>I've seen a lot of ads on the various mac sites for USB to serial
>converters. I've been wondering why, since most serial devices depend on
>two-way communication, these converters couldn't also be serial to USB
>converters, allowing us to tap into all the new USB peripherals. I'm sure
>there is a simple reason, but I'd appreciate it if someone would elucidate.
Relative speed. USB runs at a peak (burst) speed of 12 megabits/s. Without
getting into an explanation of asynchronous vs. synchronous operation, the
serial port on a Mac can be run with bursts of 230kb/s max, and the Geoport
that the 2400 and most other PCI Powermacs have can run at 2MB/s max. Data
from a USB device would simply overrun the serial port; either data bits
would be lost or performance would be abysmal with lots of retransmission
(I need to look at the USB spec s'more to know which :). It might be
possible to construct an adaptor that would negotiate the speed down or
provide a lot of internal buffering, but for the kind of performance you'd
see and the various compatibility problems from the higher latency, it
wouldn't be worth it.
The same would apply to a USB to Firewire convertor, or a USB to SCSI
convertor, because both of those standards are substantially faster than
USB-1. It'd be possible to come up with something that would work
occasionally, but it's just the wrong solution.
Localtalk to Ethertalk convertors suffer many of the same problems, but
they're more feasible because network communications are layered, meaning
that a TCP/IP or streaming Appletalk connection will guarantee reliability
by retransmitting until the data is successfully received no matter how
many ethernet packets are dropped midstream - it's a core assumption of
most network protocols and applications that many packets will go missing
or be corrupt, and that latency will be highly variable. Performance still
sucks that way, but it's workable.
--
Marc Sira | [EMAIL PROTECTED]
"If you can't play with words, what good are they?"
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