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

Robert Benward rbenward at verizon.net
Sun May 23 21:38:43 UTC 2010

Generally for a given DTE, there is a swap of 2&3 when going from DB9 to DB25 and vice-versa.  For a normal serial 
cable, DTE to DCE, pins 2&3 are swapped.   RS-232 states that pin 2 is the TX data, DCE or DTE, on a DB25.  It's TX on 
pin 3 for a DB9.  Therefore to hook a DCE to a DTE, you must swap pins 2&3.  The fact that you are going from 2-2 and 
3-3 probably means you've inserted a null modem in between, and you are using a standard serial cable.

In the old days... it was a Data Terminal Equipment  (DTE) that communicated with Data Communications Equipment (DCE) to 
the mainframe.  Stanley's assessment is correct, although I'm not sure if the mainframes of yesteryear were DTEs.

A lot of stuff since was designed as DTE, that's why all the trouble these days hooking up all this equipment, there is 
no standard as to whether a device should be designed as DTE or DCE.

My PCR-1000s are DCEs, as I use a straight RS-232 cable to hook them up.  The manual states the Z3801A is DTE, thus 
requiring a null modem in between.  That's what you built, when you ran 2-2 and 3-3 between the DB9 and DB25.  My DB25 
to DB9 adapter has 2&3 swapped.  And I know for sure that most DTE in DB9 have male pins.  The pins are a quick way to 
identify DTE or DCE.  On the back of the 3801, the DB25 is female, so like the rest of this spec, it's backwards from 


----- Original Message ----- 
From: "Stanley Reynolds" <stanley_reynolds at yahoo.com>
To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Sent: Sunday, May 23, 2010 4:56 PM
Subject: Re: [time-nuts] Z3805 utility, Was: AW: (no subject)

----- Original Message ----
From: Jeff Hook <jeffhook at comcast.net>
To: Discussion of precise time and frequency measurement <time-nuts at febo.com>
Sent: Sun, May 23, 2010 12:42:45 PM
Subject: Re: [time-nuts] Z3805 utility, Was: AW: (no subject)

My 3805 cables look like this

DB9 DB25
2 2
3 3
5 7
(Null Modem)



straight cable is 2 to 2 to 3 to 3

null modem would be

2 3
3 2

normal usage would be:

terminal (straight cable) modem (phone line) modem (straight cable) computer
computer (straight cable) modem (phone line) modem (straight cable) computer

or if no modems :

terminal (null modem) computer

As has been mentioned many exceptions to the rule.


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