Back in 2013, Bob Rosenbloom asked: Tony Duell wrote: that one side of it does not work with TTL levels. It's a level shifter too. Although the BIB functioned as a level shifter, in later devices using the 1818-2500 standalone 40-pin BPC Binary Processor Chip, including the 9872C and 9872T, HP actually used the 74LS245 instead of the BIB. The only thing they did to meet the MOS level input requirements of the BPC was to put a 10Kohm pullup resistor to +5V on each data line on the MOS side of the buffer. As noted elsewhere, the 74LS245 isn't pin compatible with the BIB, so an adapter would be needed to substitute it. The 9872A, which uses the BPC with the BIB chip, does not have pullups on the MOS side. It also uses the HP 16-bit NMOS ROM directly on the MOS side, and it's remotely possible that adding 10K pullups could be a problem with the ROM.