29 Apr
2015
29 Apr
'15
6:36 p.m.
From: Josh Dersch
it never occurred to me to just try flipping it around
since usually
that's a pretty good way to let the magic smoke out of things.
Exactly. Except for cases where I _know_ the connector was designed to be
able to withstand being plugged in backwards (e.g. IDE), I would _never_ try
reversing a cable 'on spec'. The chances are just too big one will kill
something.
(Amusing factoid: QBUS cards have the same thing, they are designed so that
you can plug them in backwards, and not kill things. I am somewhat ashamed to
admit that one one occasion, I actually did this! Luckily the engineer who
did the QBUS saved my behind.)
From: Johnny Billquist
On 2015-04-27 00:50, Mike Ross wrote:
> One does not casually reverse the installation of
cables just to see
> if maybe it will work!
Actually.... I often have done so, just because of
this.
And how many times have you melted something down, doing that? :-)
Noel
29 Apr
29 Apr
7:24 p.m.
On Wed, Apr 29, 2015 at 6:36 PM, Noel Chiappa <jnc at mercury.lcs.mit.edu> wrote:
*Mostly* on DEC equipment, reversing non-keyed cables is safe. Mostly.
From: Josh
Dersch
it never occurred to me to just try flipping it around since usually
that's a pretty good way to let the magic smoke out of things.
Exactly. Except for cases where I _know_ the connector was designed to be
able to withstand being plugged in backwards (e.g. IDE), I would _never_ try
reversing a cable 'on spec'. The chances are just too big one will kill
something. (Amusing factoid: QBUS cards have the same thing, they
are designed so that
you can plug them in backwards, and not kill things. I am somewhat ashamed to
admit that one one occasion, I actually did this! Luckily the engineer who
did the QBUS saved my behind.)
You can plug an RL02 interface cable backwards into an RL-11 and
nothing bad happens, and many, many other such examples exist, to the
quantity that back in the day, customers assumed nothing would go
wrong if it was reversed, and, again, in nearly every case, they were
correct - it wouldn't work, but it shouldn't smoke, either. In that
environment, when we were moving from making Qbus and Unibus products
to add a BI product, our design engineer had a dilemma - formerly,
there was enough room on our products to put the core functionality
and the traffic lights and EIA line drivers and 40-pin plugs right on
the edge of the board. No problems. The BI is quite different in
that respect (amount of square inches, no edge connectors allowed,
etc). His solution was for us to have a I/O plate board with two
30-pin cables that plugged into the non-BI segments of the backplane
and give the user serial indicators and do EIA level shifting off the
main board. I understand the limitations we were facing and why he
did that, but the part that got me was I told him to ensure the cables
could be reversed safely. He scoffed and said that nobody would ever
get the keyed cables upside down and backwards...
So we make the product, we ship the product, and... 100% of our
customer installs failed because each and every customer and/or 3d
party installer did just that. The swap ran +12V where it shouldn't
go and took out the low-voltage side of the receivers on the I/O
bulkhead board.
After the engineer left, I got to support that product. I ended up
socketing each and every adapter board so we could quickly repair the
damage, and for the remaining installs, between extra labels on the
cables, and several dire warnings in the docs, *and* numerous verbal
warnings on installation support calls, we got the rate down to 60%
damage.
At least the damage was less than $5 in parts, and the damage to our
reputation was minimized by the scant sales of that product because by
1990, who was buying new BI machines anyway?
I still have several of those boards - MC68010, 512K to 2MB of DRAM,
64KB of ROM, Z8530 SIO working in async or sync mode... Not sure what
one could do with that besides a comms adapter, but as long as you
don't plug in the EIA driver cable in backwards, it's a nice product.
-ethan
30 Apr
30 Apr
4:31 a.m.
On 2015-04-30 00:36, Noel Chiappa wrote:
Not a single time. DEC engineered things wisely.
Johnny
--
Johnny Billquist || "I'm on a bus
|| on a psychedelic trip
email: bqt at softjar.se || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol
From: Johnny
Billquist
On 2015-04-27 00:50, Mike Ross wrote:
> One does not casually reverse the
installation of cables just to see
> if maybe it will work!
Actually.... I often have done so, just because
of this.
And how many times have you melted something down, doing that? :-) 
5:02 a.m.
DEC sold many many systems into the OEM market and took into account
that they would be worked on by non DEC personnel.
The feedback went into making the systems more robust. I once saw a
shipment of 100 11/34A's arrive at Plessey Radar.
I have some of the Micro PDP11's that ran parts the old ATC system
before it was mordernised and moved to Swanick.
It was very reliable (it had to be)
Rod
.
On 30/04/2015 09:31, Johnny Billquist wrote:
On 2015-04-30 00:36, Noel Chiappa wrote:
Not a single time. DEC engineered things wisely.
Johnny
From:
Johnny Billquist
On 2015-04-27 00:50, Mike Ross wrote:
> One does not casually reverse the
installation of cables just
to see
> if maybe it will work!
Actually.... I often have done so, just because
of this.
And how many times have you melted something down, doing that? :-) 
6:08 a.m.
On 4/30/2015 1:31 AM, Johnny Billquist wrote:
On 2015-04-30 00:36, Noel Chiappa wrote:
Not a single time. DEC engineered things wisely.
Johnny
Heh if DEC engineered things wisely it would have been impossible to plug any
cable in backwards, and there would never had been the need to reverse cables
when a device was apparently not working correctly...
... instead of figuring out how to make the system such that it could not be
assembled incorrectly, they took the lazy way out, and did enough so that a
dumb error (reversing a cable) did not cause a fire. This still allowed for
assembling
a system that would not work, however.
From:
Johnny Billquist
On 2015-04-27 00:50, Mike Ross wrote:
> One does not casually reverse the
installation of cables just to see
> if maybe it will work!
Actually.... I often have done so, just because
of this.
And how many times have you melted something down, doing that? :-)
6:51 a.m.
On 2015-04-30 12:08, Don North wrote:
On 4/30/2015 1:31 AM, Johnny Billquist wrote:
Agreed in that keyed connectors are good. However, even so, it is wise
to make sure reversed connections don't burn anything. People make their
own cables as well, which still can reverse it all...
It would have been very stupid to just have keyed connectors and then
assume that things could never be plugged in wrong.
Johnny
--
Johnny Billquist || "I'm on a bus
|| on a psychedelic trip
email: bqt at softjar.se || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol
On 2015-04-30 00:36, Noel Chiappa wrote:
Not a single time. DEC engineered things wisely.
Johnny
Heh if DEC engineered things wisely it would have been impossible to
plug any
cable in backwards, and there would never had been the need to reverse
cables
when a device was apparently not working correctly...
... instead of figuring out how to make the system such that it could
not be
assembled incorrectly, they took the lazy way out, and did enough so that a
dumb error (reversing a cable) did not cause a fire. This still allowed
for assembling
a system that would not work, however. From:
Johnny Billquist
On 2015-04-27 00:50, Mike Ross wrote:
> One does not casually reverse the
installation of cables just
to see
> if maybe it will work!
Actually.... I often have done so, just because
of this.
And how many times have you melted something down, doing that? :-) 
10:22 a.m.
On Apr 30, 2015, at 5:51 AM, Johnny Billquist <bqt at update.uu.se> wrote:
It would have been very stupid to just have keyed
connectors and then assume that things could never be plugged in wrong.
That is *true*. We carefully keyed connectors on a mission I worked on. The technicians
at our Spacecraft vendor, who make a living making sure the right connector goes the right
way in the right socket, worked hard enough to defeat the keying preventing them from
plugging the spacecraft cable into the payload socket.(In their defense, it was about 0600
Sunday morning, but still.) We had several days of work in front of us trying to prove
(analysis and test) that nothing got shorted in a damaging way.
Johnny?s comment was as true when we got overruled on using different size connectors as
it is now. I feel for Ethan in a very very sympathetic way.
- Mark
3371
days inactive
3372
days old
6 comments
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participants (6)
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bqt＠update.uu.se -
ethan.dicks＠gmail.com -
jnc＠mercury.lcs.mit.edu -
mtapley＠swri.edu -
north＠alum.mit.edu -
rodsmallwood52＠btinternet.com