Beckhoff EL3101, Документация

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Mounting and wiring
EL31xx-00xx 79 Version: 5.9
For the single-ended 20 mA inputs the terminal series EL304x, EL305x, EL314x, EL315x, EL317x, EL318x
and EL375x they only apply with regard to technical transferability and also for devices whose analogue
input channels have a common related ground potential (and therefore the channels are not to each other
and/or not to power supply electrically isolated). Herewith an example for an electrically isolated device is the
terminal EL3174-0002.
Technical background
The internal input electronics of the terminals referred to above have the following characteristic (see Fig.
[
}
79] Internal connection diagram for 0/4..20 mA inputs ):
• Differential current measurement, i.e. concrete potential reference is primarily not required.
The system limit applies is the individual terminal EL30xx/EL31xx.
• Current measurement via a 33 Ω shunt per channel, resulting in a maximum voltage drop of 660 mV
via the shunt
• Internal resistor configuration with GND point (A) central to the shunt
The configuration of the resistors is symmetric, such that the potential of (A) is central relative to the
voltage drop via the shunt.
• All channels within the terminal have this GND
int
potential in common.
• the common GND
int
potential (A)
◦ is connected for 1 and 2 channel terminals to a terminal point and not with GND
PC
(power contact).
◦ is connected for 4 channel terminals with GND
PC
• The center point of the voltage drop over the 33 Ω shunt is referred to common mode point (CMP).
According to the technical product data, the maximum permitted U
CM
voltage (common mode) refers to
the potential between the CMP of a channel and the internal GND or the potential between the CMP of
2 channels within a terminal.
It must not exceed the specified limit (typically ±10 or ±35 V).
Accordingly, for multi-channel measurements U
CM
specifications must be followed.
Fig. 57: Internal connection diagram 0/4...20 mA inputs
The block diagram for a 2 channel terminal shows the linked GND points within the terminal ( Fig. [
}
80]
Internal connection for 0/4..20 mA inputs of a EL3xx2 ):