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Serial Link Plans
  • Schematic
  • Software You Will Need
  • Parts List
  • Tools You Will Need
  • Instructions
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    Serial link schematic

    (If you want to build a 25-pin version of the serial link, the equivalent pins are 6(6), 7(5), 2(3), 4(7), and 5(8).)

    Serial link diagram


    Before deciding to buy or build a serial link, it's a good idea to check out our chart of linking software and make sure there's a program compatible with your operating system and calculator.
    » Linking Software


    • A female 9-pin (or 25-pin) D type connector (to plug into the serial port) to which you can solder the wires.
    • A plastic cover to fit around the D connector.
    • Two zener diodes.
    • Two 3.3K ohm resistors.
    • One 330 ohm resistor.
    • One LED.
    • A piece (1 to 1.5m or so) of thin electrical three-core cable. If you can get a cable which has two wires and a copper 'screen', which is a bare copper wire wrapped around the other ones, then get some of this sort instead. Then you can use the screen as the earth to improve grounding and prevent crosstalk between the wires. Don't make your cable much longer than about 1.5m, otherwise you will increase the chance of transmission problems.
    • Either: a 2.5mm stereo jack to plug into the calculator;
    • Or: a 2.5mm stereo socket instead (recommended to ensure a good connection with the calculator's port). You then plug the calc-to-calc cable into the socket and plug the other end into the calculator. This ensures a perfect connection and is a more robust solution than using a plug - plugging and unplugging can damage fragile soldering;
    • Or: since some people have had difficulty obtaining 2.5mm sockets or plugs, your other option is to cut your calc-to-calc cable in half to use the 2.5mm plug from one end. If you choose to do so, then you will find that the tip of the 2.5mm plug is the red wire, the middle ring on the plug is the white wire, and the other (copper) wire is the ring nearest the base (see diagram above). If you don't want to sacrifice your calc-to-calc cable, you could attach a readily-available 3.5mm socket to one cut half of the calc-to-calc cable and a 3.5mm plug to the other. Make sure both the socket and plug are stereo. Your calc-to-calc cable will still work (make sure you get the connections right - use a test meter) and you can use one half to connect to your computer link via another 3.5mm plug and socket.


    • Soldering iron - it's also useful to get a clamp thing if you can find one to act as a second pair of hands.
    • Test meter - to test all the connections. Also needed to work out which part of the 2.5mm stereo jack connects to which connector on the back of it. Multimeters can act as test meters.
    • Wire strippers
    • Pliers
    • A screw driver


    • Print out the schematic: this will make it a lot easier to work with. There are two ends to the link - the calculator end and the computer end. You can make either end first it doesn't matter.

      Calculator end

      This is perhaps the most fiddly part of it. You have three options: either use a plug, a socket, or half your calc-to-calc lead.

      Calculator end - Using a plug

      • If you use a plug then (depending on its design) you may need to carefully file away some of the metal from around its base to let it fit into the calc's port fully. See if the base of your plug gets obstructed by the circular lip around your calc's port.

      • Take the plastic cover off your 2.5mm stereo jack. It will have three connectors on the back - two small ones and one large one that wraps around everything. Thread the plastic cover onto the cable now, otherwise if you have already done the other end you won't be able to thread it on to put the cover back.

        The largest of the three connectors on the back of the plug is for earth. If your cable has a copper screen as one of the wires then use this as the earth (wire shown as sleeve on diagram above). (If you have not done the computer end of this cable yet then it doesn't matter which of the other two wires you connect to which connector.)

      • Strip some of your cable and then solder each wire to each connector, cutting the wires to the correct lengths first of course. Hook the wires back after you have put them through the connector rings to ensure a strong connection. This step is the fiddliest of all.

      • You will need to know which wire you have connected to each ring on the plug. You can either find this out now or later. Use a test meter and make a note of which wire connects to what.

      • Carefully screw the cover back on.

      Calculator end - Using a socket

      • If you want to use a socket then when you use your link you will use your calc-to-calc lead to go between the socket and the calculator. The method to follow is the same as above except that to work out which connector on the back of the socket connects to each ring on the plug on the far end of the calc-to-calc lead, you need to plug one end of the calc-to-calc lead into the socket and use the test meter on this combination. The combination of calc-to-calc lead and socket functions as a plug. Personally we think that using a socket is more reliable than using a plug.

      Calculator end - Using half your calc-to-calc lead

      • In much the same way as above, to check which wire connects to what ring on the plug, you can use a test meter. You will find that the tip of the 2.5mm plug is the red wire, the middle ring on the plug is the white wire, and the other (copper) wire is the ring nearest the base.

      Computer end

      • We will connect the zener diodes, resistors and LED to this end and get everything inside the plastic housing. Investigate how your D connector attaches to the housing. It may be best to work with one half of the housing attached to the D connector so you can get everything the right length. Up to you.

      • Solder the zener diodes, LED, and resistors as shown on the diagrams above, paying attention to the polarity of the diodes. To get everything to fit you'll need to arrange things carefully and trim the legs of the components to just the right length. Make sure nothing touches that shouldn't -- put some insulation (e.g. rubber from your cable) around the components and wires.
      • Use the test meter to work out which wire in your cable goes to which part of your 2.5mm plug. Carefully strip the three wires in your cable to the required lengths and solder the wires to the appropriate points (see diagram above). Bear in mind that you will want to fit the cover over everything for neatness so don't strip them too far. Hook the ends of the wires round whatever you're attaching them to and use a good dollop of solder to make the connections strong.

      • Check the connections, then attach the plastic housing and cable grip.

      • If you haven't yet done the other end of the cable now's your chance.

      And that's it.

      Testing your link

      After double checking all your connections, you need to test out your link by sending or receiving a program with appropriate Linking Software, and hopefully it will work perfectly the first time. :-) If it doesn't work, see our hints.

      Good luck. See the link software page for where to get suitable link software.


    • Connections

      First things first: check and double check that you have got the connections correct and that no wires are touching that shouldn't. Make sure you get the pin numbering correct and ensure that no pins have become soldered to each other by mistake. Check the whole thing with a test meter to check for bad connections.

    • Use the right software

      The home-made parallel and serial links require the appropriate software to make them work. They do not work with Texas Instruments' link software. See the Linking Software section.

    • Poor contact with calculator port if you use a plug

      Some 2.5mm plugs do not fit far enough into the calc's port to make contact properly because of the circular 'lip' around the port on the calculator's case. To get around this you can either file away some of the metal from around the base of your plug, or you can carefully scrape away some of the plastic around the calc's port to let the plug fit in further. One person's link didn't work until he filed away a bit of the base of his plug.

      Also, if you look at the plug itself on some calc-to-calc leads you will see that it's smooth all the way along. The standard type of 2.5mm plug has a groove near the tip. The grooved type should connect properly with the calc's port, however if you are having connection problems consider a socket instead (see below).

    • Use a 2.5mm socket instead

      To ensure perfect connection with the calc's port you could use a 2.5mm socket and then use the lead supplied with your calculator to go between this socket and the calculator. This is what we would recommend. Just make sure you get a stereo 2.5mm socket.

    • Be careful

      Obviously treat your calculator's port (and your computer's port) with respect. Check your calc's port works by transferring something to another calculator.

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