Installation guide

20 PV Clinics Installation Manual 21/07/11 01.00 01/12
poorly terminated), which worsen the quality of the communications. The recommendation is to avoid these
and if stubs are unavoidable, make them as short as possible (only for the inner panel).
5.2.7 Termination resistances
To prevent reflections at the ends of the cable, it is recommended that a 120Ω termination resistor is in-
stalled at each end of the RS485 line. These resistances must be installed between the A and B data lines at
each end of the network and at no other location in the network.
Fig. 13: Location of the termination resistors
Depending on the lengths of the cables and their quality, the resistance values may vary slightly (generally
between 100 and 150 ohms). This determination must be made based on a detailed analysis of the line.
Improved performance may be achieved by using a ‘failsafe’ termination at one end of the network.
5.2.8 Addresses, network length and repeaters
The Modbus RTU protocol may use addresses from 1 to 247. However, the RS485 protocol is limited to 32
components in a network sub-segment. This means that a network repeater must be installed every 32 com-
ponents. Network repeaters for RS485 networks are readily available.
The maximum distance per segment is 1200m (specified in standard) although using cables at the limit is not
recommended and a suitable amount should be deducted. Repeaters can also be used to extend the length
of the network (although they will introduce an additional delay in response times).