Using I/Os in S7–400H
13.4 Connecting redundant I/O to the PROFIBUS DP interface
S7-400H
200 System Manual, 03/2012, A5E00267695-11
Redundant analog input modules for indirect current measurement
The following applies to the wiring of analog input modules:
● Suitable encoders for this circuit are active transmitters with voltage output and
thermocouples.
● The "wire break" diagnostics function in HW Config must not be activated, neither when
operating the modules with transmitters nor when thermocouples are connected.
● Suitable encoder types are active 4-wire and passive 2-wire transmitters with output
ranges +/-20 mA, 0...20 mA, and 4...20 mA. 2-wire transmitters are powered by an
external auxiliary voltage.
● Criteria for the selection of resistance and input voltage range are the measurement
accuracy, number format, maximum resolution and possible diagnostics.
● In addition to the options listed, other input resistance and voltage combinations
according to Ohm’s law are also possible. Note, however, that such combinations may
lead to loss of the number format, diagnostics function and resolution. The measurement
error also depends largely on the size of the measure resistance of certain modules.
● Use a measure resistance with a tolerance of +/- 0.1% and TC 15 ppm.
Additional conditions for specific modules
AI 8x12bit 6ES7 331–7K..02–0AB0
● Use a 50 ohm or 250 ohm resistor to map the current on a voltage:
Resistor 50 ohms 250 ohms
Current measuring range +/-20 mA +/-20 mA *) 4...20 mA
Input range to be parameterized +/-1 V +/-5 V 1...5 V
Measuring range cube position "A" "B"
Resolution 12 bits + sign 12 bits + sign 12 bits
S7 number format x x
Circuit-specific measuring error
- 2 parallel inputs
- 1 input
-
-
0.5%
0.25%
"Wire break" diagnostics - - x *)
Load for 4-wire transmitters 50 ohms 250 ohms
Input voltage for 2-wire transmitters > 1.2 V > 6 V
*) The AI 8x12bit outputs diagnostic interrupt and measured value "7FFF" in the event of wire break.
The listed measuring error results solely from the interconnection of one or two voltage
inputs with a measure resistance. Allowance has neither been made here for the tolerance
nor for the basic/operational limits of the modules.
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