The 75518 Qn outputs are a toem pole output for the VFD drive and go between
Vcc2 (up to 170V) and ground. They are most likely using extra outputs with
either diode coupling (to ignore source and only use sink output) or voltage
scaling with the 12K resistor to get the scans to a compatible voltage.

Have you tried to contact Pfaff support? I have the manual for the 2010 but
it does not go into board level schematics (iirc) that was purchased through
a Pfaff dealer. Never hurts to ask though. www.pfaff.com You might have to
go through a dealer if Pfaff won't help you direct.
Oppie

"robb" wrote in message
...

"Oppie" wrote in message
...
Looks like a scanned switch matrix. If you look at one side of the
switch,
it should have a 1 of N scan where N is the number if scan lines. One
scan
line goes active at a time and selects a switch or VFD column. The other
side of the switch is a sense that goes back somewhere else you do not
show.
The 40 pin chip you show is this one
http://focus.ti.com/docs/prod/folder...t/sn75518.html
Looks like it it giving the scans while you have two sense bits returning
somewhere (the 12K resistors).


hello thanks for the help,

i had not gotten that far along, those 12K lines go back to main board, i
was hoping the problem was isolated to this smll circuit board. *but* now
i
am doubtfull ... i think the problem is on mainboard.

those 12k lines both return directly to the I1 and I0 pins of a SN74LS251
found at
http://pdf1.alldatasheet.com/datashe...SN74LS251.html

an 8 input multiplexer w/3 state output, this chip sits near the bigg IC's
on rhe main boad.

i am having trouble understand ing the VFD chip purpose in the button
presses ? th eVFD drives the VFD as wel as sharing with some button
pressing
fnction.

thnaks again for help ,
rob