"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