[Coco] Drivewire question

[Steven Hirsch]

On Wed, 25 Mar 2009, Darren A wrote:

> On 3/25/09, Steven Hirsch wrote:
>>
>> I have some doubts about the 6551 being capable of 115.2k baud.  Even if
>> the chip itself can do it, the expansion bus may prove to be a bottleneck.
>>
> ---
>
> What do you mean by "expansion bus"?  Are you talking about the
> cartridge port and/or MultiPak?
>
> There would certainly be no bottleneck there since data can traverse
> the bus at a rate of at least 1 byte every cycle.  For 115.2k baud,
> the CPU only needs to service the 6551 roughly once every 77 cycles
> (not including any status polling).  The double density floppy
> controllers have to transfer data at more than twice that speed (250
> kbps).

Granted, but I'm thinking of a case where you are reading from one 
cartridge and writing to a device on the other.  From an electrical point 
of view, you've just doubled the bandwidth demand.  Plus, there's 
interrupt handling overhead and latency for the RS232 pack.  It will be 
interesting to see if that combination can keep up with the bit-banger 
port + cartridge port (which, indeed, is an expansion bus).

Steve


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