Re: ADT and SSC speed increase, need help!

Jeff Blakeney wrote:

On Sat, 05 Nov 2005 18:15:47 GMT, Andy McFadden <fadden@xxxxxxxxxx>
wrote:


Knut Roll-Lund <kr-lund@xxxxxxxxxxxxxxxxxxx> wrote:

I'm wondering: How fast can an Apple II receive data, buffering it without loss? I though 19200 was max. II/II+/IIe/IIc/IIgs? 

It can read a 5.25" floppy drive at 32usec/byte, and even do some
decoding.  That's 31KB/sec, equivalent to 305Kbits/sec (assuming the
serial protocol uses 10 bits per byte, which is typical).

I used ProTerm at 57600 (requires a patch?), and AppleTalk transfers
use an external clock that allows 238Kbits/sec.



The 57,600 bps is the rate at which the hardware transfers data.  A
IIgs' serial hardware can handle that speed.  This is asynchronous
communication which means that the timing of when each bit will be
transferred is based on a clock at each end of the transmission.  This
is why some IIc's can't go above 9,600 bps because the clock on the
IIc is a little off of the standard so at higher speeds the receiver
is looking for bits at the wrong times and therefore gets garbage.

Now, the reason we have hardware and software handshaking is so that
the receiver can tell the sender to stop sending data while it does
something with what it has already received and then can then tell it
to continue sending.  Like I said in my reply to Ed, even my IIgs
running at 8 MHz with the serial port set to 57,600 bps only transfers
about 45,000 bps because it has to keep telling the sender to pause
while it does something with the data it has received.  Usually this
entails disk access which really slows things down.

This is actually what I was wondering about, sustained transfer, how much throughput. Here it seems the 8MHz IIgs does sustained 57600 ok. It depends on the software and hardware. I'm thinking of this number without the saving to disk which is a considerable part of what takes time with ADT. How about an enhanced IIe and SSC would there be any gain at all with an upped baudrate (disregarding the disk access)?

> AppleTalk, on the other hand, uses synchronous communications and the
IIgs serial hardware maxes out at 230,400 bps.  In synchronous
communications, one wire of the cable is used to tell the receiver
that there is a valid bit on another line.  If I remember correctly,
there is a third wire that the receiver signals back to the sender
that they've received the bit.  Again, the receiver has the ability to
tell the sender to wait while it does something with the data.  That
is why AppleTalk on the IIgs doesn't use the full 230,400 bps transfer
rate.  The AppleTalk code itself, slows things down.

At least, this is my understanding of how all this works.  :-)


Knut
.

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