Epson PX-8 - Page 95

REV.-A
2.7.3 Key Input Control
When two or more keys are simultaneously pressed, keyboards which use a matrix structure such
as described above may not be able to identify any of the pressed keys. To remove this inconven-
ience, the 7508 is programmed such that, when two or more keys are simultaneously pressed
during one scan cycle (approximately 10 ms), the key operation is determined to be an error and
no key
input is accepted until only a single key is pressed. Assume for example that the keys A, B,
and C in Fig. 2-72 are simultaneously pressed. The KSC1 signal will pull both the KRTN1 and
KRTN6 lines
low it and will not be able to be determined which is pressed. The KRTN1 line goes
low when the KSC1 line is activated (low) because the two lines are connected via the A con-
tacts. The KRTN6 line is pulled low during the KSC1 time because the pulse signal is routed to the
KRTN6 line through A contacts, KRTN1 line, B contacts, KSC2 line, and C contacts.
KSCO
KSC1
KSC2
KSC3
KSC8
K~NK~NK~N K~NK~N
o 1 2 ......... 6 7
Fig. 2-72
2-71
key-
switch