Photo by C. Jeff Dyrek
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What is this stuff? This an aluminum coated Mylar tape used on a tally tape reader for testing the ASN-99 PMDS (Projected Map Display System).The PMDS sits in the A-7E Corsair II cockpit just on the right side of the radar display. The PMDS has a display screen which looks very much like the radar's display. The PMDS projects a sectional map image on the screen so the pilot can navigate. When the Pilot turns, the image on the screen turns. As the plane moves the image moves to follow the airplane. The PMDS receives positional data from the inertial navigation system which tells the map how to move.
The ASN-99 used a 35mm film that was 21 feet long and was rolled into a cartridge that was placed into the Display Unit. A projector lamp, just like you see in a slide projector shined through the tape and onto the screen. The tape contained two scales of the sectional chart plus a series of checklist and emergency checklist. The entire cartridge was in a roller drum so as the plane turned, the drum would turn and therefore the map display would turn along with the airplane. Data and power to the cartridge was transmitted from the rest of the display unit through a series of wiper contacts. This was the only weak link in the system. The PMDS had about a 2000 hour reliability, so it just about never broke. When it did break, the wiper contacts were almost always worn out and required replacement. Also, the salt moisture would cause oxidation of the rotary drum which the wiper contacts would ride on and this would further increase the wear.
The tape picture that you see here is an octal punched tape. An octal system has eight data bits and one parity bit, a total of nine bits, or in this case nine holes are punched into the tape. When the tape moves from reel to reel, a light passes through the holes and the data is picked up on a light sensitive device giving information to the computer. This tape was used for an automated test computer called the ASM-398 was used to automatically test and diagnose the ASN-99 PMDS. The ninth bit was called a parity bit. This system used a positive parity, meaning that if there were five data bits, then the ninth (Parity) bit would punched making it an even number of holes. When there were six data bits, there would be no parity bit, still making it an even number of holes per data word.
The tape reader used a light bulb that was exactly like a cars taillight bulb except that position of the filament had to be calibrated in the factory so that the data bits would be read properly. The light from the bulb would shine down through a circular, tubular lens, then through the holes in the tape to the detectors below.
In the A-7A and A-7B, the map was the ASN-67 Roller Map Computer system. In this system the pilot made his flight plan he actually cut out a four inch strip of the sectional maps which are used for airplane navigation. He then taped the sections together and placed them in a Roller Map display which was also placed to the right of the radar system. If he flew in an area that was off of this preplanned four inch strip, he had to get maps out of his case to figure out where he really was. This was a big disadvantage as compared to the ASN-99 Projected Map Display System.
The new map systems are much more advanced and, really, don't have any moving parts to speak of. They are part of a multi function display with a flat screen monitor as you see on the new computer systems in your home. This removes all of the tremendous number of components used to keep the PMDS system in sync with the airplane movement. Instead of using a 35mm film, the data is stored in a ROM (Read Only Memory) or an EEPROM (Electrically Erasable Programmable Read Only Memory) which can be updated very easily without removing any cartridge.
The future Jet Fighters would have no pilot at all and would have no requirement for any system of this type at all.
Going back to the picture on this page, I was told that one of these tapes cost the government about 50,000, I believe that this was the true development cost not the cost of each tape. We were replacing the tape with a new updated set so we had to destroy this tape. The data was classified, but we had fun before we destroyed the tape.
There was one big advantage to using a tape with physical holes for containing the data. That is that it cannot accidentally be erased and it was EMP (Electromagnetic Pulse) proof.
|Ok, Here's a little story. When I was in the Philippines, I brought my girlfriend and her little brother on the USS Kitty Hawk and we took a tour of the ship. When we came into our shop she knew more about the tally tape reader than I did. She took computer programming and she was proud to tell me all about how it worked. My boss was listening in on our conversation and after I took her home, then returned, he announced to everyone that there will be no civilians allowed into the shop anymore. Her name was Tessie Gamboa and I wish she would write to me if she ever reads this page. She was a very nice person and so were her family and friends.|
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