RS-232 vs. The Apple LaserWriter
Roy Smith
I just got my 2nd Apple LaserWriter and noticed that they have
re-written the manual a bit since I bought my first one 6 months ago. In
particular, they go into a bit more detail when they talk about the comm
connectors. Quoting from Appendix C, page 124:
Technically, the LaserWriter has a "DTE" type of RS-232
interface. This means it can be connected directly to a
host computer or a modem, with no signal reversals required.
Connecting to a terminal requires interposing a modem
eleminator [sic] (null modem) cable ...
Now, I don't claim to be an expert on RS-232 (although I usually do
manage to get things to work), but isn't that wrong? Shouldn't a "host
computer" also be a DTE? My Sun-3/50 has DTE ports on the back, and the
various serial ports on my Vax are also DTE. Is it common practice in
personal computers (whose owners, I assume, are the intended audience of
this manual) to have comm ports wired DCE?
--
Roy Smith, {allegra,philabs}!phri!roy
System Administrator, Public Health Research Institute
455 First Avenue, New York, NY 10016
Ben Cranston
> ... Is it common practice in personal computers ... to have comm
> ports wired DCE?
Not all that common, but sometimes it happens. Our office Apple ][+
has a CalComp serial port that is wired as a DCE. This makes it really
easy to attach a printer, but when one wants to use a modem one has to
use the Bizarro-world equivalent of a null modem cable. What would one
call such a thing? A null-terminal cable :-)
The UART (ACE) was designed to be a DTE, and cross-wiring it as a DCE
causes muy braindamage - weird in-out interactions like not being able
to disallow input with CTS while sending a break. A pox on the designer.
--
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Ben Cranston zben @ umd2.UMD.EDU Kingdom of Merryland Sperrows 1100/92
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