PROFIBUS DP slave addresses range from 0 to 125 (usable); address 126 is reserved as the factory commissioning address and 127 is the broadcast address (no data exchange).
True
Addresses 0 to 125 are the usable range shared between masters and slaves, giving 122 usable slaves once you reserve a few addresses for masters. Address 126 is the factory default: a brand-new device boots on 126, listens for a Set_Slave_Address command and does not participate in data exchange until a real address is assigned. Address 127 is broadcast: it is only used for commands (Sync, Freeze, Set_Slave_Address) and never for cyclic data exchange. Industry practice is to number slaves sequentially from 1 upwards to make diagnostics readable.
When commissioning, change the address away from 126 immediately on every new device: leaving two devices on 126 at once means the Set_Slave_Address command lands on the wrong one and creates a duplicate address.
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