The most widespread application of ROMs is in the storage of data and program codes that must be available on power-down in the microprocessor-based systems. These data and program codes are called firmware because they are firmly stored in hardware.
Refer to the display multiplexer given below. The MSD display is blank, while the LSD seems to be OK. The input and output lines to the 74157 and 7449 are checked with a scope and can be seen changing levels. The A input on the 74139 also changes; however, the 0 output on the 74139 is always LOW and the 1 output is always HIGH. The B and EN inputs on the 74139 are always LOW. What could cause the problem and what should be done to correct it?
The 74139 is defective; replace it.
The EN pin on the 74139 could have a bad connection; check the connection.
The 74139 is defective and must be replaced; or the EN pin on the 74139 could have a bad connection, which should be checked.