The following files and folders have been uploaded to the Files area of the Digitrax-Users@groups.io group.
**/ds74_rev5_13dec22.dmf**
By: AJ Ireland aj@digitrax.com
Description:
This DS74 rev5 code dmf IPL file is provided for layouts with excessive voltage pickups on the DS74 input lines.
This will manifest as false Output changes or Route triggers when no other activity is occurring on a powered Digitrax layout.
This HF noise coupling may occur on long runs of input wires that are closely spaced with other noisy e.g., track lines. Most of the tens of thousands of DS74 installs likely do not see this occasional issue.
It is suggested that on layouts with HF-induced noise causing false output or route triggers, that the DS74 needs to run with:
(1) this DS74_rev5_13dec22.dmf code or later, with
(2) OPSW44 = Closed to remove CP1 led drive, and
(3) each input paralleled with 0.01uF to 1uF input caps to ground to shunt HF noise voltages.
Interfering induced voltage is likely AC/capacitive and in the range of about +- 3V peak to peak into a hi-Z 127K resistive load, to falsely trip the input sensing logic.
There is no practical firmware filtering that can remove this impressed unpredictable frequency HF noise voltage.
OPSW44 = Closed in this rev5 code disables the CP1 led drive out, so that needed input capacitor filtering of 0.01uf to 1uF can be used with the 50Hz input sampling rate. The CP1 led drive does NOT cause this false trigger because input-sampling is synchronous, but having led drive does not allow filtering capacitors to be used on long noisy input runs.
DS74 Rev4 or earlier code does not have OPSW44 CP1 disable implemented. The default OPSW44 = T is configured for CP1 LED drive and switch sensing to run OK with the capacitance of about 250′ of Cat5 UTP cable into the DS74 127K input impedance, but this UTP does not provide strong capacitive screening of HF pickup.
Shielded twisted pairs (STP) may work better for just this usage, but would be limited to about 30% of the run length.
Digitrax typically specifies only UTP or silver-satin cabling of LocoNet because STP shield capacitance drastically lowers total network wire distance, and loads are very different to DS7x inputs.
(This post was formatted for easier reading.)
UTP=Unshielded twisted pair.
