Building a Custom Serial Interface

=Build RS232/TTL serial converter= Etching a circuit and building your own serial converter is pretty simple. The materials cost about one fourth of what a prebuilt level converter costs and it has the added benefit of not requiring the difficult step of relocating r76.

Parts
The parts required to build this are:
 * 1) A single sided copper clad circuit board.
 * 2) A small amount of ferric chloride.
 * 3) One MAX-3232CPE chip in a DIP package.
 * 4) Five .1 microfarad Tantalum capacitors.
 * 5) One 1/8 watt 10K resistor. (optional if you can relocate r76)
 * 6) One male D9 connector.
 * 7) A four pin straight header.
 * 8) 18" 22 guage wire.
 * 9) 6" 26 guage wire.

Components can be found at Digikey

Tools
The tools required are:


 * 1) 15 watt (maximum) soldering iron
 * 2) Diagonal cutters for trimming component leads
 * 3) Thin rosin core 60/40 solder.
 * 4) Wire strippers.

Build the Circuit Board
I have designed a circuit board layout. It can be downloaded from Here 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. Cut the board to the proper size. the outline of the board is part of the layout.

3. 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.

4. After the circuit board has cooled to room temperature, 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.

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) and agitating it until the exposed copper is removed 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 15 minutes). I have heard of others using muriatic acid diluted 1:1 with water (never add water to acid, add acid to water). I do not recommend this as it is quite caustic (even compared to ferric chloride). Make sure to etch your board in a well ventilated area.

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 or 0000 bronze wool to remove the toner. Do not use steel wool as it will get into the circuit board traces and cause problems later. What is left after scrubbing the board is the copper traces. It should be noted that the toner is somewhat porous and the traces will be slightly etched themselves. as long as they are not completely severed the board will work. Test the traces with an ohm meter to make sure there are no shorts or open traces. Open traces can be repaired with solder and shorts can be repaired with a scratch awl or dremel tool.

7. Drill the holes for the components in the circuit board with a .035" diameter drill bit. A dremel tool works well for this. Note that the pads for the capacitors are placed so you can use surface mount capacitors (on the trace side of the board) if you want, but I doubt you will as the object here is to make assembly easier, not more difficult.

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 type soldering iron and don't recommend anything bigger. 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 or one side of the board to the other side.

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 particular 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 plus 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.

=KuroBox Specific Parts=

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 and skip ahead to the next section. 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 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 wire that is 6 or 8" long which is much easier to hold in place while soldering. Tin the stripped end of the wire first and trim it to about 1/16" long before attempting to solder it. You should then be able to solder it in place without using any additional solder. Once the wire is soldered to the pad on the Kuro we will trim it to about 1.5" long and strip the last 1/4" of wire. We will connect it to the circuit board later.

Next, install a new 1/8 watt 10K resistor on the level shifter board in the place marked r76.

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 male D9 connector (like the one on the back of your computer) at the back of our Kurobox. We will connect these wires to the same pin numbers on the D9 connector. You will need a null modem cable (5 to 5, 2 to 3, 3 to 2) to connect the kuro to your computer but any serial accessories (modem, palm device, etc.) you use will simply plug into your Kuro.

Before we solder the level converter in place we need to connect the wire we soldered to the Kuro's motherboard (at r76) to the pad marked W1 Try to shorten this wire as much as possible as any extra length could compronise the integrity of the received data. Skip this step if you managed to relocate r75 to r76.

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, but you should make sure this is the case and add spacers if necessary.

The last step is to cut a hole in the back of your kurobox to mount the D9 connector (optionally you can just snake it out of the case) and reassemble the kurobox.

Testing Your New Serial Port
Plug the Kuro into your computer with a 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 flow control 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. If 2 way communication is working you should also be able to login through the terminal.

Here are a couple of photos of the completed board prior to installation (I didn't have a kuro without a serial converter so I built this one for waite). You will notice that the circuit traces aren't the prettiest. That isn't a problem as I beefed them up on the board layout. Spots to check carefully are where the traces travel between pins 1 & 2, 2 & 3, 6 & 7 and 12 & 13 of the MAX3232 chip.

Note the blue wire. This is the wire that attaches to the Kuro motherboard at r76.





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