Ampro Corporation 486E For cables up to 6 inches, use a high quality standard cable, such as 3M 3365/64 (64 conductor), For cables between 6 and 12 inches long, use a high quality ground plane cable, such as 3M part, Do not use cables over 12 inches long.

Little Board™/486e Technical Manual

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Expanding a Little Board/486e CPU can be accomplished by connecting short ribbon cables to the

header connectors. There are restrictions when attaching peripherals to the expansion bus with

ribbon cables. If cables are too long or improperly terminated, noise and cross talk introduced by

the ribbon cables can cause errors. Ampro strongly recommends conformance to the following

guidelines:

! Cable Length and Quality—In general, the bus expansion cable must be as short as possible.

Long cables reduce system reliability.

 For cables up to 6 inches, use a high quality standard cable, such as 3M 3365/64 (64 conductor)

and 3365/40 (40 conductor).

 For cables between 6 and 12 inches long, use a high quality ground plane cable, such as 3M part

number 3353/64 (64 conductor) and 3353/40 (40 conductor).

 Do not use cables over 12 inches long.

! Backplane Quality—If a backplane can be connected to the Little Board/486e CP U, u se a high

quality backplane that minimizes signal crosstalk. U se a backp lane that has p ower an d gro und

planes between trace layers, and run guard tr aces be twe en sensi tive bu s sign als.

! Eliminating Reset and TC Noise—Many cards have asynchronous TTL logic inputs that are

susceptible to noise and crosstalk. The active h igh R ESET and TC bus line s are e specia lly

vulnerable. These signals can be made more reliable by add ing a 20 0 pF to 500-p F c apacito r

between the signal and ground to prevent false triggering by filtering noise on the signals.

Some backplanes include bus termination to impro ve syste m reli abili ty by m atch ing b ackplan e

impedance to the rest of the system. The specification recommends the use of AC termination

sometimes called snubbers, rather than resistive termination. The recommended AC termination is

a 50 to 100 pF capacitor, in series with a 50 to 1 00 oh m resis tor, fro m each sig nal to grou nd.

Caution

Do not use resistive bus termination! If the signal requires

termination, use AC termination only.

The actual requirements for signal termination de pend on sy stem conf igurat ion, in terco nnectin g

bus cable and on the number and type of expansion modules used. It is the system engineer’s

responsibility to determine the need for termin atio n.