H009

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H009 (pronounced "who-9") is an Avionic data protocol. It is also referred to as MacAir H009. This protocol was introduced by the aircraft manufacturer McDonnell in 1967 and was among the first avionic data buses. H009 features two redundant buses, controlled by a CCC (Central Computer Complex) and up to 16 PU's (Peripheral Units)in each channel, with a 1MHz clock. The CCC polls the PU's by a "select word" and every one second sends a MBTR (Mux-Bus Test Reply) which is used to check whether the PU is working correctly. When the CCC does't get an answer, it switches automatically to the redundant bus. The hardware configuration of the bus is eight 68 Ohm shielded wires. The PU's contain special circuitry including pulse transformers, because the bus is analog. H009 was used in the F-15 Eagle A/B/C fighter jets (two buses) but due to its sensitivity to noise and other reliability problems it was succeeded by the MIL-STD-1553 protocol and is not in use in new aircraft today.

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