[time-nuts] Z3805 utility, Was: AW: (no subject)

Stanley Reynolds stanley_reynolds at yahoo.com
Mon May 24 02:08:20 UTC 2010


Bob,

The best way to get it right is to use a break out box, if both lights connected to the data pins are on then it is right else swap the pins. If no BOB then measure pin 2 to ground and pin 3 to ground one should be near zero the data receive pin and the transmit pin should read a negative voltage the transmit pin. Connect transmit on one device to receive on the other as well as ground to ground. If hardware flow control is needed then cts and rts would be done the same way as the data pins, measure voltage match neg voltage to no voltage.

Trying to understand the terminology is nice but not as easy as just looking at which pin is the source, has voltage and matching it to a pin without voltage.

Stanley



----- Original Message ----
From: Robert Benward <rbenward at verizon.net>
To: Discussion of precise time and frequency measurement <time-nuts at febo.com>
Sent: Sun, May 23, 2010 8:27:01 PM
Subject: Re: [time-nuts] Z3805 utility, Was: AW: (no subject)

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:
>> 
>> 
> 
>> <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.
> 
>> 
> 
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