1 May
2012
1 May
'12
2:02 p.m.
On May 1, 2012, at 2:58 PM, Tony Duell wrote:
it strikes me that one of the main moans about RS232
was that there were
straight cables and null-modem cables and this confused people. If it had
been agereed that all 'hosts' -- that is computers -- would be wired as
DTEs, and all 'slaves' -- modems, printers, etc -- would be wired as
DCEs, there would bne no problem. The problem arrose because you could
link 'hosts' (computers) directly togther with RS232. USB seems to avoid
this my simply making it impossible to direclty link 2 hosts. I ds not
see that as an improvement!.
I doubt that was why they did it, though. I'm sure it had much more
to do with simplifying device electronics so they could be implemented
on very cheap microcontrollers at low power consumption, rather than
anything to do with the direction; after all, it's half-duplex anyway.
There's only one pair, and it goes both ways.
Bear in mind that Ethernet has the same issues (we just call "null
modem" cables "crossover" cables instead), and people got similarly
confused. These days, though, most PHYs are smart enough to auto-
detect when they need to switch the Rx/Tx pairs, which makes crossover
cables effectively obsolete with modern hardware.
- Dave