Browse Prior Art Database

Multiplexing Switch Status Sensing

IP.com Disclosure Number: IPCOM000075918D
Original Publication Date: 1971-Dec-01
Included in the Prior Art Database: 2005-Feb-24
Document File: 4 page(s) / 67K

Publishing Venue

IBM

Related People

Cummings, JC: AUTHOR [+3]

Abstract

Input/output multiplexing switches are usually "transparent" units whose operational status is not sensed by the controlling or control unit being multiplexed. It is desirable to identify operational status including error conditions in such transparent switches, in a manner not interfering with nor detracting from the multiplexing function. This is accomplished by using negative logic status signals.

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Multiplexing Switch Status Sensing

Input/output multiplexing switches are usually "transparent" units whose operational status is not sensed by the controlling or control unit being multiplexed. It is desirable to identify operational status including error conditions in such transparent switches, in a manner not interfering with nor detracting from the multiplexing function. This is accomplished by using negative logic status signals.

Multiplexing unit 10 selectively connects any one of the four CU's (CU 1-4) to any one of a large plurality of I/O devices. A typical nodal circuit is shown which has a plurality of nodal switches 15 selectively connecting a pair of cables 16 and
17. There is one set of nodal switches for each possible connection between each CU and each I/O device. For example, if there are four CU's and four I/O devices, there are 16 sets of nodal switches.

Nodal switches 15 are selectively energized by AND circuit 18 in response to a plurality of simultaneously occurring conditions. First, a CU must send a DEVICE SELECT signal over line 19. In addition to the DEVICE SELECT signal, CU supplies address signals over cable 20 to decoder 21. AND circuit 22 is responsive to the decoded address and to either a DEVICE SELECT signal or a SWITCH SELECT signal via OR circuit 23 to energize line 24. Further, an enable condition is supplied from manually actuated enable circuit 25. The final criterion for nodal switch selection is that the I/O device has not been committed to nor held by another CU. HOLD signals on cable 30 indicate the "held" condition as to all nodes associated with the device combined by OR circuit 33. The committed condition indicates that the device is committed to operate with a given CU via a set of nodal switches whether or not those nodal switches have been in fact actuated. NOT circuit or inverter 26 inverts the combined commit signal from OR circuit 29 for enabling AND circuit 18 when no commit signals are being received.

In an attempted nodal selection by DEVICE SELECT on line 19 or SWITCH SELECT from a CU, a sensing step is performed. First, DEVICE BUSY is sensed by other nodal circuits 46 and supplied to AND/OR circuit 31. Simultaneously, this logic element receives inputs from AND circuit 22 (node address), the HOLD OR circuit 33 via NOT circuit 34, and enable circuit 25. If a HOLD signal is being received from another node, AND/OR 31 signals DEVICE BUSY. A separate HOLD signal is then supplied by AND circuit 32. The term "HOLD" indicates that there is a present nodal connection to the I/O device, but that nodal connection will shortly be released for permitting a selection. The CU may respond to such a NODE HELD signal by maintaining the DEVICE SELECT signal until HOLD is removed. HOLD's from other nodal circuits associated with the I/O device are received by OR circuit 33. Circuit 33 supplies a combined HOLD signal to AND 32 to signal CU that the I/O device is in a HOLD status. NOT circuit 34...