DCS AIU Comm Wire Length

I have one that is 35 feet.  Guys panic over this.  If you go way out, you can back feed the AIU with 12 VDC.

Matt,

The thing to be concerned about is not so much the length of any individual cable, rather, it's the total length of the cables that can be problematical.

You can send serial communication over some fairly long distances if you use the proper cable.  We had 9600 and 19.2K serial links that went six or seven hundred feet back in the 80's and 90's for our communication equipment at the NYSE, they were trouble-free links.  The flat cable that is the standard for the MTH AIU link is not an optimal cable for long distance serial communication.  Low capacitance cable with a larger wire size will take serial data a long distance.  Marty's trick of using an aux power supply for distant AIU boxes is also a good idea.

So the total length of the cabling form end to end is what matters? I am thinking I want 4 AIU's spaced about 14 feet apart along the layout for ease of wiring switches and accessories to the the AIU's and I have the TIU's essentially in the middle of the layout. if I add the 12 volt power at the head end I should be able to get reliable communication?

What you are stating should work on it's own.  I have seen that.  The back feed is a last resort if your last switch in the chain has trouble throwing.  I have used this fix and it does work.  Get the + and - right as shown in the sketch.

The long-version is it's a combination of the total cable length AND the number of AIUs involved. 

The cable carries 12V power to the AIU(s) as well as the digital serial commands.  Each AIU re-transmits (buffers) the serial commands so these signals are boosted back to "full strength" going to the next AIU.  OTOH the 12V power signal just gets smaller and smaller going down the cable(s) and passing thru each AIU.  At some point the 12V drops low enough that the relays in the AIU will not actuate.  The addition of a separate 12V power supply at the distant AIU restores the required voltage.

A confusing situation is when the downstream AIUs sometimes work depending on what the upstream AIUs are doing.  This happens because the Accessory relays in each AIU present the greatest burden on the 12V supply.  If a particular AIU has many Accessory relays turned "ON" this drags down the 12V supply that much more possibly starving downstream AIUs of power to operate their relays. 

Perhaps semantics, but in most cases the communications is not what fails; instead it's the drop in 12V supply voltage which prevents reliable relay closures.

I suggest you just hook up your system without resorting to any specialized communications-grade cabling or whatever.  Of course don't go out of your way to make the cables longer than needed.  If you have a problem, refer to Marty's suggestion; suitable 12V DC wall-warts are $2-3 (free shipping) on eBay.

Thanks guys, that is every piece of information I need. Barry, love that word "problematical"

I have applied Marty's "back feed the AIU with 12 VDC" and it works. I purchased a number of 50ft pre made flat ribbon cables, then shortened them as needed. I had one location (or one cable) where I could not get a good new connection, which caused problems to all the AIU's after the bad cable. I ended up using a 50ft pre made cable instead of a 35ft cable with a new connector applied by me at home, and it works fine.

My point is that a bad cable connection can also cause the same problem as having too much cable length. 

My total cable length starting at the TIU is significant, say 15ft+25ft+50ft+50ft+25ft, with the 12Vdc feeding the 5th AIU. Without the power boost, only the first 4 AIU's worked properly.

Hope this helps: Joe K