[Milsurplus] BC-960, all the 6SH7s

[William Donzelli]

> I'm gessing the incoming pulse stream from the interrogator was stored
> in that batch 'o tubes & the return pulse width's were the "code of the day."

It is actually not that smart. Mk III IFF interogation pulses are just one
big bang - WHO ARE YOU. The transponder is the thing that generates the
pulses, and no "memory" is needed. In the MK III boxes, it was done using
relays. Simple, but keep in mind that only six codes were available.
Adding codes would be a complete hassle. Mk IV IFF (all those BC-645s)
used coding disks spun by the dynamotor. These disks could be changed out
before a flight. At first, ten disks were given for each set (but now 
are super rare!), but later in the war, more disks with more codes were 
procured.

William Donzelli
aw288@osfn.org