When viewing levels from a 3.3volt Driver fr

A DMXter Support note

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The Open Line Detector

All DMXters include a curcuit which we call the Open Line Detector. The Open Line Detector is a proprietary Goddard Design Co. circuit to overcome a potential problem with the parent standard of DMX512, EIA485.  In EIA485 it is possible to  receive data from a cable that does not have connection of all of its pins. This can throw you off when trying to track down problems.  Since some broken cables work, we designed a circuit to try and spot the bad cables.  This circuitry flags seemingly good data when all the pins are not connected.

To do this detection reliably required special circuitry that is NOT EIA485 compliant. It has always been possible for certain driver and load combinations that are totally legal under the EIA485 specification to trigger an error message. Hence there has always been a method of disabling this feature. It is always enabled for the cable test which will not work without it.

The following functions use the Open Line Detector:
VIEW LEVELS - it is the feature we call ‘BNW’ for broken wire
VIEW PARAMETERS - error summary screen where it is also called ‘BNW’
DOUBLE ENDED CABLE TEST - it is inherent to this test and cannot be turned off.  This test will not test cables where Pin 1 is not connected at both ends.

False Errors with 3.3 Volt EIA485 Drivers

When the DMXter4 was designed it was discovered that certain 3.3 volt drivers would cause false trips of the Open Line Detector. This was not a fault of 3.3V drivers; they just have a different offset voltage. The detector was designed to spot bad cables.  It is not involved in receiving and decoding DMX data. Unfortunately an item  mentioning this behavior has never made it into the instruction manual before now.

Hardware Modification of Open Line Detector for 3.3V Drivers.

Goddard Design feels that the most consistent performance is had leaving the unit set for 5V drivers.  This also maintains compatibility with legacy equipment.

For those users experiencing problems with false trips, GDC offers a factory modification that will work on most 3.3V and 5V drivers.  Units can be returned to GDC for the modification at a nominal cost.  For users who feel confident they can perform this modification in house, we offer a modification note. (The modification involves replacing 2- 1/4watt hole thru resistors.)  Please contact the factory.

Goddard Design Company
51 Nassau Ave.
Brooklyn NY 11222 USA
718 599-0170 718 599-0172 fax

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last updated March.15, 2011