Data returned to the computer over a serial line as well. All of the microcontrollers could send data at any time. However, since having two controllers send data at the same time would corrupt the data, each microcontroller only sends when the computer asks for a message.
Because the serial data must be routed through the grid, there is potential for noise to interfere with the serial data. The potential for noise interference is high because many amperes of current are being switched quickly. The interference is reduced by using a differential current-source signaling convention. The differential configuration reduces noise by only looking at the difference between two signals. Most interference will affect the pair of signals equally, which means that the difference between the signals is unchanged. The current-source signaling also reduces interference because the feedback within the current source will compensate for the noise
The control signals start from the computer in serial RS232 format at 115.2 Kbaud. This goes to an interface board, where the signals are turned into the differential current-source signals. Three of these are created and routed to the microcontrollers in each module. The end of the cable is terminated.
Power goes to the power box in the standard 120 V AC circuit. The
power box has three cords (only one is shown) to allow for 15A circuits.
The 120 V AC is fed into a series of transformers that output 60 V AC, which
is rectified into pulsating DC. The power box also has transformers,
rectifiers, and capacitors to generate unregulated ±12 V DC, which
is regulated down to ±5 V DC for use in the grid. The difference
between 12 and 5 volts allows for cable and regulation losses.
Transmitting data back to the computer is a bit complicated because at most one microcontroller can send data. All the line drivers share a pair of wires; only one driver is enabled at a time. However, sometimes no driver is enabled, which means that both wires are at ground, which in turn means that the differential receiver will pick up any noise on the line and send it back to the computer. The circuit prevents this by having an enable line that shuts off communication when no driver is turned on.
Sending data to the computer has these steps:
1. Enable the transmit line driver so the differential lines have a valid
signal.
2. Drive the valid line. The valid line is driven by the spare second
line driver in the 75110 package.
3. Send data.
4. Stop driving the valid line.
5. Disable the transmit line driver.
The valid line has larger resistors (higher than the line impedance) for better noise immunity. Reflections aren't a problem because this is a slow signal. The valid line also has a small capacitor to reduce noise.
The figure below only shows one set of control lines; the block diagram above shows three sets. The other two sets use copies of the driver and receiver parts. The output of each set of control's AND gates are OR'ed together.
The control lines are isolated from the computer by using optoisolators (6N137's) . This will protect the computer from the relatively high voltages and currents found in the grid, and will prevent damage if there is fault. The optoisolator will probably have to be driven by something like the output of several 74LS04 inverters.
The lamp drivers need to be able to handle the following:
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The first chart shows the input waveform. The rectified
AC has pulses every 1/120 of a second. The input waveform is what drives
the string of lamps when they are fully on.
The second chart shows the waveform when the lamps are at half power. Power is only applied during the second half of each sinusoidal cycle. The lamps won't be at half brightness because the brightness isn't proportional to the amount of power. I hope to do some calibration so the main computer knows how much power to request to get a string of lamps to be at half brightness. The third chart shows the waveform at about 3/4 power. The
last chart also shows a 3/4 power waveform, but with the power on during
the first part of the cycle. When lights are on partially, I plan to
have half of the strings of lights turn on during the first part of each cycle,
and the other half turn on during the second part of each cycle. This
should make the power used be more constant, which should make the generators
happier. |
Last changed 5/18/02