Bus lengths

AN1702

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4

Bus lengths

The signal quality is evidently affected when the length of the bus increases. The most

significant factor is the attenuation of the high frequency components of the signal. This

produces slackening of the rising and falling edge of the signal as well as jitter and inter-

symbol interference. All these factors must be taken into account when considering the

cable length and signaling rate. Most e-meter applications require signaling rates below

250 kbit/s. With these rates, the signal degradation is less evident for cable lengths up to 1

km. The ST485EB is suitable for these signaling rates. Its driver outputs have a slew rate

limitation, which reduces EMI and signal reflections on the line, allowing very low signal

degradation over long distances.

5

Network isolation

Due to the long distance covered by the network, many factors such as ground loops,

external noise, spikes, electrical storms, or common-mode voltages exceeding the

maximum voltage range (-7 V to+12 V), can bother or, in some cases, damage the

equipment connected to the TTL side of each transceiver (see Figure 3).

Figure 3.

Dangerous ground shifts in a common situation

In order to avoid this, it is advisable to electrically isolate the equipment from the bus. In

Figure 4 an example is shown of isolated transceivers, requiring the use of optocouplers at

used: one for the transceivers and the related sides of the optocouplers connected to them

These two supplies should be isolated from each other, and derived from different power

generators or transformer windings.