Oldest operational computer

The problem was seen in the IFF circuits - the so called Identification Friend-Foe, or secondary radar. Basically, a radar could "interrogate" a target with a pulse of RF. The friendly target (aircraft or ship) would carry an IFF transponder that would receive these "interrogation" pulses from the radar, and act on it. The usual action was to reply, sending a stream of pulses of RF back to the radar, so the pulses could be analyzed. The pulses were coded for security reasons. If the interrogating radar found that the target in question did not have the correct pulse sequence, or no pulses at all, it was assumed to be an enemy. Friendly aircraft and ships would all be able to "give the password", so to speak. The pulse codes were very primitive by today's standards, although there were dreams during World War 2 of a binary crypto system (US NRL). The British system was really crude, using relays, but the American and German systems used rotating cams and a microswitch. Anyway, the problem was that the tubes in the IFF transponders could go hours without needing to pulse. High speed pulses were out of the question (and thus any fancy coding schemes), due to the "sleeping sickness" of the tubes. Something had to be done if IFF systems were to advance. Right after World War 2, the upcoming IFF system (Mark X) called for a sophisticated pulse coding system, consisting of 4 octal digits, called SIF (Selective ID Feature, I think). This system is basically still used today. However, it was required for the military to have almost every radar installed by the early.mid 1950s to have SIF decoders - that would be *thousands* for the US alone. So as you can see, the market for "computer tubes" was really for the military, not the few companies making data processors. -- Will