[net.micro.mac] Comm./Modem Port Warning!

harrow@exodus.DEC (Jeff Harrow, NCSE LKG1-3/F16 DTN=226-7445) (01/18/86)

This afternoon I was plugging my Modem port into an extension RS-232 (DB25) 
cable and, as  I fiddled with the plugs to get them to mate, noticed out of 
the corner of my  eye  that  the  Mac's  screen  dimmed toward black at one 
point.  I quickly realized  that this was due to a misallignment of the DB25 
connectors, where, as I attempted to mate them, the grounded portion of the 
female plug apparently shorted the +5 provided on the modem cable!!!!!

I was lucky in that (apparently, and  hopefully!)  no  damage was done, but 
***BE CAREFUL***, as I recall reading that the  current  is  NOT limited on 
these pins.


Jeff

jww@sdcsvax.UUCP (Joel West) (01/18/86)

In article <527@decwrl.DEC.COM>, harrow@exodus.DEC (Jeff Harrow) writes:
> This afternoon I was plugging my Modem port into an extension RS-232 (DB25) 
> cable and, as  I fiddled with the plugs to get them to mate, noticed out of 
> the corner of my  eye  that  the  Mac's  screen  dimmed toward black at one 
> point.  I quickly realized  that this was due to a misallignment of the DB25 
> connectors, where, as I attempted to mate them, the grounded portion of the 
> female plug apparently shorted the +5 provided on the modem cable!!!!!S

Rumor has it, this is why, when the smaller connectors (with 1 less pin)
were chosen for the Mac+, they dropped the +5V pin (Pin 2 on the DB9)
from the connector.

Unfortunately, some peripherals expect this (possibly as a power-on
indicator).  I know the Corvus net won't work.

	Joel West	CACI, Inc. - Federal (c/o UC San Diego)
	{ucbvax,decvax,ihnp4}!sdcsvax!jww
	jww@SDCSVAX.ARPA

jimb@amdcad.UUCP (Jim Budler) (01/19/86)

In article <527@decwrl.DEC.COM> harrow@exodus.DEC (Jeff Harrow, NCSE LKG1-3/F16 DTN=226-7445) writes:
>connectors, where, as I attempted to mate them, the grounded portion of the 
>female plug apparently shorted the +5 provided on the modem cable!!!!!
>
>I was lucky in that (apparently, and  hopefully!)  no  damage was done, but 
>***BE CAREFUL***, as I recall reading that the  current  is  NOT limited on 
>these pins.

Perhaps this is one of the reasons that, as I understand, the new MacPlus
does not have the 5 volts on the ports anymore.  Another is that third
party vendors were using that 5 volts to power their products.
-- 
 Jim Budler
 Advanced Micro Devices, Inc.
 (408) 749-5806
 Usenet: {ucbvax,decwrl,ihnp4,allegra,intelca}!amdcad!jimb
 Compuserve:	72415,1200

bart@reed.UUCP (Bart Massey) (01/20/86)

> This afternoon I was plugging my Modem port into an extension RS-232 (DB25) 
> cable and, as  I fiddled with the plugs to get them to mate, noticed out of 
> the corner of my  eye  that  the  Mac's  screen  dimmed toward black at one 
> point.  I quickly realized  that this was due to a misallignment of the DB25 
> connectors, where, as I attempted to mate them, the grounded portion of the 
> female plug apparently shorted the +5 provided on the modem cable!!!!!
> 
> I was lucky in that (apparently, and  hopefully!)  no  damage was done, but 
> ***BE CAREFUL***, as I recall reading that the  current  is  NOT limited on 
> these pins.
> 
> Jeff

Better yet, I once thought that it was ok to build mac-mac cables with
the +5 pins connected together.  Big mistake, as we found out when we
realized that if one mac was on and the other off, the on one powers the
logic board of the off one through that 5-volt pin!!!!  You have been
warned...

				Bart Massey
				..tektronix!reed!bart

"Wait, why is that Mac drive whirring with the power turned off?"
"Must be the serial cable."