One of the reasons for the high noise immunity of Dupline is the high CMRR of the Dupline output stage in the Channel Generator. As long as noise is induced common mode on the two Dupline wires, the differential voltage signal on the Dupline wires remains unaffected and the system operates fault-free, even in heavy noise environments. To ensure common mode operation, the 2 wires should always be floating (e.g. no earth connection of the Dupline common wire) and they should follow the same path.
The Channel Generator output stage is shown in appendix B. Please note that the output stage has a low impedance at high level where it supplies power to the bus-powered units, and a high impedance at the low level (2.1 VDC) where the transmitters send their pull-down signal and thereby draw current. Note also that this specific type is IS approved in Australia, that’s the reason for all the zener diodes.
After having started a new channel (pulse address) by making a negative transition, the Channel Generator must detect if there is an active transmitter on this address, in other words if there is a transmitter pulling down the voltage level from 2.1 VDC to 0.7 VDC. The Channel Generator waits for 90 ms before it starts sampling, because then the “ringings” from reflections have died out. The remaining 154 ms of the low-period, the Channel Generator samples the Dupline Signal constantly, and reaches app. 500 samples. The detection level between “0” and “1” is app. 1.5 VDC. If more than 40 % of the samples are below the detection level, the channel generator recognizes that there are one or more transmitters active on the address. The 40% tolerance is needed because no transmitter has an active pull-down the entire low-period. If e.g. the transmitter is 4 km away, there is a certain delay in the transmit pulse due to travel time in the cable, so in that case there might be only app. 50 % of the samples below the detection level.
Transmission of analog signals
Transmission of analogue signals on Dupline is performed by converting the analogue input signal to a digital 11-bit format, after which one Dupline pulse address is used for the transmission of each bit. In order to prevent a few analogue signals from occupying all 128 addresses, an additional layer of multiplexing can be implemented, where 16 analogue values can be transmitted on the same 16 pulse-addresses. Each analogue value simply gets access to the bus at every 16th pulse-train. In this way, up to 112 analogue signals can be transmitted on one Dupline network with a response time of 16 x 136 ms = 2.2 s.
Dupline key characteristics
As earlier mentioned, Dupline is different from other fieldbuses at sensor/actuator level, mainly due to the basic multiplexing principle and the low carrier frequency of 1 kHz. In the following, the key characteristics of Dupline will be listed and brought into application perspective.
· Transmission over long distances
Due the low carrier frequency of 1 kHz and a low current flow on the transmission line, Dupline can be used to transmit signals over distances, up to 10 km without signal Repeaters. This has been a key-feature in applications within water distribution-, sewage-, and irrigation systems, where central control and monitoring of remote pump stations (level, flow, pressure, start/stop of pumps, alarms etc.) or switching of valves with true feedback of valve-position is often required. Dupline has also been used in many railway systems for gathering of alarm- or track shifter position feedback-signals and for control of track shifter heating. Some of these applications have a 15 km long transmission line.
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