This module consists of two independent sections, each of which
can be programmed to produce one or more pulses during each revolution of the
rotor. Although we call it a timer module for simplicity, its outputs are
actually synchronised to the rotor position, so they remain in the same phase
relative to the rotor regardless of the rotor speed. Pulses can either be
generated continuously, or they can be gated so that they are generated only
when an external control pulse is present. Amongst other possibilities, this
allows one section to modify the output of another, thereby allowing the
generation of pulse waveforms of any required complexity. The output is a
standard TTL level logic pulse, implemented with CMOS devices, so that logical
0 is very close to 0V, and logical 1 is very close to +5V. Similar
characteristics are recommended for the control inputs.
The timer module is primarily intended for synchronising other equipment to the rotor. However, in some system configurations it can also be used to generate a sequence of pulses even when the rotor is not spinning; please contact us for further details if you wish to explore this possibility.
Continuous operation
The continuous mode of operation will be described first. It is selected independently for each section, by setting its output control switch to the auto position. When the rotor is spinning, the control circuitry in the system enclosure generates 200 uniformly spaced pulses for each filter position within the rotor. This signal is used by the system to generate control signals for all its internal operations, by dividing and decoding it in the appropriate manner. The timer module allows the user to generate any desired output signal in exactly the same way.
The times at which a pulse begins and ends are set independently. In other words, when a command is generated to take the output high, the output then remains high until an independently generated command takes it low. This allows pulses to be of any desired length. It also allows one to view the unit either as one that produces high level pulses on a low level baseline (output set to go high before it is set to go low) or low level pulses on a high level baseline (output set to go low before it is set to go high). Further commands to take the output high or low when it is already in that state have no effect.
The point within every filter position in the rotor at which the output potentially goes high is set by three jumpers, and a fourth set of jumpers is used to control whether or not it actually goes high at each individual filter position. A corresponding group of jumpers set the point and filter positions at which the output goes low. The jumpers are labelled on the board as the set and reset banks respectively, for each of the two sections timer 1 and timer 2. An example is given below for setting the time at which the output of the first timer section goes high, which is selected by J1, J2 and J3, and modified by J4. (J1-J4 are collectively labelled as set timer 1).
For convenience, the switching point is specified as a percentage of the way through each filter position. Since 200 pulses actually give a resolution of 0.5%, J1 is provided to provide this additional degree of resolution in case it is required. The switching point is delayed by one pulse when J1 is in the upper position. However, for most purposes, J1 can probably be ignored, and selection can be carried out using only J2 and J3. These are both ten-position jumpers, which are set by placing the shorting link between the appropriate pair of pins. They set the units (00-09%) and tens (00-90%) respectively, so that the switching point is given by the sum of the two settings, plus an additional 0.5% if J1 is in the upper position.
As outlined above, the switching point POTENTIALLY occurs for EACH filter position, and a separate jumper bank (J4) is provided to select the filter positions for which it is ACTUALLY generated. J4 is a set of two-position jumpers (on and off), with one for each possible filter position (i.e. eight altogether; for rotors with fewer than eight filters, the redundant jumpers will have no effect). Its operation is most easily explained by considering several examples. Consider the situation where J1-J3 have been configured to set the pulse low-to-high (set) point at 20%, and the corresponding jumpers J5-J7 have been configured to set the pulse high-to-low (reset) point at 70%. In the simplest case, all eight sections of J4 are set to the on (down) position. The same applies to J8, which performs the equivalent function for selecting the filter positions for which the pulse END switching point selected by J5-J7 is generated. In this case, the output goes high between 20% and 70% for EACH filter position.
Now consider the case for which the jumper link controlling filter position 2 of J4 is moved to the off position, and the same is done for J8. Now there is NO output pulse during filter position 2, but it is still generated as before for all the other filter positions. One can readily see from this example how the output pulse can be generated as required for any combination of filter positions. The simplest way to achieve this mode of operation is to set J4 and J8 identically, with the selection made as required for each filter position (although some other combinations would also be permissible).
However, by setting J4 and J8 DIFFERENTLY, it is possible to select independently the filter positions for which the on and off transitions are actually generated, thereby allowing more complicated pulse patterns to be obtained. For example, if J4 is set to be on ONLY for filter position 1, and J8 is set to be on ONLY for filter position 4, the output will go high 20% of the way through filter position 1, and it will remain high until 70% of the way through filter position 4. This allows a wide variety of output pulse waveforms.
Pulse gating
The pulse gating facilities allow still further possibilities, since they allow one timer section to modify the output waveform of the other. This will become clear once the pulse gating facilities have been described. If the output control switch for either timer stage is changed from the auto to the trig position, the output can now also be controlled by the logical state of an external signal connected to the input socket for that section.
The nature of this control depends on whether the trigger mode switch is in the edge or level position. In the level position, the pulse pattern generated by the timer stage is combined with the input signal in a logical AND operation. In other words, pulses will appear at the output socket only when the input is in a logical high state (+5V). The effect of the external pulse occurs immediately, so it can be used to generate more complex pulse patterns, by connecting the output of one timer section to the input of the second.
For example, this allows two independent pulses to be generated within a single filter position. Consider the previous example in which the timer output is high between 20% and 70% of the way through one or more filter positions. If the second timer is programmed to give an output that is LOW between 30% and 40% of the way through the same filter position(s), and is connected to the control input of the first, the resulting output waveform will go high at 20%, low at 30%, high again at 40% and low again at 70%.
Another use for the control input is to allow synchronisation with other equipment requiring only a single pulse rather than a repetitive pulse waveform - e.g. a light flash or some other stimulus. For this purpose, the trigger control switch allows the possibility of EDGE sensitive control, which gives a ONE-SHOT operation. (N.B. The nature of this control makes it suitable only for the generation of high-level pulses on a low-level baseline.) When the control input undergoes a low-to-high transition an internal latch is set, and it remains set even if the control input goes low again. When the latch is set, the next programmed low-to-high switching point will occur, i.e. the output will go high. It will go low again at the next high-to-low switching point, but the output pulse also clears the latch. Therefore no further output pulses will be generated until the control input goes low and then high to set the latch again. In this application, the circuit effectively delays a command pulse until the rotor has reached a specified position.
This is a very general-purpose module, and the notes given here do not necessarily cover all the ways in which it can be used. Please contact us for any assistance in configuring the module for other applications, or to let us know of any other uses you may find for it.
