Hi Long ago I decided to go with the terms "straight" and "null modem" for the cables I use. NM and ST are easy to mark and hard to confuse.
Bob On May 23, 2010, at 8:58 PM, jimlux wrote: > Stanley Reynolds wrote: > >> <snip> >> Dec computers / terminal servers were as I described, but many brands >> were different. Still have a BOB aka break out box with LEDs to >> indicate levels, matching transmit and receive is easy, getting the >> hardware flow control / signaling right was a little more difficult. >> straight cable = pin to pin >> crossed cable = null modem = swapped pins >> The phrase "null modem" comes from no modems or the configuration >> that allows two singular ports to be connected, this cable would >> cross the receive and transmit pins, and some would call it a cross >> over cable. A null modem cable would be used to connect two computers >> together and a program like kermit used to transfer files. > > > Yep.. DTE cable to DCE communications medium(phoneline) DCE to DTE > DCE == Modem (e.g. a Bell 202 or 212, for instance) > > There were the flow control (RTS/CTS) used to turn around a half duplex link. > And, there are also the secondary transmit and receive (for a low rate > reverse channel). If you were receiving data from the link (DCE), you'd > assert RTS, and when the modem had switched, it would tell you CTS, and off > you'd go. (fancy modems used the reverse channel to send the request to the > far end, which would acknowledge... others just use a fixed time delay) > There are also pins for the clock (since some of these modems were used on > synchronous data links). > > the "crossover" occured in the DCE to DCE link (that is, you'd transmit from > one DCE to the other DCE's receiver)... > > the nominal cable between DTE and DCE was straight through. With no real > convention on male/female.. most devices had female sockets, and the cables > usually were male male plugs. IBM PCs had male on the chassis for DTE, as > did some PDT-110 (VT-100/LSI-11 smart terminals), but most other terminals > (the LSI ADM-x, Hazeltines, etc.) all seemed to have female, as did the TI > 800 series printer/terminals. > > So, a "null modem" was a cable that emulated the DCE to DCE connection.. > > there are/were various strategies on how sophisticated the reverse is.. do > you also send the secondary channel? What about clocks? Most folks ignored > all that and used RTS/CTS > > Or you strap RTS to CTS on your side, the other side does the same. > > > > > >> I think the phrase "standard cable" which could be null or straight >> depending on the use is the confusing part. >> Phone cables RJ11 and RJ45 swap the wires which is standard. Network >> cables match the wires with the same color always on the right which >> is standard. But even when a phone cable is standard it is not >> interchangeable with a standard network cable. Again we have a need >> for cross as well as straight network cables. > > > And, to make things worse, there are different "pair" arrangements. > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
