8 Bit ISA slot. by Lee Davison
Introduction

I wanted a quick way to connect an 8 bit ISA slot, only to access the IO adresse range, to the SuprChip V microcontroller. The circuit I came up with appears on the right.

The address lines A5 to A15 can either be set using jumpers or switches (I used switches) or, if you only need one address range, can be hard wired to the corresponding address.

Any of the five interrupt lines, from IRQ3 to IRQ7, can be selected by the use of a jumper. Again if you will only need one (or none) of these then the desired interrupt can be hard wired.

This circuit could be fairly easily adapted as a memory mapped device by connecting the corresponding address and data lines (possibly through buffers), by driving the IOR and IOW pins with decoded read and write signals, and by connecting the interrupt, via an invertor, to the NMI input on the processor. The RESET line can be driven, again via an invertor, by the processor RESET line or by an output port line. This line is active high.

While experimenting with different network cards I discovered some of them need the +12v supply in addition to the +5v supply. This additional connection is shown in red on the diagram.

As I had a stack of 3Com 3C509B NICs of various types I decided to try to use them. Well it turns out that even with PnP disabled you still need to access the card's id port on $01xx. To allow this Port 6 bit 6 was connected to the A9 line, so both $03xx and $01xx can be accessed by the software. This additional connection is shown in green on the diagram.


Software

Though the slot can be accessed entirely from EhBASIC it was decided to create two short routines to make accessing the slot faster and easier. To that end one function, to read a byte, and one subroutine, to write a byte, were written.

After initialising the function and subroutine addresses thus ..


..
30 DOKE $44,$EF20 : REM USR(n) = read from IO address $xxnn	
31 rr = $EF00 : REM call address for write to IO address
..

It is a simple matter to write a byte by doing ..


20500 CALL rr,cr,dd : REM set control register			
20505 REM cr is the IO address - $00 to $1F
20510 REM dd is the byte value for that IO address

.. and read a byte by doing ..


10050 by = USR(cr)						
10055 REM cr is the IO address - $00 to $1F
10060 REM the byte value is returned to by

You can download or view the assembly source for the read and write routines.

Last page update: 7th September, 2003. e-mail me