Discussion:
grounded MMJ cables
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Phillip Helbig (undress to reply)
2021-07-18 05:22:49 UTC
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I'm testing out some MMJ cables (useful for console cables) and notice
that some (all of which are round rather than flat) have an additional
wire outside of the plug at each end, which looks like it might be
intended as ground. What's the purpose of that?
Grant Taylor
2021-07-18 08:02:20 UTC
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Post by Phillip Helbig (undress to reply)
I'm testing out some MMJ cables (useful for console cables) and
notice that some (all of which are round rather than flat) have an
additional wire outside of the plug at each end, which looks like it
might be intended as ground. What's the purpose of that?
How long is it? I'd expect it to be 3-9 inches if it's meant to be a
ground wire that is attached to something else on the chassis.

I would also have significant worries about ground loops and / or sneak
currents. Both of which can be quite dangerous from an electrical and
safety perspective. As in more amperage than the wire can safely handle
turning the wire into a heater that can melt insulation and start fires.
--
Grant. . . .
unix || die
Phillip Helbig (undress to reply)
2021-07-18 08:35:47 UTC
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Post by Grant Taylor
Post by Phillip Helbig (undress to reply)
I'm testing out some MMJ cables (useful for console cables) and
notice that some (all of which are round rather than flat) have an
additional wire outside of the plug at each end, which looks like it
might be intended as ground. What's the purpose of that?
How long is it? I'd expect it to be 3-9 inches if it's meant to be a
ground wire that is attached to something else on the chassis.
Yes, about 5 inches. It has an eyelet at the end, so one could connect
it to the chassis via a screw.
Post by Grant Taylor
I would also have significant worries about ground loops and / or sneak
currents. Both of which can be quite dangerous from an electrical and
safety perspective. As in more amperage than the wire can safely handle
turning the wire into a heater that can melt insulation and start fires.
But is that an issue when using a console cable?
chris
2021-07-18 10:40:38 UTC
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Post by Phillip Helbig (undress to reply)
Post by Grant Taylor
Post by Phillip Helbig (undress to reply)
I'm testing out some MMJ cables (useful for console cables) and
notice that some (all of which are round rather than flat) have an
additional wire outside of the plug at each end, which looks like it
might be intended as ground. What's the purpose of that?
How long is it? I'd expect it to be 3-9 inches if it's meant to be a
ground wire that is attached to something else on the chassis.
Yes, about 5 inches. It has an eyelet at the end, so one could connect
it to the chassis via a screw.
Post by Grant Taylor
I would also have significant worries about ground loops and / or sneak
currents. Both of which can be quite dangerous from an electrical and
safety perspective. As in more amperage than the wire can safely handle
turning the wire into a heater that can melt insulation and start fires.
But is that an issue when using a console cable?
Not usually from an interference to the cable point of view and iirc,
the added wire is at one end only, which would not enable earth loops.
The answer is more likely to lower any possible emc emissions from the
cable, not to it. Typical terminal baud rates are not high enough and
the rise and fall times are usually controlled, but fast edges could
still be coupled into the cable from the terminal or machine hardware...
Phillip Helbig (undress to reply)
2021-07-18 11:08:50 UTC
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Post by chris
Post by Phillip Helbig (undress to reply)
Post by Grant Taylor
Post by Phillip Helbig (undress to reply)
I'm testing out some MMJ cables (useful for console cables) and
notice that some (all of which are round rather than flat) have an
additional wire outside of the plug at each end, which looks like it
might be intended as ground. What's the purpose of that?
How long is it? I'd expect it to be 3-9 inches if it's meant to be a
ground wire that is attached to something else on the chassis.
Yes, about 5 inches. It has an eyelet at the end, so one could connect
it to the chassis via a screw.
Post by Grant Taylor
I would also have significant worries about ground loops and / or sneak
currents. Both of which can be quite dangerous from an electrical and
safety perspective. As in more amperage than the wire can safely handle
turning the wire into a heater that can melt insulation and start fires.
But is that an issue when using a console cable?
Not usually from an interference to the cable point of view and iirc,
the added wire is at one end only, which would not enable earth loops.
I have seen those, but the two I checked today have it at both ends.
Post by chris
The answer is more likely to lower any possible emc emissions from the
cable, not to it. Typical terminal baud rates are not high enough and
the rise and fall times are usually controlled, but fast edges could
still be coupled into the cable from the terminal or machine hardware...
OK.
chris
2021-07-18 12:50:33 UTC
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Post by Phillip Helbig (undress to reply)
Post by chris
Post by Phillip Helbig (undress to reply)
Post by Grant Taylor
Post by Phillip Helbig (undress to reply)
I'm testing out some MMJ cables (useful for console cables) and
notice that some (all of which are round rather than flat) have an
additional wire outside of the plug at each end, which looks like it
might be intended as ground. What's the purpose of that?
How long is it? I'd expect it to be 3-9 inches if it's meant to be a
ground wire that is attached to something else on the chassis.
Yes, about 5 inches. It has an eyelet at the end, so one could connect
it to the chassis via a screw.
Post by Grant Taylor
I would also have significant worries about ground loops and / or sneak
currents. Both of which can be quite dangerous from an electrical and
safety perspective. As in more amperage than the wire can safely handle
turning the wire into a heater that can melt insulation and start fires.
But is that an issue when using a console cable?
Not usually from an interference to the cable point of view and iirc,
the added wire is at one end only, which would not enable earth loops.
I have seen those, but the two I checked today have it at both ends.
Ok, but the one I have has just one, but maybe that was third party
item.

The frame or chassis ground and signal grounds are separate pins
on a 25 pin RS232 layout, even though they may be joined inside the
equipment. If they joined, it's often with a low value resistor, say
100 ohms, to prevent excessive earth loop currents. Don't know the
circuitry for a VT220, for example, but DEC would have done the job
right, both from an emc and safety points of view. They were obsessive
about standards...
Post by Phillip Helbig (undress to reply)
Post by chris
The answer is more likely to lower any possible emc emissions from the
cable, not to it. Typical terminal baud rates are not high enough and
the rise and fall times are usually controlled, but fast edges could
still be coupled into the cable from the terminal or machine hardware...
OK.
Scott Dorsey
2021-07-18 13:06:23 UTC
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Post by chris
The frame or chassis ground and signal grounds are separate pins
on a 25 pin RS232 layout, even though they may be joined inside the
equipment. If they joined, it's often with a low value resistor, say
100 ohms, to prevent excessive earth loop currents. Don't know the
circuitry for a VT220, for example, but DEC would have done the job
right, both from an emc and safety points of view. They were obsessive
about standards...
Correct. There is no equivalent to pin 1 on an MMJ connector, only to
pin 7. Therefore pin 1 on the DB-25 side is brought out to a lead
that can be connected to chassis shield.

Whether or not you need it depends on your installation and what you are
doing. Everything should have one and only one connection to anything
else through the ground net. The manual for the device may discuss it.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
Scott Dorsey
2021-07-18 18:01:52 UTC
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The only reason I can think of for the separate screen wire, apart
for emc reasons, is to ensure that both ends chassis are at the same
potential. The careless may use a 2 wire outlet at one end or the
other and switchers always have leakage, so a good safety feature...
Precisely. Check the original RS-232C standard, it is very intelligently
designed. Many implementations have not been so well done.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
Michael Moroney
2021-07-18 16:34:58 UTC
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Post by chris
Post by Phillip Helbig (undress to reply)
Post by Phillip Helbig (undress to reply)
I'm testing out some MMJ cables (useful for console cables) and
notice that some (all of which are round rather than flat) have an
additional wire outside of the plug at each end, which looks like it
might be intended as ground.  What's the purpose of that?
How long is it?  I'd expect it to be 3-9 inches if it's meant to be a
ground wire that is attached to something else on the chassis.
Yes, about 5 inches.  It has an eyelet at the end, so one could connect
it to the chassis via a screw.
I would also have significant worries about ground loops and / or sneak
currents.  Both of which can be quite dangerous from an electrical and
safety perspective.  As in more amperage than the wire can safely
handle
turning the wire into a heater that can melt insulation and start fires.
But is that an issue when using a console cable?
Not usually from an interference to the cable  point of view and iirc,
the added wire is at one end only, which would not enable earth loops.
I have seen those, but the two I checked today have it at both ends.
Ok, but the one I have has just one, but maybe that was third party
item.
I have never seen anything like that on a DEC MMJ cable.
Dave Froble
2021-07-18 17:59:49 UTC
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Post by Michael Moroney
Post by chris
Post by Phillip Helbig (undress to reply)
Post by chris
Post by Phillip Helbig (undress to reply)
Post by Grant Taylor
Post by Phillip Helbig (undress to reply)
I'm testing out some MMJ cables (useful for console cables) and
notice that some (all of which are round rather than flat) have an
additional wire outside of the plug at each end, which looks like it
might be intended as ground. What's the purpose of that?
How long is it? I'd expect it to be 3-9 inches if it's meant to be a
ground wire that is attached to something else on the chassis.
Yes, about 5 inches. It has an eyelet at the end, so one could connect
it to the chassis via a screw.
Post by Grant Taylor
I would also have significant worries about ground loops and / or sneak
currents. Both of which can be quite dangerous from an electrical and
safety perspective. As in more amperage than the wire can safely handle
turning the wire into a heater that can melt insulation and start fires.
But is that an issue when using a console cable?
Not usually from an interference to the cable point of view and iirc,
the added wire is at one end only, which would not enable earth loops.
I have seen those, but the two I checked today have it at both ends.
Ok, but the one I have has just one, but maybe that was third party
item.
I have never seen anything like that on a DEC MMJ cable.
Any MMJ cables from DEC that I saw were 6 conductor flat cables. Not
that I've seen everything on the planet.

Anybody can make cables with MMJ connectors. I've done many. Anybody
can add anything they want.
--
David Froble Tel: 724-529-0450
Dave Froble Enterprises, Inc. E-Mail: ***@tsoft-inc.com
DFE Ultralights, Inc.
170 Grimplin Road
Vanderbilt, PA 15486
Grant Taylor
2021-07-18 17:54:45 UTC
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Post by Phillip Helbig (undress to reply)
Yes, about 5 inches. It has an eyelet at the end, so one could
connect it to the chassis via a screw.
ACK
Post by Phillip Helbig (undress to reply)
But is that an issue when using a console cable?
I don't think so.

I would assume that the actual data lines on a console cable have some
sort of isolation to thwart things like current loops. Or at the very
least fuses to protect the rest of the system.

I know that I'm used to electro-optical isolation like that on Cisco
equipment. Specifically for this very problem.
--
Grant. . . .
unix || die
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