[GTALUG] Actual ttyS0 MIA

[James Knott]

On 2020-05-11 01:57 AM, Karen Lewellen via talk wrote:
> James remark about IRQ lines  can still ring true.  because IBM used 
> them oddly everyone else  decided to sing along. 

I used to be a computer tech, on the big systems.  I also designed and 
built an 8 serial port card for my IMSAI 8080.  In all my experience, I 
had only seen level triggered interupts or IRQs.  This meant when 
something needed service, it would pull the signal line low.  This 
allowed multiple devices to share 1 interrupt line and the OS would sort 
out which needed to be serviced.  IBM, instead of using that common 
practice, when with rising edge triggers, which cannot be easily 
shared.  The Intel interrupt controller chip that was used supported 
either mode, so there was no valid hardware reason, that I could see, 
for IBM to choose that method.  If they had gone with level trigger, 
then we wouldn't have had that IRQ sharing mess we had to deal with on 
the PC/AT bus.