Redundant Analog Input Module For Direct Current Measurements - Siemens SIMATIC S7-400H Manual

Using I/O on the S7-400H

Redundant analog input module for direct current measurements

The following applies to the analog input module circuit shown in the diagram
above:
• Suitable encoder types are: Active 4-wire and passive 2-wire measurement
transducers with an output range of +/–20 mA, 0...20 mA and 4...20 mA. 2-wire
measurement transducers are connected to an external auxiliary voltage.
• The "wire break" diagnostic function supports only the input range 4...20 mA. All
other unipolar or bipolar ranges are not supported.
• Suitable diodes: The BZX85 or 1N47..A (Zener diodes 1.3W) series, of the
voltage range specified in the module data. For the selection of other elements
you should note that the off-state forward current is as low as possible.
• In this type of circuit and when using the named diodes, a principal
measurement error is caused by the off-state forward current of maximum 1∝A.
In the 20-mA range with 16-bit resolution, this value results in an error < 2 bits.
In the circuit above, various analog inputs return an additional error. For further
information, refer to the marginal conditions. These and all other error values
specified in the module manual form a cumulative error.
• The 4-wire measurement transducers used must be capable of driving the load
of the above circuit. For further information, refer to the marginal conditions.
• Please note that the installation of a Zener diode circuit forms a heavy influence
on the voltage requirements of 2-wire measurement transducers, which is why
the necessary input voltages are specified in the technical data of the various
modules. The internal supply (
determines the minimum supply voltage and is calculated as follows:
L+ > U
in–2Dr
This module is not suitable for the use in the circuit shown above:
AI 8x12bit
7-28
) of the measurement transducer (
IS
+ U
IS–MT
6ES7 331-7KF02-0AB0
Automation System S7-400H Fault-tolerant Systems
)
MT
A5E00068197-07