Installation guide

16 PV Clinics Installation Manual 21/07/11 01.00 01/12
5. Data Communications
5.1 General
This section is a guide for the proper installation of Transclinic RS485 networks in a photovoltaic installation.
It should help, prevent possible installation errors and guarantee good communication. Most information is
valid for any RS485 network since what is explained are concepts of the standard. Transclinic data commu-
nications strictly comply with both Modbus and RS485 standards.
PV Plants are usually electrically noisy environments. Since arrays are typically very large and subject to
ground loops, induced noise on signal cables and earth potential differences around the plant can be con-
siderable.
RS485 (defined by standard EIA485) has become the workhorse for allowing multiple devices to communi-
cate over the same twisted pair. It does not describe the protocol, connectors or cable; it just describes the
physical characteristics of the network.
5.2 RS485 signals
The 485 standard is a balanced communication system. It includes 2 data lines (A and B), which are used to
transmit the signal. The system is considered ‘balanced’ because the signals are identical but inverted.
Fig. 2: Signals A and B in a 485 communication
Subsequently, both signals are compared to the reference (Signal GND) to obtain the data clearly and pre-
cisely. In the following example we can see an RS485 communication.
The data signal lines are called A and B on some equipment; D0 and D1 on some equipment; and + and −
on others. Reversing the two wires will prevent communication and there are no clear standards followed for
A/B and D0/D1 so consult the equipment manufacturer’s information to confirm. The easiest test is to simply
reverse the wires.