On Mon 06 Oct, Sean Breheny wrote: > > Hi again guys, > > I've got two more ISA bus questions for the ISA card I am working on. #1) > When I don't want to cause an IRQ, must I tri-state my lines that go to the > IRQ pins or can I just hold it low? #2) How can I cause an IRQ greater than > IRQ7? > > Also, It would be great if someone could point me to a web site with info > on the ISA bus. > > I appreciate it very much. > > BTW The ISA card I am building is a high speed bidirectional 8bit parallel > interface card, specifically designed to interface with digital circuits > that I build, such as PIC based circuits. I realise that you're probably doing this mostly because of a desire to build something yourself, but having said that there is an easier way to get a fast 8-bit bidirectional interface. In my current project at work I'm using the IEEE-1284 parallel port on a desktop machine to connect to my device. Due to how this particular desktop is designed, I can 'only' get it up to 1MB/s, on peecees you should be able to get 2MB/s. The parallel port in a PC is typically implemented with a 37C665 or one of its siblings, which isn't terribly difficult to program. Most tricky bit is getting the IEEE-1284 negotiations right, these make sure that you don't burn out your tristate drivers when you connect your box to a PC. All new PCs, and almost all of the multi-I/O cards made in the last few years will have a printer port that allows you to select EPP and ECP modes. ECP mode is designed to use interlocked handshaking and generally works very well. It will allow you to use DMA to transfer data straight into memory. The only problem from a PIC standpoint is that it requires a lot of pins (8 data+9 control). Of course if you're just designing something that you're going to use at home then you can get away with just ten signals: 8 for data and 2 for the interlocked handshake. Frank ------------------------------------------------------------------------ Frank A. Vorstenbosch Phone: +44-181-636 3391 Electronics & Software Engineer or: +44-181-636 3000 Eidos Technologies Ltd., Wimbledon, London Mobile: +44-976-430 569