Specifications

2 - 26
2.6.3 Expansion Bus Cable Considerations
Here are several points regarding the use of the board's AT Expansion Bus:
n Cable Length -- Keep the ribbon cable used as short as possible. Expansion bus cables
in excess of six inches can result in reduced system reliability. We recommend:
-- For cable lengths up to 6 inches, use a high quality ribbon cable, such as 3M part
number 3365/64 (64 conductor) and 3365/40 (40 conductor).
-- For cable lengths between 6 and 12 inches, use a high quality ribbon cable with ground
plane, such as 3M part number 3353/64 (64 conductor) and 3353/40 (40 conductor).
-- Cable lengths in excess of 12 inches are not recommended.
n Backplane Quality -- Be sure to use a high quality backplane having minimal signal
crosstalk. Use of power and ground planes, and ground guard traces between bus signals is
recommended.
n Reset Deglitching -- Some PC and AT bus expansion cards have asynchronous TTL
inputs that are especially vulnerable to cable noise and crosstalk. In particular, the active
high RESET line is one to watch out for. If this signal is found to be susceptible, a 200pf
to 500pf capacitor connected between the RESET signal and ground can be used to prevent
false expansion card resets. This RESET deglitching capacitor is included on the Ampro
MiniBackplanes.
n Bus Half-Speed Options -- Typically PC and AT expansion bus cards are designed for 8
MHz maximum expansion bus speed, though new boards and boards of your own design
may allow faster system operation. You can use the Ampro SETUP or SETSPEED
utilities to set bus operation to full- or half-speed. When set to half-speed, the "CLK"
signal on expansion bus pin J9-B20 becomes half the system clock (i.e. 8 MHz on a 16
MHz board, and 6 MHz on a 12 MHz board). Use full-speed bus operation with caution!
2.6.4 Expansion Bus Connector Pinouts
Tables 2-18A through 2-18D indicate the pinout and signal functions of the signals on the
board's AT Expansion Bus connectors. Further information about these signals is available in
numerous publications, including the IBM technical reference manuals for the PC and AT
computers, and from the reference documents listed in the beginning of Chapter 4.
The board's AT Expansion Bus header connector (J9 and J10) pins have been numbered
differently from the other header connectors on the board, so that they correspond to the scheme
normally used on PC and AT bus edgecard connectors. The even numbered pins correspond to
the "B" and "D" rows of the AT bus; and the odd numbered pins correspond to the "A" and "C"
rows (e.g. J9 pin 1 is labeled "B1"). The first 62 pins of J9 (beginning at A1 and B1) are the
ones which correspond to the 62 conductors of the PC bus subset of the AT backplane. The last
two pins are spare grounds. On J10, however, the middle 36 pins (out of 40, total) correspond
to the AT bus 36 conductor edgecard connector, with 2 spare grounds at each end of the board's
40 pin connector. The first two pins of J10 on the Little Board/ATBus have been designated
"C0" and "D0", to emphasize the fact that they are added grounds.