Siemens SIMATIC S7-400 Reference Manual page 402

Parameter sets of signal modules
A.3 Parameters of digital output modules
Byte 4*
Substitute
value
* Bytes 3 and 4 are not relevant for SM 421;DO 16 x DC 20-125 V/1.5A
402
Byte Bit
7 Enable substitute value 1 on channel 31
6 Enable substitute value 1 on channel 30
5 Enable substitute value 1 on channel 29
4 Enable substitute value 1 on channel 28
3 Enable substitute value 1 on channel 27
2 Enable substitute value 1 on channel 26
1 Enable substitute value 1 on channel 25
0 Enable substitute value 1 on channel 26
Meaning
S7-400 Automation System Module Data
Reference Manual, Ausgabe 11/2016, A5E00850736-08