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Q: Is it Possible to Limit Motor Voltage using CV5?
CV5 sets the top speed of the motor. It does not adjust the voltage applied to the motor.
Multifunction decoders use Pulse Width Modulation to control the motor. The amplitude, VMotor, is fixed.
This voltage is switched on and off by the motor driver. The ratio of time between On and Off states is called the duty cycle. At 50%, PeriodON is equal to PeriodOFF, and if measured with a voltmeter, would be approximately 50% of VMotor.
By varying the ratio between the On and Off time, the speed of the motor can be controlled.
Installing a motor rated for voltages less than that of the decoder's output will result in its destruction, and possibly damage to the decoder's motor driver circuit.
Edit FAQ Related Articles: Can Motor, Pulse Width Modulation, Speed Curve, Speed Steps, Zero Stretching Categories: Beginner, DCC, Multifunction Decoder
Q: Why allow zero stretching at all?
Zero Stretching or Analog Mode is not part of the NMRA DCC Standard.
Some manufacturers added the analog mode to create a compelling reason to adopt Digital Command Control in the early years, as modellers (particularly those with large fleets of motive power) were concerned about the costs of conversion to DCC.
This allowed DC locomotives to be controlled without adding a multifunction decoder. It is not a crucial feature today unlike 30 years earlier when DCC was introduced. It is now a legacy feature that some DCC systems still incorporate.
Analog compatibility mode is only one use for zero stretching. Other possible uses include the following:
- Providing a stretched zero after each packet for the purpose of superimposing locomotive feedback to the command station.
- Allowing generation of the command control signal with a computer using a standard serial port, which may not be possible without slightly stretching some zeros.
- There are probably other uses that will become apparent as time goes on. Note that the above-mentioned uses are still very much in the experimental stages. The point is that preserving the ability to stretch zeros allows for the possibility of some interesting things.
Edit FAQ Related Articles: Power Conversion, Zero Stretching Categories: Beginner
Q: How does the standard work with computer control of the layout?
Computer control can be accomplished by using a command station with a computer interface, or by generating the signal directly with the computer and using a power station to drive the track. The interface between a computer and a command station is outside the scope of the standard.
Q: Can One Command Station Control Multiple Layouts?
Short answer: Yes. It's possible to control two (or more) separate layouts using a single command station. For instance, if you wanted to operate an N scale layout inside and a G scale outside. Most systems combine the command station and booster into a single unit. Either case, you connect one layout as normal, that is, connect the command station/booster to one layout. Then, you simply purchase a second power supply and booster for the other layout. The second layout will receive it's commands through the throttle network (LocoNet, XpressNet, etc). This allows you to have same, or different voltages for different layout sizes.
Assume we have an N scale layout in the garage. Since power requirements are low, we purchase a DCC system that outputs about 2 to 3 amps, and the voltage is set for N scale. We setup this layout as described in other parts of this website. A year later, we want to setup a G scale, or garden railroad in the backyard. To do this, all we do is purchase another power supply and booster for the second layout. Because the power is independent of the layout, we don't need to worry about the higher voltage from the garden railway making its way to the N Scale layout. To get the commands from the command station to the garden railroad booster, we simply connect the throttle network (such as ExpressNet, or LocoNet) to the booster. We now have two railroads being controlled from any throttle, at any location, with a cost savings by not having to purchase two command stations.
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Categories: Beginner, Command Station
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