AM85C30 datasheet(176/194 Pages) AMD

176 / 194 page

SCC Application Notes

AMD

7–27

functionally distributed computers. Lengthy delays are often encountered in a centralized

system due to overloaded CPUs and slow communication lines.

f. Resource Sharing—

Remote sites in a distributed system can use each other’s facilities such as sharing

expensive peripheral devices in the system.

The advantages of distributed processing, therefore, point mainly to two important facts:

1. LANs make distributed intelligence practical.

2. Methods must be devised in order to allow large amount of data to be transported, at

high speeds, between centers of intelligence.

This discussion addresses the second factor.

7.6.2

Data Communications at the Physical Layer

Today’s data communications market can be segmented in terms of speed.

Low speed (300 baud–56 kbaud)

ii. Medium speed (0.5 Mb/s–3 Mb/s)

iii. High speed (10 Mb/s–60 Mb/s)

iv. Very high speed (100 Mb/s and up)

On medium speed applications data rate is assumed at 1 Mb/s. At this rate, the physical

interface requires a highly sophisticated transceiver. The following is a list of the require-

ments:

a. The transceivers must have good but inexpensive isolation from the cabling system

(large common-mode voltages and surge voltages are common phenomenons in any

network). Pulse transformers can provide the required isolation.

b. The transceiver must be able to support a good collision avoidance scheme, since all

devices have equal access to the network (in a CSMA type architecture). If a collision

does occur, the transceiver must detect the collision and flag the communications

controller.

c. The receiver must be able to inhibit false starts due to noise in the surrounding

environment.

d. The transmitter must control the slew rate of the output signal to reduce EMI/RFI to

surrounding electronic equipment. This is a very important criterion if the network is to

meet FCC/VDE regulations on radiation.

e. If an isolation transformer is used, some form of data encoding must also be

implemented so that a series of ‘1’s or ‘0’s do not saturate the transformer by

charging it to either voltage level.

Why the Am7960

The Am7960 Coded Data Transceiver is designated to meet all the requirements of the

medium-speed network. It is frequency agile for data rates between 0.5 Mb/s to 3 Mb/s.

Electro–Magnetic Interference

The Am7960 has an external resistor connected from the slew rate control pin (TSRC) to

ground to control the transmit slew rate. If the rise and fall times of the output signal are

each 30% of the transmit clock period, the output waveform looks approximately like a

sine wave (Figure 7–17). This has a smoothing effect on the transmitted signal, reducing

3rd, 5th and higher order harmonics. This in turn reduces energy radiating from the cable

and

minimizes the effects of electro-magnetic interference and radio frequency inter-

ference.

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