Specifications

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grounds for optional use to enhance system reliability. An independent crystal (Y1) provides J9's
14.318 MHz bus OSC signal. The timing of the OSC signal is asynchronous to other expansion
bus signals.
For compatibility with standard implementations of the AT bus, four wait states are inserted in all
expansion bus transactions for both I/O and memory cycles. Memory wait states on the bus can be
reduced, in two ways:
n If the 0 Wait State control signal on J9 is activated by the bus memory card, then no wait
states are inserted;
n If a memory card activates the MEMCS16 line on the AT Expansion Bus, only 1 wait state is
used in its access, unless the memory card adds additional wait states.
The AT bus signals on J10 include six high order address lines, the high order data byte, five
additional interrupt requests (IRQ's), four additional DMA channel handshake signal pairs
(DRQ/DACK's), and several additional control signals. It should be noted that the upper six
address lines (LA17-LA23) are unlatched; they can be latched using the expansion bus BALE
signal.
4.4 FLOPPY DISK CONTROLLER
A 37C65B floppy disk subsystem controller (U34) provides the equivalent of an entire AT
compatible floppy disk controller plug-in card within a single LSI device. This "superchip"
integrates:
n Formatter/controller
n Digital data separation
n Digital write precompensation
n Data rate selection
n Clock generation
n Drive interface drivers and receivers
n Drive motor control
n AT compatible control registers
The 37C65B is completely compatible with the industry standard NEC765 floppy controller, and
associated external devices, as implemented in a standard AT. Data rates of 125, 250, 300, and
500 Kbits/second are usable.
Two types of dual capacity drives, which allow a single drive to read both 360 Kbyte and 1.2
Megabyte diskettes, are supported:
n Standard AT drives -- the data rate shifts between 250K and 500K bps, but the drive
rotational speed stays constant.
n Dual speed drives -- the data rate shifts between 250K and 300K bps, and the drive rotational
speed also shifts between 300 and 360 rpm.
A jumper (W19) is used to select between the two types of dual capacity drives. It must be on for
dual speed drive use. It does not affect the use of normal single speed drives.
The board's 37C65B floppy controller contains a highly reliable "2nd order" digital data separator.
Write precompensation (187 nS) is also generated digitally within the 37C65B.