7 Mar
2008
7 Mar
'08
7:43 a.m.
At Wed, 5 Mar 2008 14:00:29 -0800 (PST), Herbert C. Williams wrote:
But...
Logic levels are different when using PNP transistors. IBM in its
SMS series
cards used "S" levels (+S and -S) which were -12 and 0volts. Commonly
germanium transistors were PNP, and the Vcc rail was a negative
voltage (in the
case of SMS cards -12 volts). If you have a logic analyzer, you
can TRY to use
it by connecting the ground level reference to -12 volts and hope that
something doesn't blow because the analyzer's ground is now at -12
volts and
logic levels go "up" from there.
My old Biomation K100 will take a +/- 50 volt signal and set
reference levels to -6.4 to +6.35 volts. This should be sufficient to
handle most logic levels out there. However, the newer logic
analyzers typically restrict the reference levels to +/- 5V at best.
Re-referencing ground still requires one to be able to set the
reference level, which in the case of the cited logic probably
requires +6 V.
[...]
CRC