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A WELL-LIT SOUND CHECK

Understanding the PC's MIDI Interface
by Stuart Ball

Start ı How Does MIDI Work? ı How the Circuit Works ı Sources and PDF

HOW THE CIRCUIT WORKS

The first two gates of U1 (pins 1–4) buffer the MIDI TX signal from pin 8 of the IDC header (pin 12 on the DA-15 connector) and drive the current sink pin of the MIDI OUT connector through a 330-ohm resistor. The current source pin is connected to 5 V through another 330-ohm resistor. When the MIDI OUT connector is connected to the MIDI IN on a MIDI device, the optoisolator LED at the MIDI IN end is connected between pins 4 and 5. So, when U1–4 goes low, the LED turns on. Because these LEDs have a typical forward voltage drop of about 1.6 V, the current through the diode is about 5 mA. When U1–4 is high, there is no current through the diode.

The input to the MIDI OUT driver is pulled to 5 V with a 47-kilohm resistor. This ensures that an unconnected input wonıt float, while providing a high enough impedance to avoid upsetting the MIDI output signal from the computer. Some soundcards have a resistor of 1 kilohm or so in series with the MIDI output. This prevents damage to the soundcard if a joystick directly connects pin 12 to one of the other 5-V pins, but it could cause problems with the adapter circuit if a smaller pullup is used.

Serial, current-loop data on the MIDI IN connector drives the LED in optoisolator U2. The output of U2 goes low when current is flowing in the LED, and this signal is buffered by two more gates of U1 (pins 10–13). U1–10 drives the MIDI RX signal to the computer. Diode D1 protects the LED in U2 against a miswired MIDI cable, which would apply reverse voltage to the MIDI input connector. The diode in U2 is only rated for a maximum reverse voltage of 5 V. If the leads on the MIDI cable are reversed, D1 conducts, limiting the reverse voltage to 0.6 V.

The relatively high transfer rate of the MIDI signal (32 ıs per bit) requires use of a fast optical isolator such as the 6N137. Slower devices, if they can switch the signal at all, have long turn-on and turn-off times that will distort the received waveform.

ICs U3 and U4 are ı555 timers that function as one-shot multivibrators. When a low occurs at the MIDI TX signal, U3 (pin 2) goes low and the output of U3 (pin 3) goes high for about 0.01 s. This causes LED D3 to briefly flash.

IC U4 works the same as U3, except that LED D2 flashes for incoming MIDI data. The LEDs indicate activity on the MIDI lines and can be used to troubleshoot the MIDI hookup when things aren't working right.

CONSTRUCTING THE CIRCUIT

The prototype circuit was constructed on perfboard using point-to-point wiring and housed in a plastic Radio Shack box. The MIDI connectors are mounted on one side of the box, and the LEDs on the other (see Figure 4). A slot was cut in the case cover to accommodate the ribbon cable. The female end of the ribbon, where the joystick plugs in, can be mounted to the side of the case if you choose. I brought both sides of the ribbon out through the slot.

Figure 4ıThe four signals needed by the translator circuit are taken from the ribbon cable at J1.

Figure 4 shows a right-angle connector for J1 because that's what I used. An upright connector will work just as well. I built the prototype as shown because the adapter sits on the left side of the computer. If your adapter is on the right side, you might want to reverse the circuit board in the case so the LEDs face you when the ribbon cable comes out the left side of the case and the MIDI cables come out the back.

As already mentioned, the 6N137 optoisolator at U2 is a high-speed part required for the high MIDI bit rate. Don't try to substitute the common 4N25 or other slower device because then the circuit won't respond.

The LEDs are mounted to holes drilled in the case and connected to the circuit board with wire. You can use LED panel mount holders, but I simply glued the LEDs into the case. The LEDs are optional. If you don't want them, you can leave off U3, U4, R4–R7, C3, C4, and the two LEDs. For better visibility, you can use high-intensity LEDs and make one red and the other green so that you don't have to look at the lettering on the box to tell which is active.

The bypass capacitors (C2, C5, and C6) should be located near U1, U2, U3, and U4. U1 is a 74HCT14 on the parts list. You could also use a 74ACT14 or 74HC14.

When you make the ribbon cable, clip wire 16 at both ends so the cable will fit into the DA-15 connectors. Be sure to clip wire 16, not wire 1. When you connect to the IDC-16 header on the board, make certain that you connect to the correct pins. The DA-15 connectors number their pins 1 through 8 down one side and 9 through 15 down the other. The IDC connector has the even pins on one side and the odd pins on the other, as shown in Figure 4. The connector cable manufacturers sell expensive crimpers for their cables, but a vise works well if you are careful.

If you don't want to make a ribbon cable, put both male and female DA-15 connectors on the plastic case and wire them together (pin 1 to pin 1, pin 2 to pin 2, etc.). Bring out the four wires needed for the circuit. I think the ribbon cable is easier. You can increase the LED ON time, making them more visible, by changing R4 and R6 from 100 kilohms to 220 kilohms.

CHECKOUT

You should verify all the wiring before installing the ICs and plugging the circuit into the computer. The simplest way to check the adapter is to connect it to an instrument using a MIDI cable and send a MIDI file using an audio utility that is capable of communicating with a MIDI device. Many computers come with MIDI Orchestrator or a similar program. Many soundcards come with a CD that includes audio software, including a MIDI player. If everything is working, youıll see the LEDs flash and will be able to send and receive MIDI data. Of course, you must have the instrument set up to operate over its MIDI interface as well.

If you donıt have a MIDI instrument, or if you just want to play it safe and test the circuit before you connect it to your $2000 synthesizer, connect the adapter to the joystick interface. Then, using software that can send data to the MIDI port, transmit a MIDI file. You should see the MIDI OUT LED blink. Now, use a MIDI cable to connect the MIDI IN and MIDI OUT connectors together. When you send the MIDI data again, both LEDs should be active. If you connect an oscilloscope to the MIDI RX signal on the joystick end of the ribbon cable, you should see the serial MIDI data being transmitted.

If you donıt get data, first make sure the computer is generating output to the MIDI port. Most soundcards under Windows 95 or 98 can send data either to the external MIDI device via the MIDI interface or to the speakers via a FM wavetable synthesizer. You can check this by going to control panel/multimedia/MIDI and verifying that the external MIDI device is selected.

If you still canıt get the interface to work, monitor the MIDI TX signal into the adapter circuit with an oscilloscope or logic probe. If there is no signal there, make sure that one of the ribbon cable connectors isn't attached backwards and that the connectors aren't crimped to the cable at an angle. If they are, you will usually end up with two wires shorted together.

Once the MIDI part has been verified, plug in a joystick and test it using the Windows joystick test feature (control panel/joystick/test in Windows 95). You can use the MIDI adapter to verify that a MIDI device is transmitting. Plug the MIDI OUT from the device into the MIDI IN on the MIDI adapter. When the MIDI device is sending MIDI data, the MIDI IN LED on the adapter will blink. The computer doesn't need to be running any MIDI software for this to work. As long as the computer is powered and the adapter is plugged in, the LEDs will indicate activity on the MIDI line.

You can use the same technique to troubleshoot a string of MIDI instruments when one isn't sending. Just connect the MIDI OUT or MIDI THRU from each instrument, one at a time, into MIDI IN on the adapter and check the LED.

Thatıs it. Youıre ready to control the MIDI world.

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