7.3 Module Commands

The remaining 96 characters not reserved above are usable by module designers as commands to modules, subject to the previous recommendation that only printing characters be selected when possible.

8. System Signals and Timing

8.1 Dataway System Lines

A number of dataway lines are reserved for system-required functions. The application, requirements and timing of these signals are discussed in the following sections. In all cases the dataway busses use "wiredor" logic; therefore all bus signals present on the dataway are negative-true (3.5-5 volts = logical "0"; 0-.5 volts = logical "1").

(a) CMMD (COMMAND MODE) is generated by the System Controller and indicates to all modules that the system is in the "COMMAND" mode -- that all character codes present on the Command bus are to be interpreted as commands. The CMMD signal additionally indicates that the Data bus is not being used by the controller and is therefore free to be used by the modules.

(b) CONTB (CONTROLLER BUSY) is generated by the System Controller at any time when the Controller is unable to respond to a data strobe (DATAS) generated by a module. It is present during the time required for the controller to transmit a character to, say, the teleprinter. trailing edge of CONTB indicates that the controller is ready to accept a new data character from a module.

(c) ALT CONT (ALTERNATE CONTROLLER) is generated by a module desiring to take over system control. It disables the normal System Controller, and prevents it from responding to MODD signals. Upon receipt of this signal, the System Controller clears the Command bus and relinquishes control to the module generating the signal, after which the alternate controlling module may place characters on the Command bus.

The

(a) IDSC (INHIBIT DATASTROBE TO CONTROLLER) is generated by a module when communicating data between modules. IDSC prevents the System Controller from responding to DATAS. Thus, a module would generate this signal whenever it wished to place data on the Data bus which was not intended for an action by the System Controller such as output of that character on the teleprinter.

(e) DATAS (DATA STROBE) is generated by a module when data on the Data bus is valid and may be transferred. Lines of the Data bus must be stable for at least 100 ns prior to the leading edge of DATAS. Pulse width may vary with system operating rate but must not be shorter than 1 μs nor longer than strobe S2.

(f) MODB (MODULE BUSY) is generated by a module in response to addressing that module with ADDR, to indicate that the addressed module is still busy performing an operation initiated by a previous command and is therefore incapable of receiving new commands.

(g) MODD (MODULE DONE) is a pulse generated by a module upon completion of a module operation. It may be used to restart system operations following a pause initiated by a WAIT command. The pulse width must lie between 1 and 10 μs, and the leading edge of MODD must be delayed at least 1 us after the leading edge of strobe Sl.

(h) MODEOT (MODULE END-OF-TRANSMISSION) is generated by a module having the capability of transmitting a variable-length string of data characters in response to a single command. After all characters in the data string have been transmitted, the module activates the MODEOT bus to notify the controller that the transmission is complete. The controller uses this information to synchronize and interlock communications with the programming device, recording devices, and/or remote lines.

The

(i) INH (INHIBIT) may be generated by modules, System Controller or devices external to the system. It is normally used to provide a failsafe interlock capability in standalone systems, and interrupts all activity if a limit in the external experiment has been exceeded. INH line going low causes action to both System Controller and modules. The System Controller must be inhibited from placing further commands on the Command bus, and the programming device interrupted. All modules having an operation in process as a result of previous commands must cease activity until the condition causing actuation of the INH line has been corrected.

(j) S1 (STROBE 1) indicates that data on the Command bus is valid and may be transferred. Lines comprising the Command bus must be stable for at least 100 ns prior to the leading edge of Sl. The width of Sl may vary with operating rate but must not be shorter than 1 us.

(k) S2 (STROBE 2) is identical to Sl except for timing. S2 immediately follows Sl, having its leading edge coincident with the trailing edge of Sl. The command bus lines must remain stable for a minimum of 100 ns following the trailing edge of S2. This strobe may be "turned around" by a module to strobe data onto the Data bus.

(Z) ADDR (SLOT ADDRESS) is generated by the System Controller only. There is one ADDR line from the System Controller to each slot position. The ADDR signal is used to activate a module in the addressed slot position. There is no limitation to the number of slots which may be addressed simultaneously. The MODB line will go low in response to ADDR if an operation is in process in the addressed module. Modules have no access to ADDR lines other than their own, and most therefore go through the System Controller to address another module.

(m) LAM (LOOK-AT-ME) lines are counterparts to the ADDR lines but transfer signals in the opposite direction -- from modules to the System Controller. There is a LAM line from each slot position leading only to the controller slot position. A signal generated by a module placed on

this line notifies the controller that the module requires attention. The LAM lines may thus be used as an interrupt network, giving capability to a MIDAS system run by an intelligent program source to jump to an interrupt routine to service the module requesting attention.

(n) INIT (INITIALIZE) will be present at system startup or system reset and must set all control registers and bistable elements to a defined state. It is generated by the System Controller.

(o) C1 - C18 (COMMAND BUS) consists of eighteen lines used to transfer commands from the System Controller to the modules. The least significant seven bits (C1-C7) are normally used as the MIDAS USACII-7 Command bus. The total number may be used during parallel command operations when used with a minicomputer.

(p) D1 - D18 (DATA BUS) consists of another eighteen lines reserved for transfer of data words from the modules. Again only lines. D1-D7 are used for transferring USASCII-7 data characters. The full complement may be used for parallel data transfer to a minicomputer.

(q) 60 HZ (60 HERTZ CLOCK) is an ac line frequency 5-volt square wave generated by the System Controller and available on this bus-line for use by the modules for timing purposes.

(r) EARTH (ANALOG RETURN) is an isolated "clean" ground bus intended to be used by analog circuitry as the return line. This bus must be kept free from digital switching transients. It may be left isolated or tied to digital ground or earth ground at one point as required.

Rise and fall times of dataway signals shall not be faster than 40 ns to keep cross-coupling among dat away lines to a reasonable level.

Dataway drivers shall be open-collector type capable of sinking at least 25 ma at 0.4 v maximum.

9. Power Supplies

Module designers may assume that three power supplies are available in a MIDAS crate. These are a +6 Vdc supply capable of supplying 10 or more amperes of well-regulated and filtered power for digital logic circuitry, and a positive and negative 12 Vdc supply for analog circuitry. The additional mandatory CAMAC supplies (-6 Vdc and ±24 Vdc) may be supplied if required for the particular installation or if it is planned to operate both CAMAC and MIDAS modules in the same crate, but are not specified or required for MIDAS systems.

It is optional whether the power supplies are mounted at the rear of the crate or constructed in modular plug-in form and inserted from the front of the crate as is done in the system shown in figure 1.

10. References

[1] "CAMAC: A Modular Instrumentation System for Data Handling; Revised Description and Specification", USAEC TID-25875, (July 1972).

[2] Little, John L., "Some Evolving Conventions and Standards for Character Information Coded in Six, Seven, and Eight Bits", Nat. Bur. Stand. (U.S.) Tech. Note 478, (May 1969).