22 Apr
2020
22 Apr
'20
6:35 p.m.
Back in 2013, Bob Rosenbloom asked:
I have an HP 9872 plotter that just died. According to
the internal self
test (very nice!)
one of the bib (MOS to TTL) drivers has failed.
Tony Duell wrote:
the devie is very simple (it's simialr to the
74LS245) but the problem is
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.