----- Original Message -----
Sent: 3/27/2011 6:41:37 PM
Subject: Re: Serial Connection
On Sun, 27 Mar 2011 11:40:40 -0400 (EDT), owens@netptc.net wrote:
> On Sat, 26 Mar 2011 18:52:23 -0400 (EDT, Stephen Powell wrote:
>>
>> I haven't tried this, but one thing you want to make sure of is that
>> you use a "cross-over" cable. The serial ports on PCs have what's known
>> as a DTE interface (Data Terminal Equipment). The serial ports on
>> modems have what's known as a DCE interface (Data Communications
>> Equipment). A standard serial cable is designed to connect a DTE
>> interface to a DCE interface (i.e. a computer to a modem).
>> What you are trying to do is connect two DTE interfaces together.
>> For that you need to use a special serial cable called a "cross-over"
>> cable which is specifically designed to connect a DTE interface to
>> another DTE interface. If you try to use a regular serial cable,
>> one designed to connect a DTE interface to a DCE interface, it won't work.
>>
>> An alternative to using a cross-over cable is to use a device called
>> a "null modem" on one end of your serial cable. A null modem attached
>> to a standard serial cable effectively converts it into a cross-over
>> cable.
>>
>> Connecting a serial printer to a computer also requires a cross-over
>> cable or a standard cable plus a null modem, since both devices have
>> a DTE interface.
>>
>> You also might have to use two cables and two serial ports. One serial
>> port looks like a modem, with you as the terminal. minicom
>> allocates it. The other serial port looks like a serial console,
>> with you as the host. getty allocates it. The other server sees
>> a similar pattern.
>
> There's usually more than crossing Transmit Data and Receive Data.
> There are at least two control signals that are applicable.
> Usually one "borrows" what is known as a break-out box to determine
> which signals each side of the interface are active. There are
> lots of good googles for RS-232 that will help.
That's true. But a properly-wired cross-over cable or null modem
not only crosses over TD and RD but also crosses over DTR and DSR
and also crosses over RTS and CTS. Ground, of course, is wired
to Ground. CD is normally tied to DSR on the same side of the
interface (on both sides). RI is usually left unconnected.
See, for example,
http://en.wikipedia.org/wiki/File:D9_Null_Modem_Wiring.png
--
.''`. Stephen Powell
: :' :
`. `'`
`-
Yes Stephen that's the "traditional approach". Unfortunately there are at least half-a dozen non-standard configurations (e.g. permanent RTS, connecting DSR to DTR) which drive RS-232 people nuts. I agree that the place to start is with your traditional cable or with a null modem adaptor (essentially a black box that does all the interconnections for you, allowing straight through cables from either side to the boxes) but beware this may not work in all situations.
Larry
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