The LMD-4 Load Management Decoder


The LMD-4 is a small panel the same size and shape as a standard North American wall switch plate. It is designed to disable, in a priority fashion, selected electrical equipment connected to a small hydro plant at times when the plant is not generating sufficient power to operate everything all at once. Up to 15 loads can be controlled. The unit also displays relevant information about the operation of the plant by means of several LED lamps on the front panel. These lamps and the LMD-4’s internal computer are powered from the electrical energy contained in the incoming signal, and need no other power source. It is possible to add a power transformer to the LMD-4 and operate it from an AC supply, but this is generally unnecessary and at this writing, the unit has not been approved for residential use with an AC line voltage power supply connected directly to it. For use over a very long line from the powerhouse, a modification is available that uses the familiar "wall wart" or "line lump" type of power supply to supplement the power extracted from the incoming signal with a source of 12 volts DC in a way that meets requirements for residential use.

The LMD-4 needs a signal generated by a Carson Electronics PLL small hydro controller in order to operate. Many LMD-4 panels may be connected to the same signal line. All will operate identically. The signal arrives in serial form over a single pair of wires, which must be connected to the terminals marked "SIG IN" on the LMD-4. Small cable such as 28-gauge indoor telephone wire may be used. A removable terminal board attached by four screws to the back of the LMD-4 contains screw terminals for connection of up to 15 standard solid state relays which are placed in series with the loads to be controlled. Wiring may be done easily by removing this board, wiring it and then plugging it back into the LMD-4 and replacing the four screws. The negative control wire to each solid state relay is connected to a ground terminal (15 are provided). The positive wire is connected to the terminal of the desired priority level for that load. These terminals are numbered 1 to 15, with number 1 being the most important load in terms of priority - in other words the last to be disabled and the first to be re-enabled. At the relay, connect these wires to the two small screws and the load in series with the two large screws. The LMD-4 can be flush mounted on a wall, in a cutout similar in shape to the LMD circuit board, or it can be mounted in a standard North American one-piece single wall switch box. To avoid creating "ground loops", do not connect any of the LMD’s ground screws or terminals to the building’s electrical ground system.

2. The Front Panel Display:

The green LEDs numbered 1 through 15: O O O O O O O O O O O O O O O these lights represent the status of the baseboard heaters or other electrical loads that you have connected to the LMD-4 to be managed.. If a light is out, that load has been "locked out" to prevent its operation, since there is not enough power available to run it. The user interface on your computer screen can be used to enter the power requirements of all your managed loads, so that the system will always know if enough power is available to turn oin the next managed load in order of priority. If the amount of power consumed by a managed load is set to zero, the system considers this to mean there are no more managed loads available and it will not look for any more loads to turn on.

The five LEDs in a cluster in the corner of the panel: O O O O these show what the hydro plant is doing, as follows:


O Decrease Water (yellow): lights up when the turbine water control mechanism is moving to give less water force. Corresponds to "Throttle Down" on the computer screen.

O Increase Water (yellow): lights up when the turbine water control mechanism is moving to give more water force. It corresponds to the "Throttle Up" light on the computer display if you have a computer connected to the controller.

O Synchronized (blue): lights up when the generator's speed is correct and synchronized to the quartz clock in the controller. Corresponds to "Speed OK" on computer screen. The latest versions of the controller firmware provide two more indications from the blue LED: If it is flashing, it indicates that the water tank heaters are locked out of operation (for example due to an overheated tank or if the user has locked out the heaters in order to change the water filter). Water control governing will be in effect if it is available. If the blue LED flashes quite rapidly (4 times per second) it indicates that an overvoltage condition has been detected and the plant is in the process of being shut down for protection of electrical equipmernt from damage. At this time, the conroller firmware is such that the load mamagement signal is not sent if the generator is not operating, so this indication will only be seen for a few seconds before communications with the controller is lost. Before putting the plant back in operation you will need to find the cause of the overvoltage and fix it, then turn the controller off and on to reset it.

O Overheat Alarm (red): starts flashing if the controller's water tank overheats. When this happens, the controller will attempt to reduce the water to the turbine until there is no more excess power going to the water tank, in order to be able to shut off the overheated tank without causing a power surge. It will then shut off the power to the tank. If this takes more than about 45 seconds it will shut off the tank in any case. You will need to go to the powerhouse, find out why it overheated (probably a plugged up water filter), fix the problem and then switch the controller off and on again to reset it. If the main controller in the powerhouse is running firmware that can respond to overvoltage, and if overvoltage is present at the same time as overtemperature, the overheat alarm LED will flash twice as rapidly as it does with overheat alone.

O Line Signal (green): lights up when communications are being received from the controller in the powerhouse. If the communication signals are correct, it should flash very rapidly - almost too rapidly to notice if you are some distance away. Each flash of this light represents a complete update of all the information needed for the panel to do its job. This happens 30 times each second, which is why the light flashes so rapidly. If this light is out, it means that the controller has lost communications with the main controller, probably due to a bad line or a shutdown of the plant. If other lights are on, it means the decoder has a local power supply and doesn't need to rely on using the line signal as a power source. In this case, when the signal coming from the main controller is lost, the LMD-4 will turn on all loads or just the most important four loads, depending on whether a "signal loss behavior select" jumper is installed or not.


3. Interesting (and other) Facts about the LMD-4:

The LMD-4 Load Management decoder was developed from earlier units which began with the 7-load LMD-1 in the early 1990s. Only a few prototypes of the LMD-1 were ever built. It required two pairs of wire from the controller to control 7 loads, and the PLL controller sent signals to the LMD-1 by placing 12 volts DC on 7 different combinations of one to three wires (the fourth wire was the ground). Decoding was done by the LMD-1’s digital hardware.

The LMD-2 was a variation of the LMD-1 with somewhat different circuitry and a different set of chips doing the decoding.

The LMD-3 was the first LMD decoder to utilize a serial data signal to receive commands from the small hydro controller. It used reprogrammable GAL chips and hardware timers to do the decoding. The type signals used to operate the LMD-3 are still in use today with the LMD-4, and a few LMD-3’s are still in service. The number of loads it could control was 15, up from 7 in the previous units. It was the first LMD to include a display of plant status on its front panel.

The current LMD-4 marked a big departure from previous units – in this decoder, the decoding is done entirely by a software program in a computer chip, a Motorola HC11A1 microcontroller. The program can be modified and upgraded at any time. The LMD-4 does the same job as the LMD-3 but requires less operating current, works with longer lines and is better able to withstand lightning and various electrical adventures than the LMD-3 was.

In order to upgrade the software from time to time, every LMD-4 contains a serial port which can be interfaced to a PC. The current internal program also gives a text display on a computer connected to the serial port, of the same information seen on the LEDs, but this is almost never used in practice since it is an excessively complex way of seeing what the LEDs already show.

For some special uses it may be desirable if only one of the LEDs in the row of 15 lights up at one time (sort of a dot chart versus bar chart display). A Berg Jumper may be plugged into the LMD-4 circuit board at the two pins marked JP1 to provide this function.

For completeness, the functions of the other two jumpers on the board are described below. It is not necessary to know all this as long as the jumpers are not tampered with. But it makes great bedtime reading, if your aim is to fall asleep as quickly as possible.

Another Berg Jumper at JP2 serves to change the operating mode of the internal computer, a technical point described below. When this jumper is in place, there must also be a jumper across the LMD’s serial port JP3 (2 pins erroneously marked "SPI" – should actually be "SCI" for Serial Communications Interface).

Some computer chips must operate in a different mode from others when installed in the LMD-4, even though the part numbers on the chips are the same. The reason is this: In addition to the various types of memory inside the chip which the LMD program uses, there is what the manufacturer terms a "junk ROM". This is a large permanent memory – much bigger than the one we’re actually using – which may contain various and sundry faulty programs, other customer’s unwanted obsolete software and goodness knows what else of no relevance to the operation of the LMD-4. Instead of throwing such chips away, the manufacturer simply applies a slightly different part number and sells them at a reduced price with no ROM included in the specs. In this case we place a jumper on the board (at a place marked "JP2"). This activates another built-in small program which handles the startup of the computer. We must place a second jumper on the serial port to short the incoming and outgoing data lines together. The startup program checks for this jumper whenever the LMD-4 is powered up, by sending some data out through the serial port. If the data comes right back in again, it must have traveled through the jumper, so the system now knows the jumper is there. This is seen as a command to start the LMD software. If the jumper is not there, the system is instead set up to receive a small program through the serial port, load it into the chip’s RAM and run it. In our case this small program will be a so-called "bootloader" system for installing new software into the LMD-4 in order to upgrade it. The new LMD software is sent after the bootloader, which busily installs it.

In a few chips we have found that the junk ROM contains what can only be described as good stuff, not junk - a program developer’s set of tools called the BUFFALO Monitor (BUFFALO means "Bit User’s Fast, Friendly Aid to Logical Operations", in case you were wondering). In this case we can leave the JP2 jumper off. Leaving JP2 off causes any program in the junk ROM to start – but that’s okay in this case. The BUFFALO Monitor starts – but we know enough about it to realize what it is about to do. It immediately checks to see if a certain pin on the chip is connected to ground. If so, it starts the LMD software for us. All LMD-4 boards do ground this pin. We do not need a jumper on the serial port if BUFFALO is handling the startup. Unlike the built-in start program, it does not check the serial port and will not know about any jumper there anyway.

There is no advantage at present to leaving the jumpers off. However if a more useful computer interface was designed into the LMD in future, it could be handy to have the serial port available to users instead of shorted out with a jumper all the time. The jumpers on the board simply allow us to use any HC11A1 chip in the LMD-4 to full advantage, no matter what lurks inside its junk ROM. If you receive an LMD-4 board from Carson Electronics and it does not have jumpers installed at JP2 and JP3, well… you can tell all your friends there’s a BUFFALO in your load management decoder.

One more arcane fact about the LMD-4: since it is actually a general purpose single-chip computer board, you could theoretically reprogram it to be something other than a load management decoder. Chips with up to 20 kilobytes of internal memory, such as the HC711E20 (as opposed to the HC11A1’s 512 Bytes of useful nonvolatile memory) can be plugged into the LMD-4 board with no modifications.
Other uses known to have been programmed into the LMD-4 to date:
- communications watchdog system that gives a power-off reset to a wireless network device if signal is lost
- accurate multifrequency test signal generator providing 15 different frequencies all at once
- signal generator for a true sine wave DC to AC inverter
- interface to connect a device that uses an " I-squared-C" communications interface to an RS-232 serial port on a computer
- morse-to-text converter that connects to a shortwave radio and a computer and displays morse code messages in text