Siemens Simatic S7-400 - Manual (Page 276)

Analog Modules
5-60
S7-400, M7-400 Programmable Controllers Module Specifications
A5E00069467-07
5.14 Connecting Loads/Actuators to Voltage Outputs
Connecting Loads to a Voltage Output
Connecting loads to a voltage output is possible both in a four-conductor and a
two-conductor connection.
Note
The necessary connecting cables, which result from the potential connection of the
analog output module, are not drawn in the figures shown below.
In other words, you must continue to take note of and implement Section 5.13 with
its generally applicable information for connecting loads and actuators.
Abbreviations and Mnemonics Used in the Figures Below
The abbreviations and mnemonics used in the figures below have the following
meanings:
Q
V
: Analog output voltage
S +: Detector lead (positive)
S –: Detector lead (negative)
M
ANA
: Reference potential of analog circuit
R
L
: Load impedance
L +: Terminal for 24 VDC supply voltage
M : Ground terminal
U
ISO
: Potential difference between M
ANA
and chassis ground
Four-Conductor Connection of Loads to a Voltage Output
A high accuracy at the load can be achieved through the four-conductor
connection. You must therefore connect the sensor leads (S– and S+) directly to
the load. The voltage is thus measured and corrected directly at the load.
Problems or a voltage drop can result in a potential difference between the sensor
lead S– and the reference circuit of the analog circuit MANA. This potential
difference (U
CM
) must not exceed the permissible value. If the permissible potential
difference is exceeded, the accuracy of the analog signal is impaired.