To All, Another confusing thing about DTE & DCE as it relates to RS232. There are various pins on RS232 that are either inbound or outbound depending upon whether it is DTE or DCE. For example, Pin 2 is transmit data out from a DTE device and transmit data in on a DCE device. Pin 3 is receive data in on a DTE device and receive data out on a DCE device. Another example, Pins 15 & 17 are transmit and receive clock. On a DCE device, these signals are outbound from a DCE device and inbound on a DTE device. I know it sounds confusing but once you work with it, you will get the hang of it.
Mike >>> "s vermill" 01/03/03 05:26PM >>> Duncan wrote: > > Hi > > I am busy studying for the CCNP Remote Access exam and am > really stuck > on the modem signalling bits. I think that the key to my > problem is that I > don't understand the definitions of a DCE & DTE properly and > how they relate > to the EIA/TIA-232 cabling pinouts. (which for some unknown > reason you must > learn) I hate learning anything parrot fashion, I would rather > understand > it. I have looked through the archives and there are some > pretty useful > pointers but I am still not all the way there. > > Does any one have a comprehensive description that they can > point me to, > preferably with examples of set-ups and how it all relates to > the OSI model. > > Thanks > Duncan > > I'm not sure exactly what you mean by "modem signalling bits." Since you mentioned the pinout, I'm thinking maybe you're asking about DTR/DSR, etc? I think the TIA-EIA-232 spec does a pretty good job of going over all that but it'll cost you some $$ to get your hands on a legal copy. There are actually a few generally accepted breakdowns of the acronyms DTE and DCE. These days I mostly see Data Communications Equipment and Data Terminal Equipment. The 232 spec actually goes with Data Circuit-terminating Equipment for DCE. A DCE is generally a "modem-like" piece of equipment whereas a DTE is pretty much a "terminal" or "user" device that can't interface directly to a circuit. The signals such as DTR/DSR are actually pretty straight-forward, but I seem to recall that most CP books on the subject manage to mangle it pretty badly. As an example, working strictly from memory, I recall that one or more CP book stated that asserting DSR means a modem is ready to send data to the DTE. I've never seen that in any implementation. The most common sequence that I see is this: Carrier Detect (pin 8) is asserted by the DCE when it is synchronized to the circuit or distant end (this behaviore is often strapable in that the DCE can be made to assert CD immediately upon powerup and self-check). Data Terminal Ready (pin 20) is set by the DTE upon power up and self-check. Data Set Ready (pin 6) is set by the DCE either upon power up or in response to DTR having been asserted by the DTE (this behavior is often strapable one way or the other) RTS (pin 4) is asserted by the DTE when it has data to send (this behavior is often strapable in that the DTE can be made to always assert RTS). One other use is where half-duplex circuits are present, asserting RTS can signal the DCE to switch to tx mode and de-asserting RTS can signal the DCE to switch to rx mode CTS (pin 5) is asserted by the DCE when it has allocated the resourse in response to having received RTS from the DTE (this behavior is often strapable in that the DCE can be made to respond to RTS immediately or over some delay after detecting RTS regarless of what is going on with the circuit) Sometimes the sequence of all this matters, other times it doesn't. I work with some equipment that absolutely requires that the DTE first assert DTR before RTS. If both happen at about the same time, the RTS is ignored by the DCE and a CTS is never given. Also, there are, of course, variations and additional uses. For example, some modems (DCE) refuse to answer an incoming call if a connected DTE hasn't asserted DTR. Sorry if I missed the point. You might have been asking about scripts or something? Scott Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=60523&t=60240 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
