In a number of projects I've ended up using generic RS232 to USB converters to connect a PC to various peripherals and interfaces. This is easy and cheap to do, but typically ends up with a bulky RS232 USB socket enlarging the project case, sticking out or generally getting in the way and looking unprofessional.
To resolve this, I typically strip the casing and connectors from the USB converter to give a more compact bare circuit board. This can then have wires directly attached as needed, without RS232 sockets, giving a more integrated and compact project. Having destroyed a couple of converters and now becoming more proficient, I decided to write up what I've learnt and share my tips should anyone else be encouraged to mess around in a similar way.
For this guide, I'm using a Prolific PL-2303. These cheerful converters are about £10, easy to find in computer shops and work under Windows and Linux (the drivers are present in Fedora from at least version 5). That said, FTDI also make good USB RS232 adaptors that are similar in construction and offer simplified driver installation via Windows Update, although some of these also have 2 indicator LEDs that may need to be removed depending on how compact you wish to make the adaptor.
From my meager tool box I use the following:
Getting into the case without causing any irreversible damage is best done with a large pair of waterpump or adjustable pliers. These can be sized to be just wider than the plastic shell and then used to apply gentle pressure to one half until the internal clips disengage and the casing pops apart. If it doesn't open without too much force, try the same action on the other half of the shell as on the PL-2303 the clips extend from one side only; pressing on the wrong side will close the clips further. Be careful not to use excessive force as doing so could damage the circuit board inside if the casing rapidly releases allowing the pliers to meet the delicate components or circuit board.
Depending on the exact project, you may wish to remove the USB lead and replace it with a different connector or socket. In this case I wish to use a motherboard USB connection, so I cut one off an old USB breakout back plate.
Before removing the USB cable from the board, note the colours of the individual wires connected to the PCB. Image 3 shows that the wires are coloured red, white, green, black and then black, left to right on the component side of the board. In the PL-2303's I have examined the left most black wire connects to the USB plug ground, while the outermost black wire connects to the cable shielding.
Having noted the connections, each of the small wires can then be cut. This is best done a little distance from the PCB to leave enough wire so that it can be gripped with small pliers. The five wires can be removed in turn by holding a single wire with pliers and using a soldering iron to melt the solder such that the wire can be gently removed from its hole.
Having removed all the wires, any excess solder can be removed from the contacts using solder braid so that the holes are clear for inserting and soldering new connections.
The socket header can also be removed to slim the board down even further. Removing this is fiddly since the socket is tightly soldered on both sides of the board, although the large pins and solder pads make this surprisingly simple if done correctly and with some confidence.
The underside of the board is the best side to start since it contains no components and few tracks and so is more difficult to damage, although the technique for both sides is the same. The first step is to use more desoldering braid to remove most of the solder on each of the pins.
One this has been done, the pins are easier to attack. A hacksaw can be used against the base of the pins and back of the socket to thin down and severely weaken the pins. Generally I do not completely saw through the pins as this risks damaging the board, although it is possible to carefully saw them right off if you so desire.
Finally using a soldering iron to melt the remaining solder and either a pair of pliers or scalpel the pins can be bent up away from the board to detach them one by one. If a lot of heat is applied to the pins, the plastic in the socket may also start to melt, making it even easier to pull the pins clear of the pads. If the pins do not lift easily be sure not to force them as this may rip up the solder pads on the PCB. Instead ensure heat is being applied (having completed sawing, adding solder back to the pin can help conduct heat from a soldering iron) or that enough of the pins has been sawn away. Once the pins are all bent up, more solder braid can be used to mop up any remaining solder and the pin backs can often be removed by repeated bending with pliers. Ensure that all the pins have been removed or are no longer connected to the PCB before starting on the other side of the board.
Having freed the lower side of the board, the pins on the upper side can be removed in the same way. Slightly more care is needed in this case as there are a couple of small surface mount components near to the connector which should not be damaged, although having removed all pins on the underside it is a little easier to break off these pins by bending the connector against the PCB a little.
Once all pins have been removed, the connector should fall away from the board. At this point, any metal shavings from earlier sawing should be cleared from the board (I use blue tack to pick them off), and the pads re-tinned with fresh solder ready for making new connections.
At this point, you can now solder on new cables and connectors as required. For this particular project, I soldered on a short motherboard connector for the USB, although it could easily have been a socket for a more professional fitting to a project box. Generally the USB cable colours are standardised, as given in Table 1, making it easy to wire up different plug and socket styles.
At the other end of the board, I've soldered wires that lead to a custom VGA connector to remove the need for two bulky RS232 sockets. This was easily done by placing the RS232 plug I'd previously wired next to the board and moving over each wire in turn to avoid any confusion as to which pin is which, as shown in Image 10. If however you do get confused about the pins, image 11 gives the RS232 numbering derived from the removed male 9 pin connector.
Depending on the exact final usage, you may find the cables soldered to the board are delicate and may fatigue and break away. To protect against this the board can be potted, or as I often prefer, the cable connections can be wrapped with epoxy putty. Just be sure to have checked that the connections are correct before doing this as potting is generally irreversible.
My technique is neither guaranteed or likely to be the best, although I've found it quite successful showing modification of USB adaptors to more specific needs. Other approaches are also likely to be just as good so improvise and be adventurous, particularly if other tools or USB serial converters are available.