M9047 Grant Continuity Card
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Unlike the OMNIBUS, the LSI-11 Bus does not occupy the entire backplane. Whilst the backplane is 4 slots wide (or high), the bus only occupies two slots. Many LSI-11 modules are double height, although some are quad height. The arrangement of the backplane varies from model, but an example is given below (Figure 1).
In the example, there are 8 quad slots in all. Slots 1-3 can accomodate ONE double or one quad module each. Slots 4 to 8 can accomodate TWO double modules or one quad module. The CPU is always mounted in slot 1, with memory in slot 2 and perhaps 3.
| A | B | C | D | |
| 1 | <======LSI BUS======> (1) |
C-D | C-D | |
| 2 | <======LSI BUS======> (2) |
C-D | C-D | |
| 3 | <======LSI BUS======> (3) |
C-D | C-D | |
| 4 | <======LSI BUS======> (4) |
<======LSI BUS======> (5) |
||
| 5 | <======LSI BUS======> (7) |
<======LSI BUS======> (6) |
||
| 6 | <======LSI BUS======> (8) |
<======LSI BUS======> (9) |
||
| 7 | <======LSI BUS======> (11) |
<======LSI BUS======> (10) |
||
| 8 | <======LSI BUS======> (12) |
<======LSI BUS======> (13) |
||
In the example above, the C-D slots in the first three rows are wired to allow interconnection between adjacent modules, if required.
The LSI-11 allows peripheral devices to have a position-dependent priority. The priority is indicated by the bracketed numbers in Figure 1, above. Logic on each module monitors interrupt lines and Direct Memory Access grant lines. If a line is asserted and the module has not requested a service, then that module passes the signal on to the next one down in the chain.
This means that there must be no inter-module gaps along the chain. If, for some reason a module has to be removed, then a Grant Continuity Card (Grant Card) must be inserted in its place. The Grant Card merely connects the incoming and outgoing interrupt and grant lines.
M9047 Grant Continuity Card
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Connects AM2 to AN2 and AR2 to AS2
