From: olson@mydual.uucp (Kirtland H. Olson)
Newsgroups: comp.sys.3b1
Subject: Re: 610 terminal w/ 7300 (long >190 lines)
Keywords: RS232 PINOUT SERIAL NULL TERMINAL
Date: 16 Oct 92 14:59:20 GMT
Article-I.D.: mydual.1992Oct16.145920.9547
Reply-To: olson%mydual.uucp@alliant.com
Organization: The Harvard Group, 01451-0667

In article <1992Oct15.145706.1@ulkyvx.louisville.edu> jwstok01@ulkyvx.louisville.edu writes:
>Ok, one last inquiry.
>As suggested, I connected my att610 teletype terminal to the RS232 prot on my
>7300 via a null modem cable.  It still doesn't work.  (I have also used the
>terminfo database to "update" the terminfo file (although the 610 was already
>there)).  I have 3 final questions. 
>
>1.  Is it pins 2 & 3 that are swapped on a null modem cable.  I haven't had the
>opportunity to go in search on one or one of those adapters, and don't know
>anyone who has one to test.  I found the one I have at work and bowwored it.

[stuff deleted]

>2.  Does the terminal require moreor other pins than 2-8, 20, 22 on the cable.
>If so, does anyone know which ones??

[more deletions]

First, the 610 will work with the UNIX/PC, it is a supported terminal. 
Thus the connection depends on the terminal working and using a proper
cable and null modem with the right port configuration.

Here's the basic info on UNIX/PC serial ports:

Pins supported on the built-in port *and* on EIA/RAM Combo boards are:
                        1-8,15,17,20,24

Asynchronous connections use only 1-8,20.  Pins 15,17,24 are clock pins
for use with synchronous devices.

The port is labeled DTE because *it* is a Data Terminal Equipment
configuration.  That *means* it transmits on pin 2.  It also means that
whatever hardware flow control signals are supported will have the
*terminal* nature. 

Connect modems to DTE ports with a straight-thru cable.

Connect terminals to DTE ports with a null modem.

The UNIX/PC Telephone manager manual suggests two null modem
configurations usable with terminals.  Both are symetric--the
connections are the same at both ends of the cable.

Data-only, no control signals:

   Straight-thru to corresponding pin:

     1,7      these are grounds

   Cross-over to corresponding pin

     2,3      cross the data lines

   Jumper at each end

     4&5      connect RTS to CTS

     6&8&20   connect DSR to CD to DTR

This null modem will always work since no control leads are passed
across the interface.  Because the control leads don't connect, turning
off the terminal does not log the user out.  This is equivalent to a
Black Box ME202B null modem.

Data and Control

   Straight-Thru to corresponding pin

     1,7   these are grounds

   Cross-over to corresponding pin

     2,3   cross the data lines

     4,5   cross RTS and CTS

     6,20  cross DSR and DTR

   Jumper at each end

     6&8   connect DSR to CD

Note that pin 6 has TWO connections.  This type of cable is used
primarily in terminal to UNIX/PC connections.  Turning off the terminal
disconnects the line.

End of basic data

Not all null modems use the same circuit.  As you see in the preceding,
the UNIX/PC manual shows two symetric designs.  Many kinds exist, and
some asymetric designs exist.  What's more, manufacturers seem to choose
male and female connectors without regard to any obvious standard.  Thus
you may need a gender changer as well as a null modem.

Now some hints about older terminals.  Most terminals, especially
older ones, expect the hardware handshaking lines to be operating. 
Those terminals don't function until they see the handshake.  If you can
turn handshaking off at *both* ends, you only need pins 1,2,3,7 to
communicate between two devices and only 2 & 3 need to be crossed for
like devices (two DTE's or two DCE's).  

Many terminals use a "wrap plug" for testing.  Wrap plugs look like one
end of the simple null modem *except* that pins 2 & 3 are connected in
the wrap plug.  Then the terminal can be put in it's on-line mode and
talk to itself.  (Terminal keyboards and screens are not connected to
one another except thru the attached device unless you turn on local
echo or sometimes half-duplex.)  Making the terminal end of a null modem
look like the terminal's wrap plug--except for pins 2 & 3--should always
produce transmission and reception by the terminal.

Different designers make different choices about timing and sequence of
handshake lines.  That means that some combinations of RS232 devices can
not function together with null modems, or perhaps without some special
connection that delays one signal.  These devices generate a deadly
embrace in which neither will react until the other asserts some signal
which the other won't assert before the first asserts...you get the
picture.  I've seen this mostly in early personal computers.

Terminals that type to the screen in LOCAL mode may *not* function
through their wrap plug.  The designer may choose to bypass the circuits
that drive the lines, so some defects affect only the on-line state.

When one end of the connection is a computer and the *software* says you
can turn hardware flow control off, it may only mean the software will
ignore flow control signals.  Thus the physical connection may still
need the signals before the terminal can function.

Some terminals do synchronous communications.  Some terminals have
separate connectors for synchronous and asynchronous operation.  At
least with synchronous modems connected to asynch ports, you get some
functioning--symbols do move along the connection.  The data rate
appears very slow and the error rate is very high.  If the serial port
is labeled "host," verify that this terminal has an asynchronous mode
through that port.  

FOF Cables

Found on the Floor cables often bring surprises.  Just because a cable
has all the pins does not mean it's an extension cord--maybe they're not
all wired.  Just because it's got pins 1-8 & 20 doesn't mean it's an
extension cord.  It also doesn't mean it's got 9 wires.  I've opened the
hoods of cables and found internal jumpers, crossovers between ends, and
too few conductors to make the change I wanted.  Many old cables are not
symetric, the connections differ at each end.  

In testing, I prefer ribbon cables with all connections made, fully
wired gender changers, and the little null modem kits that I can wire as
I want them.  And the gadget with the 7 2-color LED's lets me see the
transmitted and received data lines flash.  That way I know whether
there's no data to be seen or the device does not see the data. It also
shows the state of the handshake lines.

Once the testing is over and the system works, I can often use an FOF
cable for the permanent connection.  Even then, the cable usually needs
to be modified.  

For More Info

If you need more detailed info and step-by-step procedures, try Martin
Seyer's book "Complete Guide to RS232 and Parallel Connections" ISBN
0-13-783515-9 {paperback} or 0-13-160201-2 {hardcover} which actually
contains an XT to UNIX/PC example, or the sequel.

Good Luck,

         --Kirt


>--------------------------------------------------------------------------------
>John W. Stokes, Jr., Triangle Fraternity    | DISCALMER:  Unless otherwise
>Internet: jwstok01@ulkyvx.ct.louisville.edu | stated the autor claims to be 
> 	  jwstok01@vlsi.ct.louisville.edu   | right only in his own reality, 
>	  jwstok01@ulkyvm.ct.louisville.edu | not in anyone else's.  But he
>Bitnet:	  jwstok01@ulkyvx.bitnet            | is willing to be disproven in 
>Home Phone    (502) 852-3881    	    | a reality of compromise.
>


-- 
Kirtland H. Olson                     olson%mydual.uucp@alliant.com
Newsgroups: comp.sys.3b1