By Loy Spurlock © 1996 - All rights reserved
Digitrax's PR-1 decoder programmer comes with Windows software to program decoders with your PC. This software allows you to set all aspects of the decoder, then program them all at once - including the speed table which is drawn on a graph. It also allows you to save a configuration to disk, then retrieve it later for modification and/or to use on other decoders. The PR-1 plugs onto a DB25 comport. If your comport is a DB9, you'll need an adapter. And, if you're also using Digitrax's MS100, you'll either have to have an additional comport, or an AB switch to switch between them. The documentation says the PR-1 requires 400 ma at 14 to 20 VDC. But, it only drew 25 ma at 15 VDC with Bob Corak's (our PC Guru) tests. Also, the power must be clean. So, if you plan to use a typical open frame transformer, you'll have to rectify and filter the power. The cable that goes from the PR-1 to the programming track is only about two feet long. I assume the intent was for the modeler to have a programming track by his computer. But I suspect most modelers will want to keep their programming track on the layout. This means a longer cable will be needed. This cable, though, is nothing more than an RJ11-6 plug with 6-wire cable (some of the wires aren't used). The most important thing about the PR-1, though, is that it is stand-alone. That is, it does not require you to have a BigBoy system to use it. Owners of any other DCC system can use it. This is especially great news for Keller Digital users, since Keller never got around to producing a programming unit, and other systems with limited programming capability. Also, because it is stand-alone, your layout can continue to run while the computer is programming locos Ñ even with the programming track being a part of the layout. -------------------------------------------------------------------------- Our second topic this month concerns DCC rail power. Some people still think the power on the track is DC. Some even think that the signal is AC, superimposed on top of the DC power like other command control systems do it. It is neither. The rail power on DCC track is square wave AC. Think of a normal DC computer signal square wave, as illustrated here.
Computer signals like this are usually measured in milliamps, with a voltage swing from zero to 5VDC. This provides a good signal circuit, but could hardly power a train. So, the booster takes this signal, and amplifies it Ñ voltage and amperage. What you wind up with a digital signal with is enough voltage and power to run trains. Now for the AC part of the system: With DC power on the rail, a non-decodered loco would simply take off at full speed. So, the same exact signal that rises above zero is mirror imaged to the negative side, making a square wave signal that swings from negative to positive as illustrated here - square wave AC.
The amount of voltage is dependent on the booster you're using,
and which scale you have it set to operate. Regardless, a short duration pulse
represents a one (1), and a long pulse represents a zero (0). A group of eight
pulses represents one byte of an information packet. A basic packet contains a
preamble of "1" bits, a "0" start bit, an address/data byte,
a "0" start bit, an instruction byte, a "0" start bit, an
error detection byte, and a "1" end bit Ñ which leads into the
preamble of "1" bits for the next As long as the duration of the negative (-) pulse matches the duration of each positive (+) pulse, a non-decodered loco will not go. It gets an instant of positive power trying to make the loco go forward, then gets an instant of negative power trying to make it go backwards. The result is a loco that sits there buzzing Ñ as you've probably experienced. (Note: a drop of Conducta Lube on the commutator can reduce this noise.) Zero stretching is implemented to make the non-decodered loco go. That is, the zero (0) pulses are stretched longer on one side than the other. To make the loco go forward, the zeros on the positive side are stretched. This provides more positive power than negative. The loco still gets short pulses of negative power, but the longer positive pulses overpower the negative to make the loco go forward. Obviously, to make the loco go backwards, the negative zeros are stretched. As you've probably experienced, the faster the analog loco goes, the less it buzzes Ñ because the longer periods of positive power reduces the number of AC pulses per second. While this analog loco is running, all DCC locos continue to run as if nothing is else is happening to the signal. That's because the decoders are designed to recognize short pulses as a ones (1), and a long pulses, no matter how long, as a zeros (0). Ergo, stretched zeros are still read as zeros by decoders. Pretty slick huh. If you want to install meters on your track power, you have to use AC volt and amp meters. However, even though AC meters work, they won't be exactly accurate Ñ because the typical AC meter is not designed for square wave. Even so, it will be accurate enough for general layout operation purposes. |
