Installation
21
wire back to the PC or logger). For best results, especially for longer distances, use wire recom-
mended for RS-485.
Manufacturer
Part Number
AWG
Pairs
Shielded?
Impedance
Insulation
Belden
9841
24
1
Yes
120 ohms
300 V
Belden
9842
24
2
Yes
120 ohms
300 V
many
CAT 5, 5e
24
4
Optional
100 ohms
300 V
many
CAT 6
23 or 24
4
Optional
100 ohms
300 V
Table 5: Recommended RS-485 Cabling
●
Since the Modbus / RS-485 wiring may be located near line voltage wiring, use wires or
cables rated for the highest voltage present, generally 300 V or 600 V rated wire.
●
If this cable will be in the presence of bare conductors, such as bus-bars, it should be double
insulated or jacketed.
●
Use twisted-pair cable (unshielded or shielded) to prevent interference.
Because the WattNode meter uses half-duplex communication, it only needs a single twisted-
pair, but it also
needs a conductor for common
, which may be the shield or a spare conductor.
Length Limits
Under ideal conditions, using cable with a 120 ohm impedance and proper termination, it should
be possible to run RS-485 signals 1200 m (4000 ft) at up to 19,200 baud. However, a number of
factors can reduce this range, including electrical and magnetic interference (EMI), bus loading,
poor termination, etc. Repeaters are available to extend the range if necessary.
If it isn’t convenient to daisy-chain the main RS-485 bus to each meter, you may use stubs or
branches. Long stubs or branches—greater than 30 m (100 ft)—may cause signal reflections and
should be avoided.
Termination
Networks shorter than 500 m (1650 ft) should not need termination. Longer networks and net-
works in electrically noisy environments may need termination at both ends of the bus with 120
ohm resistors between the “
A-
” and “
B+
” terminals. Generally, you will put one termination resis-
tor at the PC or monitoring device and one at the meter farthest from the monitoring device.
Some EIA RS-485 PC interfaces include jumpers or switches to provide internal termination at
one end of the bus.
In some cases, termination can cause problems. It dramatically increases the load on the bus, so
that some RS-485 PC interfaces cannot handle the load (particularly port powered ones). Also,
adding 120 ohm termination resistors may require the addition of bias resistors (see next section).
Biasing
EIA RS-485 networks frequently use bias resistors to hold the bus in a “high” or logic 1 state
when no devices are transmitting. In this state, the Modbus “
A-
“ terminal is more negative than
the “
B+
” terminal. Without bias resistors, the bus can float and noise can appear as bogus data.
The WattNode meter uses an RS-485 failsafe transceiver that eliminates the need for bias resis-
tors except in noisy environments. Furthermore, many RS-485 PC interfaces include internal bias
resistors, so it is rare to need to add bias resistors.
If you determine that your network is experiencing noise problems, then you may want to add
termination and possibly bias resistors.
