Building a Custom Serial Interface

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Revision as of 02:47, 15 June 2006 by Dtaylor (Talk)

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Etching a circuit and building your own serial converter is pretty simple. The steps are:

Build the Circuit Board

I have designed a circuit board layout. It can be downloaded from Here (as soon as the ftp site is up again) and printed out. I learned a trick several years back for etching circuit boards. It is pretty easy to do yourself. Follow these steps:

1. Print the circuit design (without scaling it) on a laser printer. An inkjet printer won't work for this.

2. Use a regular iron set at its highest temperature (usually linen) and iron the circuit printout onto a single sided copper clad board. Iron the pattern on for at least 3 minutes.

3. Soak the copper board in water for at least an hour. After soaking the board most of the paper should come off without much work. What will be left is the toner and some of the paper fibers that are stuck in the laser toner. You can use an old toothbrush to remove as much of the paper fibers as possible, but make sure not to remove any of the toner. If you do remove some of the toner, you can touch it up with a sharpie.

4. Cut the board to the proper size. the outline of the board is part of the layout.

5. Etch the board by either placing it in a bath of ferric chloride (available as Radio Shack (at least it was a year ago) or you can don a pair of ruber gloves and use a sponge to wipe the board with ferric chloride. the sponge method is much faster (less than a minute as opposed to 10 minutes).

6. after the copper that is not protected by the toner has been etched away rinse the board under running water and scrub with a green scrubbing pad to remove the toner. what is left is the copper traces of the board. It should be noted that the toner is somewhat porus and the traces will be slightly etched themselves. as long as they are not completely severed the board will work.

7. Drill the holes for the components in the circuit board with a .035" diameter drill bit.

Populate the Circuit Board and Solder the Components in Place

The trick to soldering is to make sure the two items to be joined are sufficiently hot before applying the solder. Melting the solder with the iron will lead to cold solder joints which are brittle and will fail. The easiest way to accomplish this is to place the soldering iron on one side of the component lead and place the solder on the opposite side of the component lead. When everything is hot enough the solder will melt and flow around the component's lead.

One more trick is to make sure the soldering iron has a good tip and is properly tinned. The tip will erode with use and will need to be sharpened. A black tipped soldering iron is also difficult to use because the black stuff (usually burned rosin) gets into the solder and causes a poor connection. If the soldering iron tip isn't sharp (about a 60 degree angle at the tip) use a file to sharpen it and tin the tip. If the iron's tip is black it also needs to be tinned. Tinning is accomplished with a moist sponge and some solder. Use the moist sponge to wipe as much of the black stuff from the tip of the hot soldering iron. Apply solder to the tip and repeat as necessary. If you cannot get a good tin on the tip of the iron usa a file to remove the black and start over. If this fails, buy a new tip for your iron.

One last bit of information. Use the proper size soldering iron. If your iron is shaped like a gun, it is too big. I do most of my component soldering with a 15 watt pencil soldering iron. Soldering takes time. You must allow time for the component leads to heat up. a bigger iron will heat the components too fast and will damage components. Take your time and you will be soldering like a professional.

I usually install one component at a time and I wolr from the center of the board to the outside. Install the MAX3232 chip forst making sure to orient the chip properly. Pin one is marked with a small dot. By the way, the components go on the opposite side of the board from the copper traces (in this design). Solder pins 1 and 9 forst. that will hold the IC in place while you solder the remaining leads. remember. place the iron on one side of the lead and the solder on the opposite side. Hold the iron in place only long enough for the solder to flow around the component lead and only apply enough solder to cover the copper pad and fully connect the lead on all sides of the lead.

Next install the capacitors. The capacitors are polarized and must be installed in the right direction. They will have a lead marked with a + and that lead should match up with the hole that is marked with a plus on the component layout. Install all 5 of the capacitors.

Next we need to attach the 4 pin header to the board. push the short end of the header pins through the component side of the board and solder them in place.

R76 - To Install of not to Install

You definately want 2 way communication. It allows all sorts of experimentation that is not possible without 2 way communication. We now have to decide if we want to move r75 on the Kuro's motherboard to the r76 position. DO NOT TRY TO TACK ONE SIDE OF R76 IN PLACE AND BEND IT DOWN TO ATTACH THE OPPOSITE END. YOU WILL END UP PULLING THE SOLDER PADS OFF OF THE KURO'S MOTHERBOARD. If you are up for the challenge go for it. if not just apply place the tip of the iron directly on r75 and remove the chip all together. Don't worry about losing r75 as you won't need it anyway. Instead of trying to place that .4mm by .2mm resistor install r76 on the level shifter board (this component is optional if you managed to relocate r75 t r76). next solder a piece of 26 gauge single strand wire to the r76 or r75 pad closest to pin 1 of J1. This is much easier than trying to relocate that tiny chip because you can use a piece of witre that is 6 or 8" long so it is much easier to hold in place. Once the wire is soldered to the pad on the Kuro we will trim it to about 3" long and strip teh last 1/4" of wire. we will connect it to the circuit board later.

Installing the Serial Connector

Next we attach 3 pieces of wire that are about 6" long to the D9 connector pads on our circuit board marked 2, 3, and 5. You guessed it. These wires will lead to a D9 connector at the back of our Kurobox. We will connect these wires to the same pin numbers on the D9 connector.

Next we flip the level convertor over so the components are facing down and slide the jumper pins through J1 on the Kuro's motherboard. The plastic spacer should keep the components from touching the motherboard. IF it does not, you will have to make sure to raise the