The Implementing and Researching of BIT in Airborne Computer - Advanced Research on Computer Education, Simulation and Modeling - page 196

of the system would reduced. So a circle test method is proposed below. This method
designs a BIT test circuit for special digital I/O channel and special software working
together with the test of each I/O channel.
In output channel test, setting the digital value exporting though output channel first,
the output digital value should be read though BIT input channel, then comparing the
read data with the setting value. Though the circle test program, the BIT test result of
the output channel can be got. In input channel test, a group setting data output though
the BIT output channel, then reading the data though input channel, compare it with the
setting value. Though the circle test program, the BIT test result of the input channel
can be got. If the data output and input is same with the setting data, the channel is
normal. Otherwise the channel is fault and should be isolation from the system. The
BIT system block diagram of digital I/O channel test is show as fig 2.

Fig. 2. The flow chart of the digtial I/Ochannel test

3.2 A/D, D/A Channel Test and Fault Isolation

The analogy signal is input and output though A/D, D/A channel in airborn computer.
The BIT test method of A/D channel is as follow. The A/D channel of airborn computer
is connected with the reference voltage. Though running the special A/D test program,
the reference voltage is collect though the A/D channel. Comparing the collect value
with the reference value, if the difference is lower to the error tolerance, the A/D
channel is normal, otherwise send out the fault report. For example, the reference
voltage outside is +5V, 0V and -5V separate, the collected result though A/D channel in
airborn computer would be compared with FFFH, 7FFH and 0H. If the compare result
≤

Δ n bit (n is the allow error tolerance of the system n determined by system's

precision), the A/D channel is normal. Otherwise the channel is default, should be
isolation from the system.
D/A channel test method is reference voltage method. The particular test step of the
method is as follow. When the aerocraft is in ground test state, the airborn computer
output settlement data, and the assistant A/D channel collect the signal. Comparing the
collect data with the settlement data, if the difference is lower to the error tolerance, the
D/A channel is normal, otherwise send out the fault report. For example: the airborn
computer output FFFH, 7FFH and 0H three data. Using the assistant A/D channel
collect the signal, then comparing the collected data with the settlement data, if the