Specifications

Hardware Configuration
2–46
2.17.3 Bus Expansion Guidelines
One way to expand a Little Board/486i is by connecting short cables to the header connectors.
Ampro makes a small ribbon cable connector assembly that you can use to connect standard ribbon
cable connectors to the female expansion bus connectors on the Little Board/486i. Contact your
Ampro sales representative for more information about the ribbon cable adapter.
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 that you follow these guidelines.
! Cable Length and Quality—In general, keep the bus expansion cable 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 to 12 inches long, use a high quality ground plane cable, such as
3M part number 3476/64 (64 conductor) and 3476/40 (40 conductor).
− Do not use cables over 12 inches long.
WARNING
When using ground plane cable on P1, use a single drain cable
with the drain wire on pin 64 of the cable. If the drain wire is
on pin 1, IOCHK is shorted to GND, and the board will not
function.
! Backplane Quality—If you connect a backplane to the Little Board/486i, be sure to 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. You can make these signals 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. These
RESET and TC filters are included on most Ampro backplane expansion products.
Bus Termination—Some backplanes include bus termination to improve system reliability by
matching backplane impedance to the rest of the system. The IEEE-P996 draft specification for the
AT expansion bus 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. Ampro provides positions for OEM addition of
AC termination on most bus expansion products. These positions are designed to accommodate 9-pin
8-terminator Single Inline Package (SIP) terminators.
Here are some manufacturer part numbers for 9-pin, eight-terminator devices with 100 pF capacitors
in series with 100 ohm resistors:
! Dale CSRC-09C30-101J-101M
! Bourns 4609H-701-101/101