Talk:Apollo Guidance Computer

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Contents 1 CCS instruction 2 Single/double precision 3 User interface 4 Points of reference 5 What, specifically, was it used for? 6 Integrated Circuits 7 PGNCS trouble 8 Standby Mode 9 Description section 10 Technical & Generalize tags

[edit] CCS instruction

Fascinating article. Would love to know what the thinking is behind the CCS instruction - what sort of problem does it solve? What would be a modern (C, say) equivalent contruction? GRAHAMUK 04:51, 2 Sep 2003 (UTC)

The CCS instruction can be used to perform the equivalent of the C language "if" statement, "switch" statement, "for" loop, or "while" loop. I was going to put a code fragment in here as an example, but the editor really mangles the formatting... --Pultorak 07:29, 4 Sep 2003

The code fragment would be interesting, and could be adequately formatted in Wikipedia by putting a space first in every line. If applicable, you might get it back via the Page history link and put it back in. --Wernher 00:34, 3 Mar 2004 (UTC)

Designer's comment: I could probably write a small book on CCS alone. Let me know if my amplifications in November left anything unanswered. Did I mention that the basic idea was lifted from the IBM 704/7094's CAS (Compare Accumulator with Storage)? --67.75.8.160 02:46, 1 Mar 2004

First: a question before going into the subject matter proper: does "designer" in the above comment by any chance relate to "designer of the AGC and/or its instruction set"? Just wondering. And now to the question at hand: maybe the 7090/94 heritage should be mentioned in the article, in the 'quest' of educating readers about the sometimes hidden, and of varying degree and significance in each case, but still often very fascinating, continuity underlying much of computer development history? --Wernher 00:21, 3 Mar 2004 (UTC) / 21 Apr 2004

[edit] Single/double precision

The "single" and "double" precision mentioned in the article links to IEEE definitions, but surely the author meant one or two of the 15/16-bit words the machine used? --Anonymous, 21 Apr 2004

Thanks for mentioning it, I have to admit I didn't think of that when inserting the links. :-) However, the opening paragraph of the respective 'IEEE articles' does actually define sgl/sbl precision fairly generally (i.e. using one vs two words), so hopefully we'll avoid misleading the readers too much. Eventually I think we should 1) make a separate article on general single/double precision numbers, and 2) incorporate even clearer introductory info, w/links to the general article, in the IEEE definition articles. --Wernher 16:13, 21 Apr 2004 (UTC)

[edit] User interface

In this chapter there is a link to a picture and also to a diagram of the DSKY. The two do not match. [1] has there indicators:

+------------+------------+
|UPLINK ACTY |  TEMP      |
+------------+------------+
|  AUTO      |GIMBAL LOCK |
+------------+------------+
|  HOLD      |   PROG     |
+------------+------------+
|  FREE      | RESTART    |
+------------+------------+
| NO ATT     | TRACKER    |
+------------+------------+
|  STBY      |  [   ]     |
+------------+------------+
| KEY REL    | OPR ERR    |
+------------+------------+

Compare that to

+------------+------------+
|UPLINK ACTY |  TEMP      |
+------------+------------+
| NO ATT     |GIMBAL LOCK |
+------------+------------+
|  HOLD      |   PROG     |
+------------+------------+
| KEY REL    | RESTART    |
+------------+------------+
| OPR ERR    | TRACKER    |
+------------+------------+
|  [   ]     |   ALT      |
+------------+------------+
|  [   ]     |   VEL      |
+------------+------------+

I believe the top is Block I, the bottom is Block II LEM (the CSM doesn't have ALT and VEL indicators).

[edit] Points of reference

For contemporary readers who may not have been alive at the time, or who may not be intimately familiar with computer architecture, the Apollo Guidance Computer could use a comparison to later well-known computers or calculators. For example, how would it compare to the HP-65 programmable calculator, or a personal computer, such as the Apple II or IBM PC that came a decade or so later? Quicksilver 02:13, 11 November 2005 (UTC)

Good point! Perhaps we should also put in the TI-83/89 and HP-48/49g+ modern calculators, since those would be quite well-known to many college students (and engineers, scientists) today. --Wernher 17:10, 13 November 2005 (UTC)

[edit] What, specifically, was it used for?

This is a fascinating article, but I find myself wondering what exactly this computer was used for on its missions. The article says that it was used to "collect and provide flight information, and to automatically control all of the navigational functions of the Apollo spacecraft," but I'd like to see a more detailed explanation than that.

For example, what kinds of "flight information" were collected? What were the "navigational functions" of the spacecraft? Presumably the computer did not "fly the spacecraft" in a completely automatic manner. I would be interested to know more about in what way the computer was used by the astronauts to operate the craft.

DrDeke 15:29, 8 March 2006 (UTC)

For a thorough explanation, there's a 500-odd page Delco manual covering the programs used by the AGC on the Apollo 15 mission at: http://history.nasa.gov/alsj/a15/A15Delco.pdf.

While the AGC didn't fly the spacecraft completely automatically (e.g. it didn't work out when the engines needed to fire to take it to the Moon, but if given that information by the crew it could fire the engines and control the burn), it was capable of a completely automatic landing on the Moon. Typically the AGC flew the LEM until a few hundred feet above the ground, then the astronauts would use the LEM controls to adjust the programmed landing site to ensure they were going to land on a flat area and not in a crater.

The Virtual AGC page at http://www.ibiblio.org/apollo/index.html has an AGC emulator and some of the real software which ran on it. There's a video of the Virtual AGC flying a simulated Apollo CSM at http://mysite.wanadoo-members.co.uk/ncpp/CSM_DAP.wmv

The video isn't terribly exciting as it's just firing the RCS thrusters to rotate the CSM to the specified orientation (45 degrees pitch and 90 degrees roll). It does give some indication of how the real AGC was used by the astronauts though. MarkGrant

[edit] Integrated Circuits

We should probably add more information about the decision to use ICs in the AGC rather than discrete transistors. I've added links to some documents on the klabs.org site discussing this decision and it would appear to have been highly contentious at the time but extremely sensible in hindsight. MarkGrant 02:11, 9 July 2006 (UTC)

[edit] PGNCS trouble

This is an excellent article, but I question the assertion that it was the program alarms that caused Neil Armstrong to go to manual control of the Apollo 11 landing. Is there a source? As far as I know, all of the astronauts went to manual control during lunar landings, and I've never seen Armstrong's decision singled out like this before. --MLilburne 09:16, 13 July 2006 (UTC)

It does say 'more manual', but it seems odd to me too. BTW, I've also added a comment on the root cause of the 1201 alarms, which I didn't see mentioned anywhere else. Mark Grant 16:53, 13 July 2006 (UTC)

There's a good discussion of manual control and lunar landings (though on a message board) here. What "manual control" would mean in this context is, I believe, going to P66, which all of the commanders seem to have done. So I do think that the article is wrong. But I'm going to ponder a bit more before changing anything. --MLilburne 17:06, 13 July 2006 (UTC)

In First on the Moon', (Little Brown, 1970) Armstrong says he took manual control when he realized they were about to land in a boulder field. The alarm problems were an issue becaused they distracted him from looking out the window and following landmarks, but it was the realization that they were heading for a poor landing spot that caused him to take over the throtle ontrol so he could slow the rate of decent and allow more time at a higher altitude where he cold select a better spot.--agr 15:00, 8 December 2006 (UTC)

The 1201 and 1202 alarms were caused by too many events from the Rendezvous Radar (part of the Abort Guidance System) which was attempting to track the CSM in case the abort switch was pressed during decent. This Rendezvous Radar was disabled during decent in subsequent missions which is why you never hear about it after Apollo 11. --Neilrieck 03:04, 10 February 2007 (UTC)

[edit] Standby Mode

Somebody needs to correct this. It can't have been 5 to 10 kW reduction. More like tens of Watts.

http://history.nasa.gov/ap16fj/csmlc/a16Lemer1-7.gif says that some sort of standby (I don't know if it's the same) reduced consumption by 3 Watts. Does someone know if it's the same as the one mentioned in article?

Presumably it's the same. The text has been corrected already to read W not kW.--agr 14:44, 8 December 2006 (UTC)

[edit] Description section

The photo is nice but gives no indication of scale. Could anyone add to the description section indications of Power usage and Physical dimensions? Garrie 03:01, 12 December 2006 (UTC)

[edit] Technical & Generalize tags

I added the technical and generalize tags. This page is far too technical. This is obviously an amazingly important technology, but I don't know that describing clock cycles, circuit design, and other technical specifications are the best ways to convey it. Can we get more about the historical context (state of IC computers in the era), advancements that the computer made that can still be seen in modern computers, how the computer affected or resolved missions-in-flight (ie Apollo 13)?Madcoverboy 16:02, 11 March 2007 (UTC)