Building a Custom Serial Interface

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This is in Progress

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


Contents

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 and be sure to move the iron by lifting it off the copper board and replacing it in a different location every 10 seconds.


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. Note that the pads for the capacitors are placed so you can use smt capacitors if you want.


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 insure a good solder joint 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 work from the center of the board to the outside. Install the MAX3232 chip first 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 first. 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. We place a piece of masking tape over the holes for J1 on the component side and flip the level converter over so the components are facing down. We place the long ends of the 4 jumper pins into the holes (the pins will stop at the paper tape) and solder them into place.


R76 - To Install or 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 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 1mm by .4mm resistor, install a regular 10K resistor in the place marked r76 on the level shifter board (this component is optional if you managed to relocate r75 to r76). Next solder a piece of 26 gauge single strand wire across the pads for r76 on the Kuro motherboard. This is much easier than trying to relocate that tiny chip because you can use a piece of wire that is 6 or 8" long and then trim it after you solder it in place. Strip about 1/2" of the wire and tin the stripped end of the wire first. You should then be able to solder it in place without using any additional solder. Cut the extra wire as close to the pads as possible. It is okay if the bitter end is about 1/16" long. Just make sure it doesn't touch any of the other contacts on the board or the level shifter after it is installed.

If you chose to relocate r75 to r76 then you must solder a jumper wire across the pads on the level shifter that are marked r76. If not, then install the new r76 on the level shifter circuit board.

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.


We solder the opposite end of the 26 gauge wire we soldered to the Kuro's motherboard earlier to the connection marked W1 on the level shifter.


Finally we turn the level shifter over and place the other ends of the jumper pins through J1 on the Kuro's motherboard making sure that the pin numbers on the motherboard match the pin numbers on the level shifter. The plastic spacer should keep the level shifter from touching the motherboard.


The last step is to reassemble the kurobox, plug it into your computer with a straight crossover serial cable and start a terminal program on your computer (hyperterminal works well). Set your terminal program to 57600 baud, N,8,1 with no handshaking and start your Kuro. If you did everything right you should be able to see the boot messages in a terminal window on your computer. You should also be able to login through the terminal if you have relaible 2 way communication.