EIU= EICAS/EFIS Interface Unit ;-)
This is one of three computers on a 747-400 which converts a lot of the aircraft digital and analog data being provided to it by airplane sensors into recognizable glass cockpit flight and engine displays. It also sends signals to a DFDAC (computer card) which converts a lot of the EIU aircraft data into a format compatible with the DFDR data input circuits (Harvard Biphase).
The data is stored on a 25 hour continuous magnetic tape. As new stuff comes in, the 25 hour old stuff is erased. There are a series of 4-track record and erase heads. The loop is driven on our aircraft by a "stepper motor" (which I thought to be interesting, as it would not provide overly smooth rotation).
The DFDAC is also provided with acceleration data (c/o of a dedicated accelerometer) and radio keying pulses (so that the Voice Recorder tapes can be synched with the DFDR tapes. This helps the crash investigators to better figure out the chain of events which led to the accident or incident.
Our non-digital DFDR weighs 30lbs (probably because it is made of stainless steel) and the tape is contained in a crashproof/fireproof/waterproof enclosure.
On the front of the DFDR there is an Ultrasonic Underwater Locator Beacon (automatically activated when submersed in water). It operates to a depth of 20,000' and the signals can be detected 1800~3000meters away. The transmissions last up to 30 days. It transmits on a frequency of 37.5Khz.
Hope this helps (I'll let someone else respond to the OQAR question
).
Rgds.
Q.