View source for M782 Interrupt Control

[[Image:M7820InterruptControl.jpg|250px|thumb|right|M7820 FLIP CHIP]]

The '''M782 Interrupt Control''' (later versions M7820 and M7821) is a [[Digital Equipment Corporation|DEC]] [[FLIP CHIP]] which implements the '[[interrupt]] control' function for the [[UNIBUS]]. It contains two separate interrupt [[circuit]] blocks; they can be wired to produce separate interrupts, or either can be used to provide the device with [[Direct Memory Access|DMA]] capability.

It was used, usually along with an [[M105 Address Selector]], in early [[PDP-11]] [[device controller]]s which plugged into an [[Small Peripheral Controller|SPC]] slot. It was also used in a number of early devices which were built out of a custom [[backplane]] and a large number of FLIP CHIPs, such as the [[RK11|RK11-C]]. Some later devices which no longer used a large number of small Flip Chips (such as the [[TM11|TMB11]]) also used it.

==Jumpers==

[[Image:M7821InterruptControl.jpg|250px|thumb|left|M7821 FLIP CHIP]]

The boards contained a number of [[jumper]]s which could be used to set confguration options.

===Vectors===

One set of jumpers controls the [[interrupt vector]]. The M782 can set vector [[address]] [[bit]]s 3-7 (i.e. vectors from 0 through 0374); bit 2 (04) is controlled by the device's circuitry. The M7820 adds bit 8 (0400; i.e. vectors of the form 0xx0 and 0xx4).

('''''Note:''''' The 1972 Edition of the 'Digital Logic Handbook' shows a jumper on address bit 2; however neither the prints, nor actual boards, agree with this.)

The M7821 adds a jumper on address bit 2 (04); that bit can be controlled be either the device's electronics, or the jumper. (The sense of the vector jumpers is inverted between the M7821 and the two previous versions.)

===Operation===

The M7821 has a number of operational improvements over the M7820.

It will decline interrupt grants when there is a DMA request pending, reducing the DMA latency. Not all PDP-11 [[Central Processing Unit|CPUs]] can handle this properly; a jumper (un-numbered; it is up near the handle) disables this behaviour when it is removed.

The A interrupt block can allow a device to do multiple DMA cycles with a single [[bus grant]], when pin J2 is left high; [[ground]]ing pin J2 signals that the DMA cycles are finished with.

The grant-passing circuity is quite different between the M7820 and M7821; it is possible that it was changed to reduce a [[meta-stability|race]] issue, but the import of the changes is not clear.

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