Re: Positioning Block Detectors for AIU-01

John Cahill

Mark, thanks a lot for that detailed technical information and the practical advice! In particular the suggestion of using bell wire is a cheap and practical wiring solution and will make the rewiring effort easier and I will definitely be using that approach.  I may need more AIU's than originally planned, but it will be worth it to avoid trouble in the future.

The advice (now and in the past) from you and Don and other contributors has been invaluable!
Thanks again!

On Fri, Nov 13, 2015 at 6:23 PM, Mark Gurries gurriesm@... [WiringForDCC] <WiringForDCC@...> wrote:

Don is right on….. 

Clarification…The output of a BD20 itself is not that sensitive to noise.  From a noise standpoint Its got a huge capacitor (0.47uF) directly across its output.  AKA a noise short circuit.   The AIU input also has some RC filtering but has a very low threshold of detection (TTL) that is not the same as the LED indication threshold (CMOS).   It is more sensitive than the LED indications would indicated.   Anyway, what this means is the noise gets in is the cable running between the BD20 and the AIU and it can create false/random states.

But to be clear, do not move a BD20 further away from it detected block out of interest of getting it closer to the AIU.  Optimize the location of the BD20 first and then after that is done, given the locations of the BD20, figure out a common central place for a AIU to pick up a lot of them.  

Steady state problems are easy to fix.  But what typically happen on a layout is they are not steady.  They appear to be random and mysterious for it depends on what is running or not on the layout and where the trains are. It like there is a ghost in the wiring.   There is no ghost.  It just understanding what if happening with regard to the wiring.  The BD20 to AIU wiring is less of a problematic issue than the BD20 to the block wiring basic detection reliability issues.  Also if there is a problem, its a lot easier to fix the BD20 to AIU cable wiring than it is to fix the track block to BD20 cable wiring.   Never run/bundle a BD20/AIU cable in with a DCC track bus cable paths.  It is perfectly OK to run one or multiple BD20/AIU cable along or in the Cab bus cable path.

AIU Utilization:  Remember your AIU also typically include turnout position feedback.  If you have a multi level layout, think about a given AIU picking those blocks and turnouts that are not that far away wiring distance wise.  HOWEVER, is is better to have multiple AIU than to FORCE one AIU to pick up all the BD20 and Turnouts on a large layout.   If you running more than a 1/3 of the AUI connection a long distance of 25 feet or longer, then it time to START thinking about getting another AIU and/or twisted pair wiring.  Twisted pair wiring address the noise pickup issue between the BD20 and the AIU up to a point.   Just use twisted pair wiring up front and help yourself in the long run at the time of installation.

Door bell wire historically has been sold in basic 2 wire twisted pair format without a jacket.  Red and White are common wire pair colors.  Being a unique combo color code, it will be easy to identify the cable under the layout in the future.

On Nov 13, 2015, at 9:22 AM, John Cahill johncahill25@... [WiringForDCC] <WiringForDCC@...> wrote:

Thanks Don. That is very helpful! I had been trying to figure out using a schematic diagram but what you suggest tells me distances are critical so would seem I need to figure out rough positions of BDs & AIUs on a scale drawing first. It is good to have a starting point and direction to go in!

Best Regards,

On Nov 13, 2015, at 15:29, "'Vollrath, Don' dvollrath@... [WiringForDCC]" <WiringForDCC@...> wrote:




The occupancy detectors like the BD20 want to be in relatively close proximity of the track sections being monitored. The output of the BD20’s are low level circuits and subject to electrical noise. But the NCE AIU talks to the command station via the cab bus. And the length of the cab bus back to the PowerPro command station is not that critical.

I would place each AIU unit somewhat centrally located around the cluster of BD20’s it is monitoring, and simply route/wire and daisy chain the cab bus to each AIU.

If you find that there is 1 or 2 BD20’s that end up being remotely mounted from its monitoring AIU, use a twisted pair to wire it back to the AIU to minimize interference. [The BD20 really doesn’t need a power supply for operation. The power supply is there to only light an LED if you think you need it.]




From: WiringForDCC@... [mailto:WiringForDCC@...]
Sent: Thursday, November 12, 2015 5:10 PM
To: WiringForDCC@...
Subject: [WiringForDCC] Positioning Block Detectors for AIU-01


I have a HO layout which measures 25' X 13' in a U footprint (base is 13', legs 25') and has a new terminus station which was built with block detection wired in as it was constructed, using a mix of BD20 and HDL hdM14 4-way units.  The rest of the layout is about to be rewired for block detection but due to its physical form, fiddle-yard, two through stations and mainline runs, there will be a lot (30+) of detectors scattered across the layout.

What is the ideal way to optimize the wiring back to 3 AIU-01's?  Is it OK to have long runs back to a single base?  Should the AIU's all be in the same place? How close to the PowerPro does the AIU-01 need to be? Ultimately the AIUs will connect back to a laptop.  Obviously, avoiding interference is a priority, while minimizing wiring runs is also desirable.  Any thoughts, ideas and general rules would be appreciated. Thanks in advance,

John Cahill


Best Regards,

Mark Gurries
Electrical Engineer
DCC Website & NMRA DCC Clinics:

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