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參數(shù)資料
| 型號： | PCSYSCON |
| 英文描述： | PC(ISA)BUS I/O CARD |
| 中文描述： | 電腦（ISA）總線的I / O卡 |
| 文件頁數(shù)： | 9/36頁 |
| 文件大小： | 383K |
| 代理商： | PCSYSCON |
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Section 2. The PCSYSCON I/O Map
The I/O Pointer Scheme
There is a serious shortage of I/O space in most PCs. This can be a
real limitation if I/O boards have lots of functions and hence lots of
registers, like the PCSYSCON. The I/O pointer scheme used on the
PCSYSCON and other Arcom PCbus boards solves this problem.
In outline, to access a register on the PCSYSCON you must first set
up a pointer to it by writing a byte to the 'base address' of the
PCSYSCON. After that, you can read from and write to the register
that is pointed to, by accessing the byte at the base address + 1. The
base address is the address that is set up on the address switches,
subject to the constraint that it must be an even address (an odd
address set up on the switches is treated by the PCSYSCON as the
next lower even address).
Given that the pointer value is a byte, there are 256 possible
registers on a PCSYSCON. Obviously, not all of them are actually
used. In fact, on most boards very few of them are used, but the
possibilities for expansion are there.
In order to allow standardisation of software some register
addresses have been defined for all Arcom PCbus I/O boards. In
particular, the top half of the 256 byte space has been defined as
'special function' register space, and the bottom half as 'I/O' register
space. The special function registers are mostly devoted to self-test,
checking, security and diagnostics. The I/O registers are the ones
which the board is there for: in this case monitoring, alarm and
input/output functions.
The next two sub-sections describe the register allocations in detail.
How the PCSYSCON Appears in PCbus I/O Space
The PCSYSCON occupies two bytes in PCbus I/O space. They start
on an even byte boundary. The lower byte contains the pointer and
can only be written to. The upper byte contains the data and can be
read from or written to.
The address switches define where these two bytes are in PCbus
I/O space. The switches set the address of the lower of the two bytes
(the base address); the upper byte is one byte up from the lower.
Another way of saying this is that the board can only be addressed
at even byte boundaries and takes two consecutive bytes of I/O
space.
To set the address simply rotate the switches until the desired base
address is visible, reading them left to right, as text is normally
viewed. For example, to set up an address of 10CH, the left-hand
switch is set to 1, the centre switch to 0 and the right-hand switch to
C. Note that the addresses are in hexadecimal.
The problem comes in finding what to set the switches to. Many PCs
are not supplied with any information about what I/O devices are
2192-09065-000-000
Section 2. The PCSYSCON I/O Map
Page 9
J278 PCSYSCON
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