Serial interfaces (was Re: Any former Psion 5 owners out there?)

Tony Duell ard at
Mon Jul 26 13:14:47 CDT 2010

> >> DOS-based software containing its own UART driver.  I don't even want to 
> >> think about trying to make that work on a PCI card RS232 port.
> > 
> > What is the great problem with doing this?
> To be honest, I've never really looked into it in detail.  The PCI bus 
> interrupt sharing thing always sort of scared me off from going any 

All decent buses allow you to share interrupts -- the ''design' of the 
interrupt suystem on the ISA bus is one reason to hate that bus.. 

Very well designed buses (Unibus :-)) have each card send an interrupt 
vector which directs the CPU to the right ISR. One others (e.g. HP's 
DIO-1 bus, the ISR has poll each card using that interrupt level to see 
if it's the source of the interrupt. I assume PCI does something like 

> further.  Maybe it's not as bad as I visualize.  Actually, it's probably 
> worse than I imagine.  I do recall that a contractor friend of mine 
> wrote a Windows driver for a custom PCI card designed by the EE's at a 
> company I once worked for.  His discussion of that effort did not make 
> me very optimistic either.  I'll have to ask him more about it some 
> time.  There is obviously a great deal I don't know about this.  I

Nor me. as I said, I don't own a PCI machine (and probably never will)
> probably should become more informed before exposing my vast ignorance 
> any further.  :-)
> OK, just for grins I Googled "PCI bus I/O address" (without the quotes). 
>   Looks like there is some sort of scheme where BIOS talks to each PCI 
> card on power up and dynamically assigns the I/O addresses for each card 
> within the 32-bit PCI I/O address space - whatever/wherever that is.

YEs, that was part of the 'no configuration links/switches' idea. I am 
not sure thats' a Good Thing, but it can't be too hard to work with.

> Must have been invented by the same folks who invented USB enumeration! 

Plenty of machines, including many classics, had some form of 
autoconfiguration. Having doen battle -- and won -- with the HP71's 
automaticlaly coffiguriuig nmemory, I can't beelive this is any worse.

> > Having seen bits of PCI card design, I am glad I design cards for my 
> > classics with ISA, Unibus, DIO-1, etc buses. Those I can understand and 
> > wire up a card for any one of them in a couple of hours...
> I feel rather the same way about the ISA bus.  It surely was/is a nice 
> and easy interface.  I suppose I'd have to admit the PCI bus has its 

The ISA bus is easy to design cards for (it's basically just the 80x86 
bus) but it does have that horrible interrupt system. The idiot who came 
up with the idea of active high, edge triggered, interrupt should be 
condemned to spenmd eternity in a part of hades having to design complex 
ISA cards and write drivers for them. He would then realise wby everyone 
else used active low level triggering.

> advantages, such as solving the "out of interrupts" problem (I think).
> I'm pretty sure it was not designed with the electronics hobbyist in 
> mind, but then not a lot of things are - the economic impetus is just 
> not there I'm afraid.

Alas not. But PCI is notably harder to design for than many of the older 
buses, which may well also not have been designed with the hobbyist in 
mind,but at least a serious hobbyist could wire-wrap an interface card 
and expect it to work. 


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