AM85C30 datasheet(55/194 Pages) AMD

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4–3

Data Communication Modes

Functional Description

CHAPTER 4

4.1

INTRODUCTION

The SCC provides two independent full-duplex channels programmable for use in any

common asynchronous or synchronous data communication protocol. This includes:

Asynchronous, Synchronous MONOSYNC (8-bit sync character), Synchronous BISYNC

(16-bit sync character), normal SDLC, and SDLC Loop Mode.

4.2

PROTOCOLS

A communication protocol defines a set of rules for the orderly transfer of information be-

tween two communicating devices. All communication line protocols in the industry today

exchange data in either an asynchronous or synchronous manner. Asynchronous trans-

mission is used in several protocols including the TTY protocol while synchronous trans-

mission is used in protocols which include: IBM BISYNC, Synchronous Data Link Control

(SDLC), High-Level Data Link Control (HDLC), and Advance Data Communication Con-

trol Procedures (ADCCP).

This section provides a brief overview of these protocols; however, if further information is

desired the book titled “Technical Aspects of Data Communications” by John E.

McNamara, published by Digital Press (DEC) 1982, is a good reference.

4.2.1

Asynchronous

In Asynchronous transmission, as the name implies, each character is transmitted as an

independent entity; that is, the time between the last bit of one character and the first bit

of another character can be variable.

Since the receiver must be able to detect the beginning of each character transmitted,

this mode requires that at least one bit be added at the start and end of each character

for synchronization purposes.

Synchronization at the receiver is accomplished by sensing the transition of the Start-bit

for each character transmitted. The first data bit of the character is typically sampled one

and one-half bit times after the High-to-Low transition of the Start-bit, and each subse-

quent bit is sampled one bit time thereafter. The sampling of the bit occurs near the cen-

ter of each bit to allow correct data recovery and typically occurs at some multiple of the

data rate. Larger multiples allow a closer approximation to the middle sampling.

Figure 4–1 depicts a typical Asynchronous 11-bit format. Each 8-bit character is preceded

by a Start-bit and followed by a Parity check bit and one Stop-bit. The Start-bit of the next

character can occur anytime after the first character’s Stop-bit. The idle state of the trans-

mission line between characters is always in a mark idle condition (i.e., TxD pulled High).

Asynchronous communication channels are found in most distributed computer systems

for terminal-to-computer comunications. The common “serial port” found on personal

computers is an asynchronous port. It is used to attach external modems and printers,

and to interface the personal computer to a minicomputer for use as a terminal.

Stop

Start

Stop

Start

X = High or Low

Figure 4–1. Asynchronous Format

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