisition system, and other numerous combinations. It is quite commonplace to share process signals between two different systems. In this part, we will go beyond the basics to look at some more advanced applications.Īrea isolation (divert and protect signals) In Part 1 of this article we covered some of the basics of signal isolation and conversion. By using the right signal interface instruments, in the right ways, potential problems can be easily avoided, well before they boil over.
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