How to convert a four-wire switch signal into a two-wire output
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The level switch originally used a two-wire system and transmitted NUMAR signals. The problem now is that the instruments on site need to be converted to a four-wire system, but the wires themselves cannot be changed – those two wires cannot be replaced. I’m wondering if there’s a way to install something at the site that can transmit the 24V power supply and the relay signals through these two wires; then, on the system side, another device could be used to convert those signals into NUMAR format
Or instead of converting it to NUMAR, it’s also possible to convert it into a 2-wire signal of 8/16 mA
It’s just that if you can’t redo the wiring, then re-running the wires becomes simple
On these two wires, it is necessary to supply power to the meter head as well as transmit the (on/off) signal from the relay; so how can power be maintained when the contacts are disconnected?
This post was last edited by hxch150804 on 2021-11-11 08:06. I guess it’s due to an error in the procurement process; the original ones were proximity switches that met the NUMAR standard, but such switches are not readily available in the Chinese market. We’re all used to regular proximity switches, which come in three-wire and four-wire versions. So, the items purchased do not match what’s needed. Whether it’s because the technical specifications weren’t clearly communicated or because the procurement process was flawed, the best solution is to buy a new one and replace it – there are no other options. You won’t allow additional wires to be used. If it’s possible to obtain power supply nearby, then a 24V switching power supply could also be used; the safety barrier can be removed, and the signal can be transmitted via the existing two wires to the control unit, where it enters the DI module. That’s about it for the ideas
Then you might as well simply replace it with a two-wire level transmitter to get a 4-20mA signal
There are 2-line to 4-line converters; I haven’t used them before, so I don’t know exactly how they work or whether they’re suitable.
This post was last edited by qugd on 2021-11-11 at 12:05. Since rewiring is not possible, then why did the two-wire system change to a four-wire system? With two extra wires, isn’t rewiring necessary? We don’t discuss issues in such a dogmatic way, do we? The four-wire active output signal is converted into a passive signal through a signal converter, and this passive signal can then be received by the original DCS card that is powered externally. In this case, wiring must be added; at least power cables need to be added, right? In this way, no changes are required on the DCS side; power supply is added on-site (to power both the instruments and the signal cards), and a signal conversion card is added (this card can also be placed in the cabinet or at the field station).
Using a shielded wire as the common ground GND is acceptable for temporary use. But the wiring inside the cabinet needs to be changed