Assembling the Interface/Charger Board


This is the beginning of the Handy Board assembly process. We will begin with the Interface/Charger board because it's the simpler of the two boards, and because we will use it when testing the Handy Board itself. During this assembly sequence the entire Interface/Charger Board will be constructed.

For this assembly sequence, collect the following parts:

In the instructions that follow, component names indicated with an asterisk (e.g., D2*) are polarized and must be installed in the proper orientation. POLARIZED COMPONENTS THAT ARE INSTALLED BACKWARD WILL NOT WORK; please take care that all components indicated with an asterisk are installed correctly. Refer to the component installation instructions given in Assembly Tips before proceeding.


Interface/Charger Board Power Circuit

In this step the power supply circuitry for the Interface/Charger Board is installed. The above photograph shows how the board should look after this step is completed.

The power circuit uses a component known as a bridge rectifier (D2) to normalize the power coming in from the DC adapter. Then this unregulated voltage, which might be as high as 17 volts, is given to U17, a five-volt regulator. Two capacitors (C15 and C16) are used to smooth out ripples in the power supply voltage. A red LED (LED13) provides a visual indication of U17's voltage output.

Installation

Install D2*, C15*, C16*, R10, LED13R*, U17*, and J11*.

Testing

Apply power to the Interface/Charger Board using the 12 volt DC adapter by plugging into J11. The power indicator LED, labelled ``PWR'' (LED13R) should light.

Now measure the board voltage. A convenient spot is the holes for component C14. Place the voltmeter's black lead onto the square hole (the minus terminal) and the red lead onto the round hole (the plus terminal). You should see a voltage between 4.9 and 5.1 volts.

If the voltage reading across the C14 terminals checks out, proceed to the next assembly step; otherwise, follow the debugging instructions below.

Debugging

If the LED does not light, or if no voltage is present at the C14 terminals, verify that the DC power adapter is providing power by measuring the voltage at the two terminals labelled ``~'' on D2. If there is no voltage here, then the adapter is not working (check that it is plugged in).

If power is present, check for voltage on the output terminals of D2. There should be between 15 and 18 volts DC across the terminals marked + and -. If there is no power here, then D2 is suspect.

If D2 is outputting the correct voltage, then U17 is suspect. Check that it is installed correctly. If it is very hot, there is a good chance it's installed backward.

Finally, check the polarity (orientation) of the two ripple-smoothing capacitors (C15 and C16). If either of these are installed backward, the circuit will not function.


Interface/Charger Serial Circuit

In this assembly step, the MAX232 serial level converter chip (U16) and its associated components are installed. This chip performs the function of translating between the ``RS-232'' voltages that a computer uses on its serial port (signals between -12 and +12 volts) and the voltage required by the 6811 microprocessor (signals between 0 and 5 volts).

Installation

Install DIP6, the 16--pin socket for U16. It is convenient to install the socket so that its ``pin 1 marking''---the notch in one of its shorter lengths---lines up above the corresponding notch in the component placement marking on the circuit board. This notch indicates the correct orientation for the chip that is plugged into the socket.

Install capacitors C10*, C11*, C12*, and C13*, making sure to observe polarity. Install C14, R9, and LED12G*.

Install the MAX232CPE chip, U16*. As indicated in the diagram, the pin 1 notch is to the right.

Testing

To verify that U16 is operating correctly, use a voltmeter to test the following voltages. Make sure that the DC adapter is plugged into the board and the red PWR LED is on. Using pin 7 of J9 as a ground reference (i.e., touch the black voltmeter probe to pin 7), measure the voltage at pin 2 of U16. It should read +10 volts.

Now measure the voltage at pin 6 of U16. It should read -10 volts. The presence of these two voltages demonstrate that the MAX232 chip is properly generating the voltages required for RS-232 communication. If these two voltages are not present, check the orientation of capacitors C10 through C13 and the orientation of U16.

The next test exercises the MAX232's ability to transmit data to the Handy Board. Using a scrap piece of wire, temporarily make a connection between pin 2 of J9 (the serial input line to the MAX232) and pin 6 of the MAX232. This simulates the host computer transmitting data to the Handy Board. The green LED labelled ``SER'' (LED12G) should light.

Finally, the MAX232's ability to transmit data to the host computer is tested. Again using pin 7 of J9 as a ground reference, measure the voltage at pin 3 of J9. This is the MAX232's serial output. It should read -10 to -12 volts. Now temporarily jumper the -12 volt supply (pin 6 of the MAX232) to pin 10 of the MAX232, which is the input side of this output stage. Again measure the voltage at pin 3 of J9---it should now read +10 to +12 volts.

Debugging

Not yet prepared

Interface/Charger Board, Charge Circuit and Jacks

In this step, the circuitry for charging the Handy Board's battery is installed, along with the remaining jacks.

The charge circuit is a based on R11, a 47 ohm resistor, as a current-limiting device. The power supply provides between 12 and 15 volts to the charge circuit of R11 and the Handy Board's 9.6 volt battery. There will then be a voltage drop of about 2 to 5 volts across R11, yielding a charge current between 40 and 100 mA (milliamps).

SW4, the charge rate switch, is used to short out R11, providing a ``zap charge mode'' in which charge current is limited only by the performance of the DC adapter and any current-limiting effect that D2, the bridge rectifier, may have. In practice, this works out to about 1 ampere, which is quite a lot, since the standard charge rate of the battery pack is 50 mA. The Handy Board should never be left on zap charge unattended; after the battery pack begins to get warm, it's time to remove it from zap charge.

Assembly

Install the remaining components of the Interface/Charger Board: R11, R12, LED14Y*, SW4, J9, and J10. Make sure J10 is the side-entry (not top-entry) style of RJ11 jack (unless the kit has two top-entry models). If the RJ11 jack has only four wires, that's fine; the circuit only uses the center four conductors of the jack, and board has six holes so that it is physically compatible with six-wire jacks.

Testing

Using the appropriate modem cable, plug the Interface/Charger Board into the host computer. The green SER LED may light. Connect power to the board, and verify that the red PWR LED is lit.

Run the 6811 downloader software. At this point, the green SER LED should light up, if it was not before. The software will display an error message, since there is no Handy Board present for it to communicate with.

Check-out of the charge circuit (yellow LED and switch) will be done later.

Debugging

Not yet prepared
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Fred Martin / MIT Media Laboratory / fredm@media.mit.edu / Mon Jun 3 16:26:42 1996