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Without D2, the input signal voltage could exceed the supply voltage of the processor, which may cause the processor to mal-function. D2 is a signal diode, therefore the voltage drop across it is much lower than an ordinary diode. D2 is necessary to ensure that the low level voltage at the input pin to the processor meets the specification.
The component RES is a three terminal ceramic resonator to provide the timing reference for the processor. It provides a frequency stability of +/- 0.5%, making this the accuracy of the timing measurements performed by the processor. The processor measures the input pulse to +/- 1 uS. However, the processing of the input pulse is more accurate, since the output pulse is based on the same timing reference, causing any error in measuring the input pulse width to be reproduced on the output. The output pulse width is therefore very close to the input pulse width.
The remainder of the circuit is simply the five option links used to configure the operation of the program. The processor provides internal pull up resistors, so the option links connect these input pins to ground.

Construction.
The complete circuit can be fitted on a small piece of stripboard, 13 holes by 9 holes with the strips running lengthways. First, cut the strips, where indicated in figure 2, using a spot face cutter, or a drill bit. This shows the board with the copper strips facing.
Figure 2.
After cutting the strips, clean off any swarf and carefully inspect the cuts to ensure they have completely broken through the copper track.
Next, turn the board over, taking care to keep row 1 at the top. Now, add wire links to the board where shown in figure 3, noting the two special cases described below. Use solid, tinned copper wire for these links.

Component leg offcuts are often useful here. There are two special cases. One link is bent to avoid other components. This link starts at J5, goes diagonally over the top of I6, and is connected at I9. The other special case is the link which starts at B1, is bent around the link between A1 and A3, passes through the board at A5, and then travels on the copper side of the board (shown dotted in figure 3) connecting to each of A6, A7, A8, and A9. This provides the ground connection for four of the option links. If a pack of switches is to be fitted instead of using wire links, do not solder this link to positions a6 to A9 yet, as these holes are needed for the switch connections.
Figure 3.
Now, all the actual components may be added. Their positions are shown in figure 4. Note carefully the orientation of the two diodes, and the capacitor. The positive lead on the capacitor is usually longer than the negative lead.

Figure 4.
The wire links are shown to help position the components. Care should be taken with IC1 as this is sensitive to static electricity. If fitting a set of dual in line switches instead of wire links for OP1-4, then these are fitted through holes D6-9 and A6-9, soldering the wire link at the same time. Wire links for the five options OP1-OP5 may be fitted later when the function is being defined.