Installation guide

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5.3 Rules for reliable RS485 networks
Some simple rules must be followed for reliable communications using RS485:
No more than 32 Loads: RS485 does not specify the maximum number of transceivers but allows for
the connection of a maximum of 32 ‘unit loads’. A unit load is typically a single device although some
transceivers are now available with ½ and ¼ unit load ratings. If you are unsure it is best to stick to 32
especially if you are using higher speeds.
Terminate each end of the bus: to avoid errors due to signal reflections and line coupling, it is neces-
sary to terminate the bus at the beginning and at the end. Terminate the bus by inserting a 120Ω ½W
5% resistor between wire B and A (positive and negative signals) at the last and first instrument. A com-
mon mistake is to terminate the bus at each node, which will cause problems when there are more than
three of four nodes. The network termination is necessary even for point-to-point connections and con-
nections over short distances.
Maximum bus length 1200m: for data rates above 100k, you will have to restrict the cable length even
further.
Use twisted pair, cable for the bus connections: the characteristic impedance of the cable should be
100-120Ω. A 120Ω cable presents a lighter load, so it is preferred. Use cables with wire gauge AWG24
at minimum, thicker cable like AWG22 can improve results over long runs.
Interconnect the 3 terminals (+, - and GND) between all the devices in the network. Use a twisted
pair for + and - and another wire (or both wires) from a different pair for signal GND connection.
Use shielded cable: Shielded cable can provide protection from interference in noisy environments.
Connect cable shields to a single clean communications ground point at one end of the cable only.
Keep communications ground and electrical earth separate: communications ground points are
separate from electrical earths which are typically very noisy. If the communications ground point is not
completely independent, or you are not certain, use a gas arrester between the shield and the communi-
cations ground point.
Interconnect the cable shield: The shield should be continuous for the full length of the network but
earthed (optionally via a spark gap arrestor) at a single point only. Do not connect the shield to any
Transclinic or OVP device.
Keep signal ground wire and electrical earth/cable shield separate: Do not connect the cable shield
to any of the signal wires - especially the RS485 signal ground wire (GND). Digital and analogue signals
often have wires (called signal grounds) that are used to provide a reference point for signal voltages –
they are not related to the (typically very noisy) ground/earth points used in electrical circuits and the two
should never be connected.
Use unique addresses: assign a unique Modbus slave address to each Modbus Slave device on the
network. Although Modbus allows for Modbus slave addresses from 1 to 247, you can still only connect
32 unit loads (including the PV Plant PC). A duplicate address will prevent communication because two
units transmitting at the same time will cause a collision.
Connect the devices using Daisy-chain topography: avoid stubs when possible and reduce their size
as much as possible if they cannot be entirely avoided (inside the panel).
Build an independent Network for the Transclinics: Do not install equipment from other manufactur-
ers in the Transclinics RS485 network. Since Weidmuller cannot guarantee that other equipment avail-
able in the same line is compatible with the speed and characteristics of the Transclinics. Additionally,
this separation of networks simplifies fault finding.
Set all devices to the same speed (bps); parity; data-bits (8) and stop bits (1): Check that all net-
work components (the masters as well as all of the slaves) have the same configuration otherwise the
communication will not operate properly.