Siemens SIMATIC NET PROFIBUS Owner's Manual page 53

Rules
The following rules must be taken into account when extending PROFIBUS-PA:
S There can be a maximum of 31 PA field devices in a PROFIBUS-PA system
S Only one device supplying power (=DP/PA coupler) can be connected in a
physical PROFIBUS-PA segment.
S A maximum of 31 PA field devices can be attached to a DP/PA link. The
maximum number of attachable PA field devices per physical PROFIBUS-PA
segment or per DP/PA coupler is limited by the maximum output current of the
DP/PA coupler and the I/O data to be transferred.
PROFIBUS Networks SIMATIC NET
6GK1970-5CA20-0AA1 Release 2 05/2000
Topologies of SIMATIC NET PROFIBUS Networks
2-27