I/O Configuration
VL-486-4 Reference Manual
Configuration – 25
I/O Configuration
U
SING
8-B
IT
I/O C
ARDS
I/O cards which only decode 8 address bits (A0 - A7) will work properly with the VL-486-4
provided the STD Bus signal IOEXP is decoded low on the I/O card. IOEXP will be driven low
in the I/O address range FC00h to FFFFh. The I/O card can be configured to use any 8-bit
address in the range 00h to FFh.
•
00h
−
FFh (With IOEXP decoded low)
A card which does not support IOEXP will repeat every 256 (100h) bytes throughout the entire
64K I/O space. This will cause conflict with reserved I/O addresses used for on-board devices.
Operation in this manner is not recommended.
Application software should be written to communicate with the I/O cards using the addresses
listed above as X+FF00h. For example if your I/O card is addressed at 38h, the software should
use FF38h as the I/O port address.
U
SING
10-B
IT
I/O C
ARDS
I/O cards which only decode 10 address bits (A0 - A9) will work properly with the VL-486-4
when addressed in the following I/O ranges:
•
100h
−
1EFh
•
200h
−
27Fh
•
300h
−
3AFh
A card which does not decode IOEXP low will repeat every 1024 (400h) bytes throughout the
entire 64K I/O space. This means the CPU will see the I/O addresses listed above as X+0000h,
X+0400h, X+0800h, X+0C00h, X+1000h, X+1400h, etc.
If IOEXP is decoded low, the card will only appear in the FF00h to FFFFh range (assuming the
card is addressed at 300h to 3FFh). Operation in this manner is not recommended.
Application software should be written to communicate with the I/O cards using the exact
addresses listed above (i.e., X+0000h). For example if your I/O card is addressed at 220h, the
software should use 0220h as the I/O port address.