Product Description

The SCA 1270 System Crate Adaptor or System Crate to ACB Gateway is a unique module which allows ACB Auxiliary Controllers access to Parallel and Serial Branch Systems based on the Hytec/GEC Sytems Crate Architecture. This will allow computers which do have an ACB Interface (e.g. VAXstation, IBM PC/XT/AT) to be connected to existing multi–crate systems based on the Hytec System Crate architecture.

Application

The System Crate consists of an Executive Controller in stations 24 and 25, Parallel and Serial Couplers in defined stations, and Computer Interfaces or ’Program Sources’ in other stations. The Auxiliary Controller can be added by changing the standard MXCTR3 to a MXCTR5 (Executive Controller and ACB Master), then the Auxiliary can talk to modules in the System Crate, but not down the branches. If the System Crate Adaptor (SCA) is added, then the Auxiliary can address it as a user module and give it a branch command to do. The SCA then acts as a System Crate Program Source and executes the command, storing X and Q, and Read Data if applicable.

If the command involves writing data, then the SCA waits for this to be written by a separate command before proceeding. In this case, writing more data causes the same command to be repeated. If a read command is actioned, then the Auxiliary gets this read data from the SCA after the branch cycle has been completed and may also read the Control and Status Register (CSR) to obtain the state of Q and X bits. When data is read from the SCA, the Q response of the last cycle is passed, so it will not always be necessary to check the CSR. One bit in the CSR allows the user to specify an automatic repeat of the last read command so that after loading once, repetitive readout requires only one operation.

SCA 1270 Principle of Operation

Having received the command, it will then either wait for the Write Data or start straight away trying to gain access to the System Crate via the Priority Arbitration Highway. For further information on the method of operation of the System Crate see publications ’HYTEC System Crate Philosophy’ and ’Configuring Hytec System Crate Controllers’. Having achieved Mastership, it then does its cycle, storing the state of Q and X from that cycle and the Read Data (if any).

Looking at the System Configuration shown in figure 1 we can see that our Auxiliary can use the ACB to access modules in the System Crate. One of these modules is the System crate Adaptor (SCA) which is a System Crate Program Source, addressed by the Auxiliary Controller (and anyone else for that matter) and told what to do in terms of the command, a 24 bit word in the form BCFNA - B (branch), C (crate), F (function code), N (station number), A (subaddress); and the Write Data (if any).

In multi–user systems, we must have a means of ’booking’ the module, so that competing prospective users can tell if another computer is already using the SCA or not. This is done in the same way as in the List Processor LP1340, ie, by ’Test available’ command and a ’Reset’ command. The Q response to the ’Test’ – F(27),A(0) – indicates whether the unit is in use, (Q = 1 means ’Available’) and if it was available, then it will now become booked, and give Q = 0 to all further tests until given a ’Reset’ command.

Having given the Command to the SCA, the Auxiliary will then wish to check the result. If the Command was dataless or involved Writing Data, then it can read the CSR in the SCA to check that the cycle completed OK (i.e. DONE is SET and NO S2 is not SET) and also check that Q and X were received as expected. Note that even if the Auxiliary attempted this access IMMEDIATELY after giving the SCA the Command, it would not be able to gain access to the crate since the SCA is assured of first service by the MXCR5. If the Auxiliary loaded a Read Command, then it may simply read the data register in the SCA, at which time it will be given the Q response as received by the SCA as it did the branch cycle.

If the SCA was asked to address a non–existent branch or crate (or if the crate were off–line) then the NO S2 bit would be set in the CSR. If the LAM ENABLE bit is set in the CSR then the NO S2 flag will cause the SCA to generate a LAM. If the X response was not present during the SCA cycle, then a LAM would be generated in the same way. Using this process, the Auxiliary would not need to read the CSR response since it would receive an Interrupt if anything “went wrong”.

Physical

The unit is a single width CAMAC module, with 2 x 4 way LEMO connectors on the rear panel, for the Priority Arbitration Highway cables. Two front panel LED indicators show ’N’ (addressed) and ’busy’ (doing a cycle), both stretched to 20 mSec, a further 24 LEDs show the currently loaded command in the form BCNAF, and 4 further LEDs show ’booked’, Q and X from the last dataway cycle and ’NO S2’.