Siemens SIMATIC S7-400H System Manual page 202

Using I/Os in S7–400H
13.4 Connecting redundant I/O to the PROFIBUS DP interface
Redundant analog input modules for direct current measurement
Requirements for wiring analog input modules according to Figure 8-10:
● Suitable encoder types are active 4-wire and passive 2-wire transmitters with output
● The "wire break" diagnostics function supports only the 4...20 mA input range. All other
● Suitable diodes include the types of the BZX85 or 1N47..A series (Zener diodes 1.3 W)
● A fundamental measuring error of max. 1 µA results from this type of circuit and the
● The 4-wire transmitters used must be capable of driving the load resulting from the circuit
● When connecting up 2-wire transmitters, note that the Zener diode circuit weighs heavily
Redundant analog input modules with redundant encoders
With double-redundant encoders, it is better to use fail-safe analog input modules in a 1-out-
of-2 configuration:
Figure 13-11 Fault-tolerant analog input modules in 1-out-of-2 configuration with two encoders
The use of redundant encoders also increases their availability.
A discrepancy analysis also detects external errors, except for the failure of a non-redundant
load voltage supply.
You will find connection examples in Appendix Connection examples for redundant I/Os
(Page 461).
The general comments made at the beginning of this documentation apply.
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ranges +/-20 mA, 0...20 mA, and 4...20 mA. 2-wire transmitters are powered by an
external auxiliary voltage.
unipolar or bipolar ranges are excluded in this case.
with the voltages specified for the modules. When selecting other elements, make sure
that the reverse current is as low as possible.
specified diodes due to the reverse current. In the 20 mA range and at a resolution of 16
bits, this value leads to an error of < 2 bits. Individual analog inputs in the circuit above
lead to an additional error, which may be listed in the constraints. The errors specified in
the manual must be added to these errors for all modules.
above. You will find details in the technical specifications of the individual modules.
in the power budget of the transmitter. The required input voltages are therefore included
in the technical specifications of the individual modules. Together with the inherent supply
specified on the transmitter data sheet, the minimum supply voltage is calculated to L+ >
U + U
e-2w IS-TR
System Manual, 03/2012, A5E00267695-11
S7-400H