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Issue #4, September 1997

Finding the time to write The Prescription has been harder than anticipated. Isn't everything? But anyway... here is issue #4.

DMX tip of the month.

I have received some questions as to the wiring of DMX512 cables. The most common being: Should pin 1 be connected at both ends? YES! A primary function of Pin 1 is to provide a signal reference common. Without pin 1 connected at both ends, the receiver and transmitter circuits can float apart by more than the 7 volts allowed in the RS-485 chip designs. Worse, some "DMX512" receivers (and probably some transmitters) do not use RS-485 chips. These designs do not even allow for the 7 volt difference before they stop working.

But what about "Ground Loops" these people ask. In most DMX512 designs, pin 1 (signal common) is isolated from Earth Ground, either completely (no connection at all) or at low frequencies (by a high frequency capacitor between common and earth). In cases where pin 1 is tied directly to earth at both ends, a ground loop will occur. A system may work fine during rehearsal if the ground difference is only a few volts but at show time, when the elevators are running, the air conditioning is on full, and Murphy is watching, the ground difference may be sufficient to cause problems. So what can you do?

First you should determine if pin 1 is tied to earth at either end. This is done by measuring the resistance between the ground pin of the power cord to pin 1 of the DMX connector. The resistance should be several thousand ohms or more. If either end is less than a thousand ohms between earth and signal common, I would suggest you isolate the DMX512 signal common between the source (console) and the receiver (dimmer, etc.). Any of our splitters, as well as our $200.00 in-line isolator, can serve this function. The in- line isolator is designed to be used right at the console whereas our splitters (and isolation amplifier) can be used anywhere in the DMX line.

Three Pin vs. Five Pin.

The standard states that a five pin XLR style connector be used for DMX512. High End Systems has used a three pin XLR connector since before they offered DMX512 control. Now their equipment is commonly controlled by DMX512, on their 3 pin connector. The pin numbers are the same (1=Common, 2=Data-, 3=Data+) but beware: the pins are not in numerical order on a three pin connector. Martin also uses 3 pin connectors but has reversed the pin numbering (Martin uses 2=Data+, 3=Data-). Doug Fleenor Design Isolators and Splitters are available with 3 pin connectors and work with High End and Martin protocol as well as DMX512. We will even mix 3 and 5 pin connectors at no charge on a special order basis.

New Product.

We introduced our DMX Terminator at LDI'95. Priced at $49.00, we didn't sell many the first year. At LDI'96 we introduced T2, an economy terminator, priced at $25.00. The economy version did not have the surge protection, signal LED, or RFI (Radio Frequency Interference) protection of the original, but it was cheaper. Now we sell four times as many expensive terminators as we do cheaper ones! Either terminator will improve the signal integrity by preventing reflections and lowering the load impedance. We have a paper titled Why Terminate (available free of charge) which explains more about the necessities of terminating the DMX control cable.