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. Α 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 mini computer 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 in cost. The crate pictured in figure 1 has 16 slots, eight of which are single-width, and 8 of which are double-width. The remaining slot position (No. 25), has a special connector to which a plug-in Power Supply makes connection when plugged into the crate. This particular power supply has quintuple-width dimensions, and when inserted into position, renders 2 double-width slots inaccessible. These slots may be recovered if necessary by mounting the unit outboard of the crate and making connections by cable. 5. Dataway and Bus Assignments The dataway consists of a number of conductors interconnecting the modules with each other and with the control module. Each slot position is terminated with an 86-pin etched-circuit type connector [1] which mates to the etched-circuit extension at the rear of each module. The connectors are supported and interconnected by means of the "backplane", which may be either of etched circuit construction or wire-wrap construction, or a combination of the two. The backplane consists of the dataway, connectors and optional patch pins. All digital communication occurs along "bus-lines" connecting corresponding pins together at all slot positions along the dataway. Two slot positions at the extreme right-hand side of the crate are reserved for the use of the System Controller. The rightmost slot is unique, having connections to all individual slot positions, but not having access to the data bus-lines. The second slot position from the right is normal in all respects but as with CAMAC, is reserved for the use of the controller. The remaining 14 slot positions are available for use by any module. Additional bus-lines bring power to all slot positions, and provide power-return and "clean ground" bussing throughout the crate. There are five uncommitted contacts at each slot position. Two of these are "free bus-lines" and are connected across all normal slot positions. The remaining three are "patch-points" and may be employed arbitrarily to establish nonstandard interconnections at the user's option. So far, the backplane and dataway layout have followed the CAMAC specification exactly. The use of the various bus-lines and MIDAS signal assignments do not necessarily follow the CAMAC us age; however, for comparison, CAMAC pin assignments may be found in the Appendix and reference 1. MIDAS pin assignments are detailed in table 1 for "normal" module slots, and in table 2 for the Controller position. It is seen that although many of the functions are identical, differences in operating philosophy preclude a one-to-one correspondence between the two systems. 6. System Controller While the individual modules provide interfacing between external instruments or devices to be monitored and controlled and the MIDAS system, the System Controller provides the interface between programming and recording devices and the system, and is necessary for directing communication between modules. It may indeed be considered to be the nerve center of the system, being the only module having access to all lines and busses on the backplane. The architecture of the MIDAS system may best be understood by reference to figure 8, in which the general functions of the System Controller are schematically outlined. Specific minimum functional requirements of the System Controller are described below. 6.1 Dataway Terminations All dataway busses are "wired-or" logic driven by open collector drivers in the modules. The System Controller must terminate all busses with approximately 3 k ohms to +5 V. 6.2 System Initialization and Reset The System Controller is responsible for generating a bus signal upon initial application of power to the system. This initialization may also optionally be generated by system command or manual switch closure. 6.3 Slot Addresses Slot addresses are decoded by the controller corresponding to the system commands A through 0. These are used to activate the proper line to the addressed module, enabling it to respond to succeeding commands on the Command bus. System Commands to accomplish simultaneous addressing of two or more modules must be decoded and interpreted by the controller. 6.4 Code Conversion All commands placed onto the command bus and data received from the data bus are in 7-bit parallel ASCII code. The controller must perform any conversions necessary to interface with external programming and recording devices which use other codes. Similarly, Similarly, serial-to-parallel and parallel-to-serial conversions are performed in the controller when required to interface with serial input/output devices. 6.5 Strobes The controller must generate strobes for each command directed to modules. These strobes are to be generated only when the logic lines have settled sufficiently to produce a valid command character and correct parity is established. |