From: John E. Malmberg [wb8tyw@qsl.net]
Sent: Tuesday, November 23, 1999 4:11 PM
To: Info-VAX@Mvb.Saic.Com
Subject: Re: MMJ Connector Pinout

 Ebinger . Eric <EEbinger#drc.com> wrote:


>There is a diagram in the back of the Digital Server 3300 Service Manual
>(http://www.digital.com/alphaserver/archive/3300/k7fwwsg.pdf) of the COM1
>MMJ connector.
>
>The diagram shows something like the following:
>
>     ----------
>     |        1 = DTR
>     |        2 = -TXD
>     |        3 = CHAS GND
>   ---        4 = -RXRTN
>   |          5 = -RXD
>   ---        6 = DSR
>     |---------
>
>Does that help?
>
>Eric Ebinger


Is this document correct?  It does not agree with other published MMJ or
MOD-TAP wiring documentation I have seen.

It was my understanding that with MOD-TAP the PIN 3 was the TXRTN, for
connections to RS-423 devices.

It may be possible that the TXRTN is also tied to the CHAS GND, but that
should be stated.  The chasis ground is intended for shielding purposes, not
carrying a signal, and connecting equipment in this way may result in a
ground loop.


Also when connecting MOD-TAP to an RS232 device, both the TXRTN and the
RXRTN need to be connected to the signal ground.

I have seen some interesting problems occur when this was not done, and some
beings that should have known better totally puzzled when they occured.
They could not believe it was wired wrong, when it worked over 90% of the
time.  That last 10% they could not understand.



Also I would not recommend using the built in serial ports on a VAX 4000-100
at moderate to high data rates, as this will be CPU Interrupt intensive.
You will get better through put with a DECSERVER or a CXnnn controller on an
attached Q-BUS expansion box.

-John