Optoscan - CAIRN RESEARCH - #9

_Optoscan Control

When a program is running, and the external program control option has been chosen, then the logic levels on the two program control input pins determine which of the four possible programs is being run at that time. Specifically, these pins are monitored at the transition to the last position in each cycle, and the corresponding program number will be run, starting from the first position of the next cycle. However, the cycle counter will not be reset by switching programs in this way. This facility can be used to interpose either drastic or quite subtle changes to a wavelength sequence, depending on exactly how different the individual programs are. This may sound like a solution in search of a problem, but we foresee a number of useful applications, and we expect that users will be able to devise many others.

The other major feature here is the control and display of transition times between positions. The galvo drive electronics provides three logic-level signals to inform the microprocessor system when the grating and slits have reached their new positions. These signals are ANDed together by the microprocessor to produce an overall "ready" output. As well as being available on the 37-way D connector, it is also sent to the internal backplane (if fitted) for the system modules, where it inhibits input signal integration when in the low (not ready) state. It is important to switch off signal detection during wavelength transitions, since the grating will be scanning across intermediate wavelengths during this time. The effects of acquiring data while slit widths are changing are likely to be less serious, but in general the slits are likely to reach their new positions before the grating does, so there is usually no additional time penalty for waiting for all three ready signals.

We realise that some applications may also require receipt of a "ready" signal from external equipment. We have therefore provided an external ready input on the 37-way D connector, which is combined with the three galvo inputs to derive the overall ready signal. This pin is an exception to the rule for the other inputs, in that it is held at a logic high level if not connected. However, as this corresponds to the ready state, the rule remains the same as for the other input pins, i.e. nothing needs to be connected here if this facility is not required.

The transition time between positions clearly depends on how much each galvo needs to move, so it will be different for each transition in the program cycle. However, the time for any given transition will be essentially constant from one cycle to the next. A key question now arises, as to whether or not the transition time should count as part of the total time spent at each position. The decision is up to the user, as the software allows either mode of operation, although our preference is to count the transition time as part of the total time. This has the advantage that the overall cycle times are entirely regular and predictable, allowing the various outputs to be used as accurate timebases for controlling other equipment.

On the other hand, there will clearly be problems if the transition time (however specified) exceeds the total time at any position, since it is impossible for the system to operate correctly under these conditions. It is the responsibility of the user to avoid such a situation! We have therefore also provided the possibility of keeping the transition times independent from the sampling times, so that the system will wait for the transition to be completed before acquiring photometric or other data for the ENTIRE specified sampling time at each successive position.

What does this choice mean in practice? Imagine that the monochromator is programmed to spend 10.0msec at each of three positions, and that the transition times to reach each position are 0.3 0.5 and 0.6msec respectively. If we include the transition times within the time at each position, then the actual sampling times are 9.7, 9.5 and 9.4msec respectively, and the total cycle time is 30.0msec. On the other hand, if the transition times are not included, then the sampling times at each wavelength are all exactly 10.0msec, but the total cycle time is extended by the sum of the transition times, to 31.4msec.

As stated above, we prefer the first option, but purists may feel unhappy with either. We have therefore provided the additional facility of transition time extension. This allows a minimum transition time value to be specified, which is the same for all positions. In the above case, we could