Siemens Sinamics S110 Function Manual page 459

Dimensioning of load resistors - example 1:
According to manufacturer documentation, the leakage current of an F-DO on a safety PLC
is 1 mA for the P and F channel; in other words, it is 0.5 mA higher than is permissible for the
F-DI.
The necessary load resistance is therefore R = 5 V/0.5 mA = 10 kΩ.
At maximum supply voltage, the power loss for this resistor is:
P = (28.8 V)²/R = 83 mW. The resistor is to be permanently dimensioned for this power loss.
Dimensioning of load resistors - example 2:
If further conditions for the digital output (e.g. a minimum load or a maximum load
resistance) are specified in the manufacturer documentation, these must also be taken into
account.
For example, a load between 12 Ω and 1 kΩ is specified for the SIMATIC ET200S 4 F-DO
I/O module (6ES7138-4FB02-0AB0).
Therefore, two additional 1 kΩ load resistors and a continuous load capacity of at least P =
(28.8 V)²/R = 830 mW are required to connect an F-DO of this kind to a CU305 F-DI.
If a controlled 24 V power supply is used (e.g. SITOP), a resistor with a significantly lower
power loss is sufficient.
Note
Open-circuit detection for pull-up resistors
If the pull-up resistor has a resistance greater than 1 kΩ, open-circuit detection is no longer
reliable and has to be switched off.
Function Manual
Function Manual, 01/2011, 6SL3097-4AB10-0BP3
Safety Integrated Functions
8.8 Application examples
457