PC cable on a MAC.
Gustavo A. Arroyo Torres
My PC is broke, but I have access to a Mac. So,
It is possible to connect my PC serial cable to a Mac port?
I also need to know in wich port of a Mac can the calculator be attached?
THANXS.
I know, i know... my english is not so good! (sorry).
Gustavo A. Arroyo T.
email: arr...@ibt.unam.mx
John Michael Sauder
In article <Pine.SOL.3.96.970703145422.19298A-100000@pbr322> "Gustavo A. Arroyo Torres" <arr...@ibt.unam.mx> writes:
>
>My PC is broke, but I have access to a Mac. So,
>
>It is possible to connect my PC serial cable to a Mac port?
>I also need to know in wich port of a Mac can the calculator be attached?
>
> email: arr...@ibt.unam.mx
I also had a similar question if anyone knows the answer.
I had tried connecting my PC cable to a Mac null-modem cable (intended
for connecting a Mac serial port to a PC serial port), but it didn't work.
Does the null-modem cable change the pinouts so that connecting my HP48
PC cable wouldn't work?
As far as Gustavo's second question: you can attach the cable
to either the modem or the printer port -- it doesn't matter as long
as you configure it properly on the Mac. I'd probably use the modem
port for consistency.
John H Meyers
In article <5q0l1a$qc8$1...@taurus.fccc.edu>,
sau...@polaris.fccc.edu (John Michael Sauder) writes:
> I had tried connecting my PC cable to a Mac null-modem cable (intended
> for connecting a Mac serial port to a PC serial port), but it didn't work.
> Does the null-modem cable change the pinouts so that connecting my HP48
> PC cable wouldn't work?
I'd expect so.
RS-232 provides for simultaneous "talking" in both directions between
two devices, by using two separate data transfer wires (in addition
to one common "signal ground"). However, in order to communicate,
the two devices must "complement" each other, so that the wire
on which device A "talks" is the wire on which device B "listens,"
and vice-versa.
Two computers, each having a "modem port," are naturally going to
be wired up identically; thus, if you connect the corresponding
wires (pin numbers) of each one "straight through," both of them
will attempt to talk on the same wire, meanwhile listening on
the opposite wire; result: conflict, no data transfer.
The very same thing would happen, of course, if you connected
two identical modems in the same way.
If you have one computer ("modem port" or "COM port") and one modem,
however, the modem manufacturer always wires up the modem to
"listen" to the pin number on which the computer manufacturer
is going to "talk," and vice-versa. Result: perfect communication.
But what if we want two identical computers to talk to each other?
Why, it's simple, Simon -- somewhere between the two ports, just
reverse the wires, so that computer A's "talk" pin gets connected
to computer B's "listen" pin, and vice-versa. This trick, also
known as a "cross-over," is part of the little gadget you get
from Radio Shack (or wherever) called a "null modem adaptor,"
which you simply plug into either end of an ordinary "straight through"
cable when you want to connect two "identical" sorts of devices.
Now, when HP makes up a cable for connecting the HP48, should they
wire it up the same way as computers are wired, or should they
wire it up the same way as modems are wired? Well, most people
want to connect the HP48 at one end of the cable to a computer
at the other end, so of course HP wires up that cable as the
opposite of a computer, as if the HP48 itself were a modem,
and thus the HP cable is ready to be plugged *directly* into
a computer's communications port, with *no* "crossover."
Thus, if you outsmart the folks at HP by inserting an extra
"null modem" or "crossover" of your own, it will no longer work!
OTOH, those few of you who want to attach the free end of your
HP48 cable to a modem, rather than to a computer's port, will
actually *require* a cross-over or null-modem to make this work.
BTW, engineers use some common "buzz-words" to label the two opposite
ways in which any device may wire up its RS-232 connector; if the
RS-232 connector is wired up the same way as a modem, the engineers
call it a "DCE" device (Data Communications Equipment); if the
RS-232 connector is wired up the way same way as a computer port,
then it's called a "DTE" device (Data Terminal Equipment).
Ordinary "terminals" (VT-100 etc.) are also "Data Terminal Equipment"
(what a coincidence!), just as are RS-232 "serial" printers.
Because of the complementary way in which their RS-232 connectors are
wired, if you want to connect two *different* device types (one DCE,
one DTE), then a standard "straight through" cable does the job nicely;
if you want to connect "like" devices ( DTE<->DTE, DCE<->DCE ),
then you need to insert a cross-over or null-modem adaptor.
The adaptor also connects a couple of other wires (RS-232 plugs have
either 9 or 25 pins, so they have plenty of other wires to worry about),
but those other wires are for functions which the HP48 doesn't have.
Okay, here's a question to test whether you "got" all this:
what should we call the HP48 -- DCE or DTE?
Since we can directly connect a computer port (DTE) to an HP48 cable
and have these talk to one another with *no* cross-over, the RS-232
end of an HP48 cable is indeed wired up the same way as a modem (DCE),
just as we already said, many paragraphs ago.
A whole gang of people said just the opposite when we last
dared to bring up this delicate subject, but as you know,
the big crowd is often found betting on the wrong horse :)
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With best wishes from: John H Meyers <jhme...@mum.edu>