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From: Chris.Gehlker@p12.f56.n114.z1.fidonet.org (Chris Gehlker)
Newsgroups: comp.sys.mac.programmer
Subject: RE: What's the differance between serial Port A & Port B?
Message-ID: <29484.27342F46@stjhmc.fidonet.org>
Date: 4 Nov 90 14:53:28 GMT
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> "Port A and Port B are identical except that Port A (the modem port) has a
> higher interrupt priority, making it MORE suitable for high-speed
> communication"
> 
> What should I make from this? Does it mean I should be able to configure 
port
> B
> for 1200 baud input at my own risk of hardware overrun errors, or is such
a
> configuration internally restricted? I get the feeling the former is true,
> because when I try to configure port B for 300 baud I get the same error.
> What's going on? It's got to be a bug in my code but I can't find it 
anywhere.


To quote from page 2-9 of the Zilog Z8030 Z-Bus SCC/ Z8530 SCC Serial
Communications Controller Manual:

"Transmit interrupts, receive interrupts, and external/status
interrupts are the main sources of interrupts. Each interrupt source
is enabled under program control with channel A having a higher priority
than channel B and with receive, transmit, and external/status interrupts
pripritized respectively whithin each channel."

I think Apple is just trying to warn you that it's inherent in the design
of the SCC that port A will always be serviced before the printer port.
I wrote the device driver for the Kurta serial graphics tablets which run at
9600 baud.  They work fine on port B even if the user is communicating on
port A at up to 2400 baud. I can afford to loose a packet now and then.


 

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