Siemens SIMATIC S7-400H System Manual page 192

Using I/Os in S7–400H
13.4 Connecting redundant I/O to the PROFIBUS DP interface
Library V5.x Library V4.x Library V3.x Module
X X
X
192
X
AI 8x16Bit
Use in voltage measurement
The "wire break" diagnostics function in HW Config must not be activated

when operating the modules with transmitters.
Use in indirect current measurement
When using indirect current measurement, ensure a reliable connection

between the sensor resistances and the actual inputs, because a reliable
wire break detection cannot be guaranteed in the case of a wire break of
individual cables of this connection.
Use a 250 ohm resistor (measuring range 1 - 5 V) to map the current on a

voltage; see figure 10-9.
Use in direct current measurement
Suitable Zener diode: BZX85C8v2

Circuit-specific additional error: If one module fails, the other may suddenly

show an additional error of approx. 0.1%.
Load capability of 4-wire transmitters: R

(determined for worst case scenario: 1 input + 1 Zener diode at an S7
overload value 24 mA to R
Input voltage in the circuit when operating with a 2-wire transmitter: U

15 V
(determined for worst case: 1 input + 1 Zener diode at an S7 overshoot value
24 mA to U
X AI 8x16Bit
Use in voltage measurement
The "wire break" diagnostics function in HW Config must not be activated,

neither when operating the modules with transmitters nor when
thermocouples are connected.
Use in indirect current measurement
Use a 250 ohm resistor (measuring range 1 - 5 V) to map the current on a

voltage; see figure 10-9.
Use in direct current measurement
Suitable Zener diode: BZX85C8v2

Load capability of 4-wire transmitters: R

(determined for worst case scenario: 1 input + 1 Zener diode at an S7
overload value 24 mA to R
Input voltage in the circuit when operating with a 2-wire transmitter:

U < 15 V (determined for worst case: 1 input + 1 Zener diode at an S7
e-2w
overshoot value 24 mA to U
= (R * I
B E
= R * I + U )
e-2w E max z max
= (R * I
B E
= R
e-2w E
Order number
6ES7 331–7NF00–0AB0
> 610 ohms
B
+ U )/I
max z max max)
6ES7 331–7NF10–0AB0
> 610 ohms
B
+ U )/I
max z max max)
* I + U )
max z max
System Manual, 03/2012, A5E00267695-11
<
e-2w
S7-400H