[time-nuts] Z3805 utility, Was: AW: (no subject)
rbenward at verizon.net
Mon May 24 01:27:01 UTC 2010
OK. I think I know where some of this confusion I'm having is coming from.. All these descriptions are misleading in
that they (Wiki) call pin 2 TX. On wiki's page, both DCE and DTE are TX=2. The table below shows the same except that
TX is defined from the DTE's point of view. Thus DTE to DTE needs 2&3 crossed, and DTE and DCE needs 2-2 & 3-3. If your
device is a DCE, then pin 2 is really a RX input, not a TX output.
DB-25 DB-9 Common EIA
Pin # Pin # Name Name CCITT DTE-DCE Formal Name
----- ----- ----- ----- ----- ------- -------------------
1 FG AA 101 - Frame Ground
2 3 TD BA 103 ----> Transmitted Data, TxD
3 2 RD BB 104 <---- Received Data, RxD
4 7 RTS CA 105 ----> Request To Send
5 8 CTS CB 106 <---- Clear To Send
6 6 DSR CC 107 <---- Data Set Ready
7 5 SG AB 102 ---- Signal Ground, GND
8 1 DCD CF 109 <---- Data Carrier Detect
Stanley, Did I get it right this time?Sorry to all for my previous ramblings.Bob----- Original Message ----- From:
"jimlux" <jimlux at earthlink.net>
To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Sent: Sunday, May 23, 2010 8:58 PM
Subject: Re: [time-nuts] Z3805 utility, Was: AW: (no subject)
> Stanley Reynolds wrote:
>> 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.
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