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Issue #3, May 1996
No, you didn't miss receiving the April issue. We have been so busy we never
quite got around to writing it.
DMX tip of the month.
When troubleshooting a DMX system, and removal of the terminator causes the
system to start working (or placing a terminator on a working system causes it
to stop working) a possible cause is a break (or high resistance) in either the
-Data or +Data lines (pin 2 or pin 3). This is because the DMX signal is based
on the difference between the -Data and +Data lines: When +Data is above -Data,
a digital ONE is being sent, when the +Data is below the -Data, a digital ZERO
is being sent. A DMX receiver will usually cause an open input to float to some
median voltage. The data line which is still connected will go above and below
the open input, allowing the receiver to detect the signal. Placing a 120 ohm
terminator across the lines causes the two lines to move up and down together
(since one is disconnected). Since neither input is above or below the other,
the receiver can not detect the signal.
Note that I've implied that a DMX system will often work with either the
+Data or -Data line broken (until you terminate the line). Although this is
often true, the system will be unreliable and intermitant; a difficult problem
to troubleshoot. Termination will help determine if an open data line exists.
Yet another reason for terminating DMX lines!
New Products.
We designed a product for a custom sign at an industrial show that chased 84
seven watt lamps. We used the microprocessor circuit from our DMX to Analog
converter to control the outputs. This circuit already had a DMX input section
so we went ahead and put it on the board. Our hardware design allows for up to
96 outputs so we went ahead and laid out the board to control up to 96 ten watt
loads. Now we have a 96 output, DMX controlled, ten watt per channel, 120 volt
output, solid state relay pack! This may be used to control low wattage lamp,
120 volt coil relays or contactors, larger triacs or SCRs, or (by bypassing the
120 volt section) solid state relays. We don't anticipate selling many of these
as stock products but if you have a project that this could be a custom part of,
we've got 'em.
The Prescription via E-Mail.
Some of you have asked for the Prescription to be sent via E- Mail. Starting with the next issue (will it be June, July, December?) we will offer this service. If you would prefer receiving this newsletter as an e-mail document instead of a fax, please send your name and e-mail address to
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Our Web Site.
Speaking of electronic mail and hi-tech stuff, we have a site on the internet. It's not much compared to the graphic intensive beauties maintained by the big companies, but it's ours. Come visit us at: www.dfd.com.
Replacement Hardware.
If you lose a screw from one of our products, please call. We will gladly send
you replacements at no charge. We realize it is sometimes hard to find matching
hardware at the local store. If you should ever strip the threads on the
extruded chassis of one of our products, we have extra long screws available to
extend the threads into new metal.
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