Concept
Cable Selection (Legrand Power Meters)
This is one of the most important selections having significant impact on the performance and reliability of the RS-485 network being installed. An incorrect cable selection can be difficult and expensive to reverse. The decision should not be made on previous examples of seeing some alternate non-compliant cable work.
With RS-485, two conductors are used to pass the differential data (A+ and B- signals) from one node to the next. To maintain the balanced characteristics between the two wires, the cable must provide twisted pairs and be specified for data communications. As the twisted pair passes along side other cables and equipment in the facility, there are a multitude of noise sources, radiated EMI and electromagnetic fields that will impose noise on to the twisted pair cable.
The shield does not provide any form of total protection. The shield inhibits a significant portion of high frequency radiated noise, but other fields may simply pass through. As long as the twisted pair of wires remains balanced, the noise will be imposed on the two wires equally. An imbalance of impedance to ground of the differential pair determines in part the susceptibility of the network to interference, regardless of it being inductive or capacitive coupled. When balanced, a noise appearing equally on both wires is called common mode noise. The RS-485 receiver only looks at the differential voltage seen between the two conductors and ignores the common mode noise. This is true up to the common mode voltage limits of the transceiver, which should be a minimum range of -7 V to +12 V. A reduction in this range, such as from fully exhausting transmitter CMV support with excess unit load, can make the bus susceptible to induced common mode noise.
The balanced performance of the cable requires more than just the twisted pair characteristics, although it is definitely the most important. The twisted pair cable can become unbalanced when encountering discontinuities in the capacitance between the two wires, or the capacitance from conductor to shield, or the impedance of the wires. This makes it important to select quality cable specified for RS-485 data communications. The cable supplier must provide cable specification that includes all of the characteristics seen in the table below. The recommended specification for these characteristics is also listed. These will provide the best results. You should avoid a cable where the manufacturer/supplier cannot provide the full cable specifications.
Multi-conductor cable with a twist of the conductors under the jacket in not twisted pair cable. Such a casual twist of the wire bundle is a characteristic on most cables. Such cable has no balanced pair characteristics.
|
Characteristics |
Recommendations |
|
Type |
Shielded Twisted Pair Low Capacitance |
|
Twisted Wire Size |
22 AWG to 24 AWG (0.33 mm² to 0.20 mm²) |
|
Impedance |
120 ohm |
|
Capacitance (wire to shield) |
<82 pF/m (<25 pF/ft) |
|
Capacitance (wire to wire) |
<46 pF/m (<14 pF/ft) |
|
Maximum Length |
1200 m (4000 ft) depending on termination and bias restrictions |
|
Cable |
Size |
Pairs |
Imp. |
Cap1 a |
Cap2 b |
Vel |
Plenum |
|
Belden 3107A |
22 AWG Str (0.33 mm²) |
2 |
120 ohm |
36.1 pF/m (11 pF/ft) |
68.6 pF/m (20.9 pF/ft) |
78% |
|
|
Belden 9842 |
24 AWG Str (0.20 mm²) |
2 |
120 ohm |
42.0 pF/m (12.8 pF/ft) |
75.5 pF/m (23 pF/ft) |
66% |
|
|
Belden 82842 |
24 AWG Str (0.20 mm²) |
2 |
120 ohm |
39.4 pF/m (12 pF/ft) |
72.2 pF/m (22 pF/ft) |
76% |
Y |
|
Belden 89842 |
24 AWG Str (0.20 mm²) |
2 |
120 ohm |
39.4 pF/m (12 pF/ft) |
72.2 pF/m (22 pF/ft) |
76% |
Y |
|
Alpha Wire 6455 |
22 AWG Str (0.33 mm²) |
2 |
120 ohm |
36.1 pF/m (11 pF/ft) |
68.6 pF/m (20.9 pF/ft) |
78% |
|
|
Alpha Wire 6413 |
24 AWG Str (0.20 mm²) |
2 |
120 ohm |
42.0 pF/m (12.8 pF/ft) |
75.5 pF/m (23 pF/ft) |
|
|
|
General Cable C0842A |
24 AWG Str (0.20 mm²) |
2 |
120 ohm |
37.4 pF/m (11.4 pF/ft) |
67.3 pF/m (20.5 pF/ft) |
66% |
|
- Cap1 = Capacitance between the two conductors of the pair(s)
- Cap2 = Capacitance from each signal conductor to shield
All cables recommended for Legrand are two pair cables. This allocates one twisted pair for data and one twisted pair for use in providing a large gauge shielded RS-485 return conductor from the isolated commons on all of the Legrand meters. This scheme is consistent with the recommendations detailed in the Legrand communications guide. The Legrand guide recommends the use of Belden 9842 cable (or equivalent twisted pair cable).
RS-485 Communications
Legrand Power Meters