Servo - June 2011

FIGURE 3. Lid with holes cut.

The first version of the code allowed a straight dumping of the SPI contents to the terminal. Now, we will add a very simple menu to accommodate some simple configurations to complete the unit.

Hardware

The first thing to do is to drop the circuit board into a box. I found a small inexpensive box from Hammond and decided to mount the board in the lid. Having the great benefit of hindsight, it probably would have been wiser to have figured out hole positions and such before building the circuit, but where is the fun in that? I decided to remove the connectors for the SPI ports and the UART, and place them on the other side of the board. This way, the board could be mounted in the lid with the connectors passing through holes in the lid. This approach provides a secure and convenient way to mount the board and still access the connectors easily.

I chose headers with locking ramps for the SPI and UART connectors in a mostly self-defense choice since I have inserted too many un-keyed connectors backwards in my history and generally did not enjoy the results. Also, since the connectors that plug in here are color-coded clips, I did not want to confuse things by allowing more than one orientation.

The replacement of the headers required a little bit of rewiring because I flipped them to the other side of the board and moved the position of two of them to have a better layout in the lid. This was managed quite easily and quickly. This is shown in Figure 1 and Figure 2.

Next, I laid out and drilled holes in the lid so that the connectors would pass through, and I also created mounting holes for the board. I found some appropriate locations for some 4-40 screws in the board corners and drilled holes in that. Assembly of the box was as simple as securing the 4-40 screws through the mounting holes in the lid, tightening a nut on the inside to act as a standoff for the board, and placing the board in place. I then secured it with another nut on each screw. Figure 3 shows the lid

FIGURE 4.
Cable
entry and
inside view.

after cutting. The oval slots were made with drill holes and I milled the slot between the holes with a Dremel milling bit, my drill press, and some patience.

There are basically two more wired connections to the board from the outside world which are the power supply connections and the RS-232 port. Both of these were handled by drilling holes in the side panel, passing a cable through the panel with a grommet clinching the cable. I chose to solder these wires directly to the existing pins to save a connector, since this was to be a fairly permanent installation. Figure 4 shows a view of this.

The connectors were configured the same as they were originally, just oriented in different locations. Figure 5 shows the new positions. Operation is the same as in last month’s article (Part 1) with respect to connector attachment and configuration.