Re: Cable Question

[Tim Meushaw <tmeush1@gl.umbc.edu>]

On Tue, 29 Nov 1994, David Empson wrote:
> In article <Pine.SGI.3.90.941128081445.25213C-100000@umbc9.umbc.edu> you write:
> > On Mon, 28 Nov 1994, David Empson wrote:
> > > Another possibility would be a Mac to DB-25 modem cable, with a null
> > > modem cable between that and the PC.
> That was a neat trick - I'm sure I replied to you in E-Mail, yet your
> reply turns up as a posting on comp.sys.apple2.  This is NOT normal
> practise.  If I did reply as a post, it hasn't turned up on my own site!
> Did you post it deliberately, or is there a problem with your
> news/mail software, or am I just confused?  :-)

Pine version 3.90 can send mail to a NNTP server if I so desire.  The
newsgroup name where the original post was taken shows up, and when I
reply, I can choose to post my reply to the newsgroup...makes things
quite handy.  :-)

> > How come the null modem cable in there?
> Because if the cable is designed to plug into a modem, you need to
> swap all the flow control signals and data lines over if you want to
> plug it into a PC.
> The PC's serial port is a DTE (Data Terminal Equipment), as is a
> terminal or a serial printer.
> A modem is a DCE (Data Communications Equipment or Data
> Circuit-terminating Equipment).
> The DTE/DCE designation detemines which direction most of the data
> signals flow: a DTE transmits data on pin 2 and receives on pin 3; a
> DCE is the other way around.  All the flow control signals are
> similarly swapped (DTE outputs on pins 4 and 20, inputs on pins 5, 6
> and 8; DCE is the opposite for each pin).  Pin 7 (signal ground) is
> common to both.
> A DTE can be plugged into a DCE using a direct pin-to-pin connection
> for the desired signals.
> Plugging a DTE into a DTE, or a DCE into a DCE requires a null modem
> cable with the desired signals swapped over (e.g. pins 2 and 3, the
> data lines).
> A cable that plug into a DCE (as in this case) is effectively a DTE
> (the cable will output data on pin 2, where the modem expects to
> receive it).  You therefore need a null modem cable to swap the data
> and flow control lines over so that the PC receives data on pin 3, and
> similarly for the other signals.
> If you use a Mac to ImageWriter I cable, you will not need a null
> modem cable, because the cable is designed to plug into a DTE (and its
> connector is therefore a DCE).

Well, I'm using a Mac to a Hayes modem, so I guess I'll run out after
class to the bookstore and buy the null cable (please, PLEASE let them
have one....)
Thanks for all your help.
Hope you don't mind if I post this message to the newsgroup, for any
who want to read the explanation who might be having my problem (yeah,
right, I'm one of only a few who have attempted to hook an Apple
printer to an IBM computer.... :-)
Thank you for replying in E-mail, though, I greatly appreciate that.

------------------------
Timothy A. Meushaw   tmeush1@gl.umbc.edu   http://umbc8.umbc.edu/~tmeush1/
University of Maryland, Baltimore County
"Give you everything I got for a little piece of mind!"  (The Beatles)