FN6363 Rev.4.00

Page 21 of 34

Apr 19, 2018

ISL3150E, ISL3152E, ISL3153E, ISL3155E, ISL3156E, ISL3158E

6. Application Information

6.

Application Information

6.1

Network Design

Designing a reliable RS-485 network requires the consideration of a variety of factors that ultimately determine the

network performance. These include network topology, cable type, data rate and/or cable length, stub length,

distance between network nodes, and line termination.

The main difference between network designs is dictated by their modes of data exchange between bus nodes,

which can be half-duplex or full-duplex (Figures 35 and 36).

Half-duplex networks use only a single signal-pair of cables between one master node and multiple slave nodes,

which allows the nodes to either transmit or receive data, but never both at the same time. Its reduced cabling effort

makes these networks well suited for covering long distances of up to several thousands of feet. To maintain high

signal integrity, the applied data rates range from as low as 9.6kbps up to 115kbps. This requires transceivers with

long driver output transition times, typically in the range of microseconds, to ensure low EMI in the presence of

large cable inductances.

Full-duplex networks, on the other hand, aim for high data throughput. These networks use two signal-pairs to

support the simultaneous transmitting and receiving of data. The signal pair denoted as the transmit path connects

the driver output of the master node to the receiver inputs of multiple slave nodes. The other pair connects the

driver outputs of the slave nodes with the receiver input of the master node.

Because the data flow in the transmit path is unidirectional, the transmit path requires only one termination at the

remote cable end, opposite the master node. Data flow in the receive path, however, is bidirectional, thus requiring

line termination at both cable ends. Commonly, high data throughput also calls for higher data rates in the 1Mbps to

10Mbps range. As cable losses increase with frequency, most full-duplex networks are limited to shorter bus cable

lengths of a few hundred feet to maintain signal integrity.

The following sections discuss the aforementioned parameters that impact network performance. This discussion

applies to both half-and full-duplex network designs.

6.1.1

Cable Type

RS-485 networks use differential signaling over Unshielded Twisted Pair (UTP) cable. The conductors of a

twisted pair are equally exposed to external noise. They pick up noise and other electromagnetically induced

voltages as common-mode signals, which are effectively rejected by the differential receivers.

For best performance use industrial RS-485 cables, which are of the sheathed, shielded, twisted pair type,

(STP), with a characteristic impedance of 120Ω and conductor sizes of 22 to 24 AWG (equivalent to diameters

of 0.65mm and 0.51mm, respectively). They are available in single, two, and four signal-pair versions to

Figure 35. Half-Duplex Bus

Figure 36. Full-Duplex Bus

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