More sophisticated Programming Modules may be designed to further narrow the distinction between stand-alone and computer-controlled MIDAS configurations. There is no reason why Programming Modules should not have decimal computation capability with conditional jumps dependent on arithmetical or logical comparisons. A MIDAS system operating at this level approximates a special-purpose computer, with the program potential limited only by the imagination of the module designer and experiment programmer. The major disadvantage of this configuration is that programming of computational code is bound to be somewhat onerous due to limitations imposed by the simplicity of such a "computing module" when compared to a true computer. Since all computation must be "microprogrammed" in machine code, it would presumably be limited to simple tasks such as summing, averaging, scaling or arithmetical comparisons. 3.3 Computer-Controlled System Remote Time-Shared Computer When the decision has been made to go to computer control, it may be obtained via the nearest telephone. Figure 4 illustrates schematically this form of operation. Notice that the MIDAS system is interposed between the telephone coupler and the remote terminal, and once again, the operating speed of the system is determined by the teletypewriter (or other) terminal. There are a number of major advantages inherent in this configuration. First, the extra investment required is quite small when computer control is only occasionally required the system is operated as a stand-alone (sec. 3.1) configuration the majority of the time. Second, this configuration is quite portable and may be used in turn to control several experiments either stand-alone or under computer supervision. Sharing the system would be more difficult if the computer and associated peripherals would have to be moved along with the MIDAS hardware. Third, it allows great programming simplicity, since it is now possible to program both commands to MIDAS and sophisticated computations in an easy-to-use, higher-level language such as BASIC or interactive FORTRAN. These languages are universally available and are already familiar to a large segment of the research community. Fourth, the experimenter may also take advantage of all of the benefits of a large computer program storage, large data file storage, editing facilities, adequate user core, and possibly high-speed input/output peripherals. - The disadvantages encountered in using a remote time-shared computer for control are minimal; continuous operation over long periods can run up sizeable computer bills, and one is, of course, limited to the operating speed of the terminal and timesharing system, although some systems can operate at several different data transmission rates. There is an additional uncertainty inherent in the response time of a timesharing system when heavily loaded, the computer may take several seconds to respond to output from MIDAS. If operating speed is critical, this configuration may seem intolerably slow under peak load conditions. The MIDAS equipment may be plugged in directly into the teletypewriter port of a mini computer with no additional interfacing, as minicomputers almost invariably have provision for teletypewriter input/ output on a 20 ma current loop interface. Once again the MIDAS system is simply placed in between the computer and the teletypewriter, as shown schematically in figure 5. The operating speed is determined by the teletypewriter speed, generally 10 characters per second. However, the system may now be operated fully closed-loop and programmed in a higherlevel language such as BASIC. BASIC is available on a number of minicomputers and will run short programs with only 4 K words of core memory. The advantages offered by operating in this configuration include rapid and interactive program generation, debugging and modification, with provision for considerable amounts of computation. A MIDAS system operating through the TTY port functions as a universal interface from any minicomputer to almost any instrument, facilitating expansion or modification of the system or program to suit the changing requirements of the experimenter. If higher operating speeds are required, the MIDAS system may be plugged into a second teletypewriter interface with the teletypewriter plugged into the customary TTY interface (fig. 6). The clock rate of the second interface may be adjusted to run at any desired rate up to at least 9600 baud. Data and commands are still transmitted by serial start-stop codes, but in this configuration operation proceeds at nearly 100 times normal teletypewriter speeds. The teletypewriter is used to communicate with the computer program, which may again be written in BASIC or equivalent. It is necessary to modify the BASIC system program slightly to provide for additional commands which address the second teletypewriter interface as an additional peripheral device of the computer having an independent device number. In this way, communication with the teletypewriter may occur at teletypewriter rate while MIDAS may communicate with the same BASIC program at a much higher rate. After reduction of the data brought in through MIDAS, the results may be printed on the teleprinter at low speeds. Figure 6. MIDAS System Operated Serially through 3.6 Minicomputer Controlled System Parallel Communication The ultimate speed of operation may be realized by employing parallel data and command transfer between the computer and the MIDAS system. The Basic System Controller is designed primarily for serial communication; therefore a Parallel System Controller is required to interface with the minicomputers parallel I/0 bus. This controller may be considerably less complex than the Basic System Controller due to the reduced need for timing and serial-to-parallel conversion functions. A system operating in this configuration is shown in figure 7 which enables MIDAS to be operated at computer speeds. The penalty paid for parallel operation lies in the increased difficulty in programming the system, as parallel data transfer must be programmed in machine or assembly language. It is possible with BASIC interpreters offered by several minicomputer manufacturers to write machine-language subprograms that may be called from a BASIC main program to transmit commands to MIDAS or to input data into the computer through the MIDAS system. Using this technique, complex calculations could still be performed entirely in BASIC, but commands to MIDAS would take the form of CALL subroutines using the USASCII command as the calling parameter. One possible implementation of this configuration would employ a single universal Parallel System Controller and a number of separate interface cards designed to interface to specific computers. The alternative is to have an individual Parallel System Controller designed to interface directly by cable and plug to the I/0 bus of each desired specific minicomputer family. The mechanical characteristics and dimensions of the modules and crate are entirely according to CAMAC specifications [1]. Excerpts from this document containing descriptions and details of the CAMAC mechanical specification are reproduced here in the Appendix; therefore the discussion of this section will be concise. MIDAS modules are constructed in CAMAC "plug-in unit" hardware -- there is no physical difference between these units and indeed, MIDAS modules could be operated in a CAMAC system with the proper programming. In addition, MIDAS systems may be operated in standard CAMAC crates. Standard CAMAC crates, however, have provision for 25 slots or "stations", somewhat more than may be addressed by a MIDAS system, which is limited to 16 slots. It is economically expeditious to construct special crates for MIDAS to CAMAC specifications, but with 16 instead of 25 slots, effecting some savings |