Specifications
Advanced Topics
4-9
4.3.3 Speaker Interface
The CoreModule/PC provides a PC compatible speaker port. The speaker logic signal is buffered by
a transistor amplifier, and provides approximately 100 mW of power to an external 8 ohm speaker.
As in the standard PC, the speaker circuit's output frequency is based on two control signals: the
output of Timer 2; and the programming of two bits, 0 and 1, in I/O port 61h. Bit 1 of I/O port 61h
is one term of a 2-input AND gate in the control logic, the other term being the output from Timer
2. Thus, setting bit 1 to a logic 1 enables the output of Timer 2 to the speaker, and a logic 0
disables it. If Timer 2 is disabled (by setting bit 0 of port 61h to a 0), then toggling bit 1 of port 61h
will directly pulse the speaker.
4.3.4 PC/104 Bus
An I/O channel, compatible with the PC bus found on standard PCs, is provided on a 64-pin dual-
row header connector labeled "P1" on the board. This I/O channel contains an 8-bit bidirectional
data bus, 20 address lines, 6 levels of interrupt, three DMA channel handshake lines, a number of
other control lines, and power and ground for expansion cards. The bus also includes two additional
ground pins (A32, B32) for enhanced reliability. The CoreModule/PC expansion bus provides 6 mA
drive capability on each line. All signal levels are TTL compatible.
To a great degree, the signals on this interface match their PC counterparts. However, since the
CoreModule/PC uses a 7.16 MHz clock and the original PC uses a 4.77 MHz clock, there are some
timing differences. The signal timings do, however, comply with the IEEE P996 Draft Specification
for the PC bus. The address for obtaining a copy of this spec is given at the beginning of this
chapter.
For compatibility with standard implementations of the PC bus, wait states have been inserted in all
expansion bus transactions for both I/O and memory cycles. If you wish to change bus timing,
contact Ampro Technical Support.
A 3.58 MHz bus clock is brought out to the PC expansion bus as the "CLK" signal (pin B20).
Specifications for the original PC indicate that this signal can be used for "synchronization," but the
actual signal frequency is not specified. Most standard PCs provide 4.77 MHz, "ATs" provide 8
MHz, and "turbo" PCs often provide 7.16 MHz, 8 MHz, or 8.33 MHz. The duty cycle of this signal
is also unspecified; the CoreModule/PC supplies approximately 50% duty cycle. The CLK signal is
active only during expansion bus accesses.
Another bus clock signal, "OSC" (pin B30), is generally specified to have a frequency of 14.31818
MHz, a frequency chosen because it is a useful timebase for video controllers, since it is four times
the standard NTSC color burst frequency. The IBM PC/AT computer provides this signal, even
though it serves no specific purpose on the AT motherboard. The synchronization of this clock
signal is not specified, and expansion card designers generally do not count on its being
synchronous with any other PC bus signal. In the CoreModule/PC this signal is 14.31818 MHz,
with a duty cycle of approximately 50%. Note that this clock is asynchronous, that is, no specific
phase relationship to any other timing on the module is specified.