17 Jun
2013
17 Jun
'13
8:53 p.m.
On 06/17/2013 07:52 PM, Mouse wrote:
I'm not sure what's supposed to be unfriendly about RS-232C. The major
unfriendly part there, it seems to me, is the space in the middle, -3V
to +3V IIRC, for which the logical signal state is undefined.
Well, the problem is that to go from mark to space, you need to swing AT
LEAST 6 volts to be defined. Given that most logic is either 3V or 5V,
that's a bit of a problem.
OTOH, USB is differential, not single-ended. Had you specified RS-422
or RS-486 as a comparison, I would admit that both were competitive with
USB.
Every citation of the TIA/EIA/RS-232 spec talks about the range of
voltages used for signaling, but I can't find a citation of "voltage
into what LOAD" defined. Is 2 ohms too small; is 2 megohms too big?
How much current at the specified voltage range is to be supplied by the
driver?
RS?EIA?TIA-232 is a single-ended protocol, which makes it less tolerant
to various signal issues, such as ground loops, induced noise, etc.
I remember that the old MC1488 driver were power-hungry devices; about
half a watt per package if memory serves. The 1489 receiver was nearly
as bad in terms of power.
'232 was a good interface for the 1960s and 70s, but it's not a great
choice today.
--Chuck
232C and USB are both bit serial, but USB has higher
speeds,
friendly voltage levels, power from a host included and well-defined
protocol.
I question friendly voltage levels. 3.3 volt powered logic and chips
came just after them if I recall correctly.