Re: Apple IIc+ @ 19200

[dempson@actrix.gen.nz (David Empson)]

In article <tolsenCrLMLp.572@netcom.com>,
Terry R. Olsen <tolsen@netcom.com> wrote:
> Matthew Pearce (mpearce@crl.com) wrote:
> :  >Does anyone have an Apple IIc+ hooked up to a high speed modem?
> :  >I have a 14400 modem hooked up but anything above 9600 loses
> :  >characters (although file transfers go off without a problem).
> :  
> : Make sure your cable is designed for "high speed" or hardware handshaking.
> : I've heard that the true test is to transfer files using Z-modem.
> :  
> 
> The cable is a hardware handshaking cable.  I use it on my GS as well 
> with no problems at high speed.  Just can't understand why it doesn't 
> work well on the IIc+...

The IIc and IIc+ are not really able to deal with hardware
handshaking, due to limitations of the 6551 ACIA (the serial chip used
in the IIc and IIc+).

Each flow control line is supposed to tell the other device when to
stop transmitting, without affecting received data.

The IIc and IIc+ use the RTS and DCD pins on the 6551 as the flow
control lines.

If RTS (output from IIc) is deactivated, the IIc will stop
transmitting characters.  It will also chop off a character which is
being transmitted, probably causing data corruption (this will
probably cause a block retransmission if you're using an error
detecting protocol such as XModem, YModem or ZModem).

If DCD (input from IIc) is deactivated, the IIc will stop receiving
characters.  The modem will be unaware of this, and will keep sending
characters, which the IIc will completely ignore.  This will also
require retransmission once everything is back to normal.

In addition, the software in the IIc will notice that the incoming
flow control line is off, and will stop transmitting.

In effect, while either flow control line is off, no data will pass in
either direction (in the case of DCD, the modem may be transmitting,
but the IIc will ignore it).


These problems don't exist on the IIgs - deactivating flow control
lines doesn't directly affect the transmitter or receiver: the
change of state in flow control is handled by software.
-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand