DP8344 datasheet(8/184 Pages) NSC

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10 Communications Processor Introduction

The increased demand for computer connectivity has driven

National Semiconductor to develop the next generation of

special purpose microprocessors The DP8344B is the first

example of a ‘‘Communications Processor’’ for the IBM en-

vironment It integrates a very fast full function microproc-

essor with highly specialized transceiver circuitry The com-

bination of speed power and features allows the designer

to easily implement a state-of-the-art communications inter-

face Typical applications for a communications processor

are terminal emulation boards for PCs stand-alone termi-

nals printer interfaces and cluster controllers

The transceiver is designed to simplify the handling of spe-

cific communication protocols This feature makes it possi-

ble to quickly develop interfaces and software with little con-

cern for the ‘‘housekeeping’’ details of the protocol being

used

11 COMMUNICATIONS PROTOCOLS

A communication protocol is a set of rules which defines the

physical electrical and software specifications required to

successfully transfer data between two systems

The physical specification includes the network architec-

ture as well as the type of connecting medium the connec-

tors used and the maximum distance between connections

Networks may be configured in ‘‘loops’’ ‘‘stars’’ or ‘‘daisy

chains’’ and they often use standard coaxial or twisted-pair

cable

The electrical specification includes the polarity and ampli-

tude of the signal the frequency (bit rate) and encoding

technique One common method of encoding is called ‘‘bi-

phase’’ or ‘‘Manchester II’’ This technique combines the

clock and data information into one transmission by encod-

ing data as a ‘‘mid-bit’’ transition

Figure 1-1 shows how the

data transition is related to the bit boundary in a typical

transmission The polarity of the ‘‘mid-bit’’ transition en-

codes the data value other transitions lie on bit boundaries

Bit boundaries are not always indicated by transitions so

techniques employing start sequences and sync bits are

used with bi-phase transmissions to ensure proper frame

alignment and synchronization

The software specification covers the use of start se-

quences and sync bits as well as defining the message

format Parity bits may be used to ensure data integrity The

message format is the ‘‘language’’ that is used to exchange

information across the connecting medium It defines com-

mand and control words response times and expected re-

sponses

The DP8344B Bi-phase Communications Processor sup-

ports both the IBM 3270 and 5250 communication proto-

cols as well as IBM 3299 and a general purpose 8-bit proto-

col The specialized transceiver is combined with a micro-

processor whose instruction set is optimized for use in a

communications environment This makes the DP8344 a

powerful single-chip solution to a wide range of communica-

tion applications

An example of an IBM 3270 message is shown in

Figure

1-2 The transmission begins with a very specific start se-

quence and sync pulse for synchronization This is followed

by the data command and parity bits Finally the end se-

quence defines the end of the transmission

The IBM 3270 and 5250 are two widely used protocols The

3270 protocol was developed for the 370 class mainframe

and it employs coaxial cable in a ‘‘star’’ configuration The

5250 protocol was developed for the System3x machines

and it uses a ‘‘daisy-chain’’ of twin-ax cable A good over-

view of both of these environments may be found in the

‘‘Multi-Protocol Adapter System User Guide’’ from National

Semiconductor and in the Transceiver section of this docu-

ment

TLF9336 – B7

FIGURE 1-1 Biphase Encoding

TLF9336 – B8

FIGURE 1-2 IBM 3270 Message Format

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