Types of PALs There have been a number of different types of PALs used over the years.
Combination lock
The earliest control mechanism was a three-digit combination lock. Later versions were four-digit locks designed to accommodate split-knowledge, where two different individuals could each have half the key. The combination lock can do different things. Some block the volume into which firing components must be inserted, others block electrical circuits, while still others prevent access to the fuzing and arming mechanisms.
These locks were in use at least as recently as 1987. In 1981 -- almost 20 years after PALs were invented -- about half of the U.S. nuclear weapons in Europe were still protected by mechanical locks [SF87]. CAT A
CAT A PALs, intended for use on missiles, were electromechanical switches. The arming input was a 4-digit decimal number. (Some sources say it was a 5-digit number.) Crews used a portable electronic device that plugged into the weapon to arm it. CAT B
The CAT B PAL, used on bombs, was similar in spirit to the CAT A, but used fewer wires. This permitted remote control of the PAL from an airplane cockpit. With the CAT B, it is also possible to check the code, relock the weapon, or rekey it. Later models of the CAT B included a limited-try feature, rekeying, and a code-controlled lock. CAT C
The CAT C PAL accepts 6-digit keys. A limited-try feature disables the bomb if too many incorrect keys are entered. Most references omit the CAT C. It may just be a later model of the CAT B. CAT D
The CAT D PAL accepts 6-digit keys. A given PAL can accept a number of different keys, permitting different groups of weapons to be unlocked with one transmission. Some keys are used for training; others are used to disarm the weapon or to disable it. One source [CAH84] suggests that PAL codes can also be used to vary the yield on some weapons. There are a number of selectable mechanisms to disable the bomb. In addition, there are "violent or nonviolent methods for destroying the warhead or making it irreparably nonfunctional" [C87c]. (One report, which I have not yet seen confirmed in the literature, is that the violent option involves a shaped charge which destroys the symmetry of the pit. It is thus no longer able to fission until it has been remachined -- and machining plutonium is non-trivial.) One reference suggests that there is a remote disable option on some PALs. CAT F
The CAT F PAL appears to be similar to the CAT D, but it accepts a 12-digit key.
The 1984 price for a CAT D PAL was $50,000 [CAH84].
I haven't yet found anything about setting C.R.M.-114 discriminators to "FGD 135", let alone "OPE"...
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Jesus. Christ. http://www.gamespot.com/features/6112889/p-18.html
You've got to love that 'Dr. Strangelove: Or How I Learned To Stop Worrying And Love The Bomb' reference at http://www.cs.columbia.edu/~smb/nsam-160/pal.html
Hint: Last line...
Types of PALs
There have been a number of different types of PALs used over the years.
Combination lock
The earliest control mechanism was a three-digit combination lock. Later versions were four-digit locks designed to accommodate split-knowledge, where two different individuals could each have half the key. The combination lock can do different things. Some block the volume into which firing components must be inserted, others block electrical circuits, while still others prevent access to the fuzing and arming mechanisms.
These locks were in use at least as recently as 1987. In 1981 -- almost 20 years after PALs were invented -- about half of the U.S. nuclear weapons in Europe were still protected by mechanical locks [SF87].
CAT A
CAT A PALs, intended for use on missiles, were electromechanical switches. The arming input was a 4-digit decimal number. (Some sources say it was a 5-digit number.) Crews used a portable electronic device that plugged into the weapon to arm it.
CAT B
The CAT B PAL, used on bombs, was similar in spirit to the CAT A, but used fewer wires. This permitted remote control of the PAL from an airplane cockpit. With the CAT B, it is also possible to check the code, relock the weapon, or rekey it. Later models of the CAT B included a limited-try feature, rekeying, and a code-controlled lock.
CAT C
The CAT C PAL accepts 6-digit keys. A limited-try feature disables the bomb if too many incorrect keys are entered. Most references omit the CAT C. It may just be a later model of the CAT B.
CAT D
The CAT D PAL accepts 6-digit keys. A given PAL can accept a number of different keys, permitting different groups of weapons to be unlocked with one transmission. Some keys are used for training; others are used to disarm the weapon or to disable it. One source [CAH84] suggests that PAL codes can also be used to vary the yield on some weapons. There are a number of selectable mechanisms to disable the bomb. In addition, there are "violent or nonviolent methods for destroying the warhead or making it irreparably nonfunctional" [C87c]. (One report, which I have not yet seen confirmed in the literature, is that the violent option involves a shaped charge which destroys the symmetry of the pit. It is thus no longer able to fission until it has been remachined -- and machining plutonium is non-trivial.) One reference suggests that there is a remote disable option on some PALs.
CAT F
The CAT F PAL appears to be similar to the CAT D, but it accepts a 12-digit key.
The 1984 price for a CAT D PAL was $50,000 [CAH84].
I haven't yet found anything about setting C.R.M.-114 discriminators to "FGD 135", let alone "OPE"...