DP8344 datasheet(59/184 Pages) NSC

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30 Transceiver (Continued)

325 Line Interface

3251 3270 Line Interface

In the 3270 environment data is transmitted between a con-

trol unit and a device via a single coax cable or twisted pair

cable The coax type is RG62AU with a maximum length of

15 kilometers The twisted pair cable has become more

prevalent to reduce cabling and routing costs Typically a

24 AWG unshielded twisted pair is used to achieve the cost

reduction goals The length of the twisted pair cable is a

minimum of 100 feet to a maximum of 900 feet The 3270

protocol utilizes a transformer to isolate the peripheral from

the cabling system

An effective line interface design must be able to accept

either coax or twisted pair cabling and compensate for

noise jitter and reflections in the cabling system There

must be an adequate amount of jitter tolerance to offset the

effects of filtering and noise Some filtering is needed to

reduce ambient noise caused by surrounding hardware

Such filtering must not introduce transients that the receiver

comparator translates into data jitter

An effective driver design should also attempt to compen-

sate for the filtering effects of the cable Higher data fre-

quencies become attenuated more than lower frequency

signals as cable length is increased yielding greater dispari-

ty in the amplitudes of these signals This effect generates

greater jitter at the receiver The 3270 signal format allows

for a high voltage (predistorted) magnitude and a low volt-

age (nondistorted) magnitude within each data bit time In-

creasing the predistorted-to-nondistorted signal level ratio

counteracts the filtering phenomenon because the lower

frequency signals contain less predistortion than do higher

frequency signals Thus the amplitude of the higher fre-

quency signals is ‘‘boosted’’ more than the lower frequency

signals Unfortunately a low signal level is more susceptible

to reflection-induced errors at short cable length Proper im-

pedance matching and slower edge rates must be utilized to

eliminate as much reflection as possible at these lengths

Additionally shielded or balanced operation must be ade-

quately supported Shielded operation implies the use of

coax cable where balanced implies the use of twisted pair

cable Proper termination should be employed and a termi-

nation slightly greater than the characteristic impedance of

theline may actually provide more desirable waveforms

than a perfectly matched termination Board layout should

make the comparator lines as short as possible Lines

should be placed closely together to avoid the introduction

of differential noise These lines should not pass near

‘‘noisy’’ lines A ground plane should isolate all ‘‘noisy’’

lines

BCP Design

The line interface design for the receiver is shown in

Figure

3-12 An offset of approximately 17 mV separates the com-

parator inputs making the receiver more immune to ambi-

ent noise present on the circuit board A 211 (arranged as

a 31) transformer increases any voltage sensitivity lost by

introducing the offset A bandpass filter is employed to re-

duce edge rate to the comparator and eliminate ambient

noise The bandwidth (30 kHz to 30 MHz) was chosen to

provide sufficient attenuation for noise while producing mini-

mum data jitter

The driver design

Figure 3-13

incorporates a National

Semiconductor DS3487 and a resistor network to generate

the proper signal levels The predistorted-to-nondistorted

ratio was chosen to be about 3 to 1 The coaxtwisted pair

front end

Figure 3-14 includes an ADC brand connector to

switch between coax and twisted pair cable The coax inter-

face has the shield capacitively coupled to ground The

510X resistor and the filter loading produce a termination of

about 95X The twisted pair interface balances both lines

and possesses an input impedance of about 100X This

termination is somewhat higher than the characteristic im-

pedance (about 96X) of twisted pair Terminations of this

type produce reflections that do not tend to generate mid-bit

errors Such terminations have the benefit of creating a larg-

er voltage at the receiver over longer cable lengths For a

more detailed explanation of the 3270 line interface see

Application Note ‘‘A Combined CoaxTwisted Pair 3270

Line Interface for the DP8344 Biphase Communications

Processor’’

3252 5250 Line Interface

The 5250 environment utilizes twinax in a multi-drop config-

uration where eight devices can be ‘‘daisy-chained’’ over a

total distance of 5000 feet and eleven splices (each physi-

cal device is considered a splice) Twinax connectors are

bulky and expensive but are very sturdy Twinaxial cable is

a shielded twisted pair that is nearly

of an inch thick

Legend

f To coaxtwisted pair front end

f To line driver circuitry

f To BCP comparator

Includes board capacitance

TLF9336 – G1

FIGURE 3-12 BCP Receiver Design

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