Ampro Corporation 486E manual Bus Expansion Guidelines, Bus Termination

Little Board™/486e Technical Manual

Bus Expansion Guidelines

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.

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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 CPU, use a high quality backplane that minimizes signal crosstalk. Use a backplane that has power and ground planes between trace layers, and run guard traces between sensitive bus signals.

!Eliminating Reset and TC Noise—Many cards have asynchronous TTL logic inputs that are susceptible to noise and crosstalk. The active high RESET and TC bus lines are especially vulnerable. These signals can be made more reliable by adding a 200 pF to 500-pF capacitor between the signal and ground to prevent false triggering by filtering noise on the signals.

Bus Termination

Some backplanes include bus termination to improve system reliability by matching backplane 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 100 ohm resistor, from each signal to ground.

Caution

Do not use resistive bus termination! If the signal requires termination, use AC termination only.

The actual requirements for signal termination depend on system configuration, interconnecting bus cable and on the number and type of expansion modules used. It is the system engineer’s responsibility to determine the need for termination.

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