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.

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, 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.

6.6 Synchronization

The controller must synchronize all data and command transfer between system modules and programming/recording devices. Transfer must be in full-duplex mode since there is no general limitation on simultaneous transmission and reception by the modules. A signal is generated by the controller to inform any module wishing to transmit data that the controller is busy and cannot receive. This signal may be used to synchronize the transfer of strings of characters from the modules. When a module has finished its transmission of a string of data, it generates another signal to inform the controller of this condition.

If it is required that the programming device be temporarily halted to allow completion of a module operation before resumption of the program command sequence, the controller must recognize system commands to enable the "waiting" function, and must respond to a completion signal from the module to restart the programming device.

The controller must clear the command bus and relinquish control over the system when a signal from an alternate programming device is present. It will, however, continue to transmit data to the recording device as dictated by the data strobe.

6.7 Inhibit

An additional responsibility of the controller is to generate an "inhibit" bus signal which inhibits sequencing of the programming device and halts all in-progress operations in the modules. A front panel connector must be available to allow a signal to be brought in from the outside world to inhibit system operation.

7. Command Structure

7.1 USASCII-7 Command Set

There are 256 possible code combinations formable using all eight bits of the USASCII-8 code, X3.4-1967 [2]. Unfortunately, the complete USASCII-8 code set is able to be utilized on only a very few types of equipment. Additionally, there are obvious advantages to using the eighth bit of an 8-bit code to provide a capability to check parity. Such practical considerations lead to the limitation of the valid code combinations for MIDAS to the lower-order 7 bits, the widely-used USASCII-7 code, consisting of 128 possible characters. The only character which is inherently unusable as a MIDAS command character is the rubout character "DEL" (octal 177), since it must be blocked from the system to allow correction of program tapes and to provide a code signifying "no-operation".

Of the USASCII-7 code, 32 characters (octal 000 through 037) represent control functions and are therefore non-printing. The lower-case font (octal 140-176) is unusable on many devices including less-expensive

teletypewriters. These characters are all potentially usable as commands to MIDAS equipment, and as such are not excluded from the possible command set. It is strongly recommended, however, that only the printing characters (octal 40 through 137) be used as program commands, since programs composed of this set may be listed on the teletypewriter for program verification and debugging.

7.2 System Commands

Of the printing character set, 32 characters are reserved for MIDAS system commands and are interpreted by the System Controller. The reserved set includes the punctuation marks from octal 40 through 57 and the characters octal 100 through 117. The latter group includes the alphabetical letters A through O which designate the slot addresses. The use of the remaining system commands is arbitrary and is left to the discretion of the controller designer. The reserved System Commands and their octal representations are listed below: