8.12 Class test 1_25th_march_2011

Class Test 1_25.02.2011

Give the design, layout and fabrication steps for the dual-3-INPUT wide RTL-NOR Gate as shown in the Figure 1. This IC Chip was used successfully in Apollo Guidance Computer.

The Apollo Guidance Computer ( AGC ) provided onboard computation and control for guidance, navigation, and control of the Command Module (CM) and Lunar Module (LM) spacecraft of the Apollo program . It is notable for having been one of the first IC -based computers.

Each flight to the Moon (with the exception of Apollo 8, which didn't take a Lunar Module on its lunar orbit mission) had two AGCs, one each in the Command Module and the Lunar Module . The AGC in the Command Module was at the center of that spacecraft's guidance&navigation system (G&C). The AGC in the Lunar Module ran its Primary Guidance, Navigation and Control System , called by the acronym PGNCS (pronounced pings ).

The AGC was designed at the MIT Instrumentation Laboratory under Charles Stark Draper , with hardware design led by Eldon C. Hall . ^[1] Early architectural work came from J.H. Laning Jr., Albert Hopkins , Ramon Alonso , ^{[2] [3]} and Hugh Blair-Smith . ^[4] The flight hardware was fabricated by Raytheon , whose Herb Thaler ^[5] was also on the architectural team.

The Apollo flight computer was the first to use integrated circuits (ICs) . While the Block I version used 4,100 ICs, each containing a single 3-input NOR gate , the later Block II version (used in the crewed flights) used 2,800 ICs, each with two 3-input NOR gates. ^{[1] :34} The ICs, from Fairchild Semiconductor , were implemented using resistor-transistor logic (RTL) in a flat-pack . They were connected via wire wrap , and the wiring was then embedded in cast epoxy plastic. The use of a single type of IC (the dual NOR3) throughout the AGC avoided problems that plagued another early IC computer design, the Minuteman II guidance computer , which used a mix of diode-transistor logic and diode logic gates.

PGNC generated unanticipated warnings during Apollo 11's lunar descent , with the AGC showing a 1201 alarm ("Executive overflow - no vacant areas") and a 1202 alarm ("Executive overflow - no core sets"). ^[8] The cause was a rapid, steady stream of spurious cycle steals from the rendezvous radar, intentionally left on standby during the descent in case it was needed for an abort. ^{[9] [10]}

During this part of the approach the processor would normally be almost 85% loaded. The extra 6400 cycle steals per second added the equivalent of 13% load, leaving just enough time for all scheduled tasks to run to completion. Five minutes into the descent Buzz Aldrin gave the computer the command 1668 which instructed it to calculate and display DELTAH (the difference between altitude sensed by the radar and the computed altitude). This added an additional 10% to the processor workload causing executive overflow and a 1202 alarm. After being given the "GO" from Houston Aldrin entered 1668 again and another 1202 alarm occurred. When reporting the second alarm Aldrin added the comment "It appears to come up when we have a 1668 up". Happily for Apollo 11 the AGC software had been designed with priority scheduling. Just as it had been designed to do, the software automatically recovered, deleting lower priority tasks including the 1668 display task, to complete its critical guidance and control tasks. Guidance controller Steve Bales and his support team that included Jack Garman issued several "GO" calls and the landing was successful. For his role, Bales received the US Medal of Freedom on behalf of the entire control center team and the three Apollo astronauts. ^[11]