MMJ cable length?

Tothwolf tothwolf at
Mon Nov 27 11:57:56 CST 2006

On Mon, 27 Nov 2006, Kevin Handy wrote:
> Tothwolf wrote:
>> I was thinking about putting together some standard MMJ cables, but I 
>> realized I didn't know what the standard lengths were. Can anyone tell 
>> me which were the most common/useful lengths for the original DEC 
>> cables? I've seen and use cables as short as 18" and as long as 50ft. 
>> I've used a lot of them that were somewhere in the 12-15ft range, but 
>> I'm not sure what length would be the most versatile.
> The 5-6' cables were probably the most common, but you could go up to 
> about 300' if you ran it at a slow enough speed. Avoid outside runs as 
> much as possible (lightning damage can be very impressive).

Ok. I'm just not sure what length are the most useful. I know the 12-15ft 
cables always worked ok for me to connect systems/terminals/terminal 
servers, but I'm just not sure if shorter cables would still work as well 
length wise.

Lightning can do some pretty strange things. One of the more interesting 
lightning damaged installations I found was where someone had run a 
thicknet drop cable across a building roof. I have no idea what the person 
responsible for that cable run was thinking.

> It really depends on the enviornment you are in on how long your average 
> cable will be. If you are setting up a terminal room, you will probably 
> use short cables. If you are setting up in an office enviornment, you 
> will probably want to wire the walls, and place MMJ jacks on the wall. 
> Then you will need to have cables long enough to reach from the wall to 
> the desk (5' to 20'), depending on weither they are willing to have 
> their desk against the wall with the jacks.
> I wired up a lot of long runs from a computer room to individual 
> offices, and you cannot get the cable through an existing wall with the 
> ends attached. They almost always get trashed on the way through the 
> wall, or get hung up on the way through.

I mainly just need to figure out what length will be the most useful for 
equipment to equipment connections. None of these will be going in the 
walls. Flat cord isn't rated for in-wall use anyway and in a commercial 
building, the building inspectors would throw a fit.

I've also had a couple of people ask me off-list about making a few extra 
MMJ cables since my initial email, so figuring out the most useful/common 
length would help with those too. I think I only have 100ft or so of extra 
cable on hand right now.

In any case, 6C flat cord is fairly cheap, but the plugs aren't (at least 
the AMP brand ones anyway). Anyone happen to know a source for inexpensive 
AMP brand MMJ plugs? :)

> Later on, I used regular CAT-5 connectors on the wallplates, and wired 
> cables up as CAT5-MMJ to the terminals. This allowed for later 
> transition to a network, as wll as using easier to find CAT5 wallplates.

I've done the Cat5 to MMJ cables lots of times myself. I've also used 6P6C 
RJ11 to MMJ cables. I actually just made some more of those to use with 
RJ11 to DE9F adapters, as I've yet to find genuine H8571-J adapters in 
quantity at anything resembling a reasonable price.


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