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BE-+2520
ROM1
ROM2
RAMi1 (RAM 1 select signal)
RAM2 (Not used)
(P-ROM select signal)
Not used
Fig, 7
74LS138 Function Table
| Enable
| Select |
iIrtrtrtirTitiitr
x
=
Outputs
Y2 Y3 Y4 Y5 Y6 Y7
x=
x
x
x=
<=
x
x=
x
POP
ee
or
Se
mrrmrrereree.x
Lr
en
mr
kK
Zreitrztrgmwre;.x
Zrrrtrrre:
a
eS
ees
mrrririr3zririsre
rrrrrtrgrg.4
Lorne
Ere
sr
rrrrrririe}e;
Irerrrrixze:
rrrrirriric
=
=z
© G2 = G2A
+ G2B
® Chip select Decoder (SN74LS138)
1ORQ:
1/0 Request
PRS:
Printer select
SGS:
Display segment select
MCS:
Display segment (DP. COM. W ), buzzer select
KRS; .. Key read select
.CLKR: . Clock IC Read
CLKW: - Clock IC write.
SCS:
Display digits select
KRS: __ Key scan signal
0000 cal
am
@
Memory addresses used in BE-2520
MREQ: Low
{1K: 1024 bytes)
Memory address area (64K)
QFFF
BFFF
FFFF (Address)
a
ROM1:
Address 0000 ~ 3FFF
(16384 Addresses) ... 16K
RAM1:
Address 8000 ~ 87FF
(2048 Addresses)
.
2K
FOR THE TIMING
CHART,
REFER
TO CHAPTER
10
(CPU
Z-80)
OF
THE
"CASH
REGISTER
BASIC
MANUAL".
5-5. Signal description table
(Key, mode switch,
display)
Description"
SCS
Display digit and key scan data strobe
(Enabling signal for the (4514) 4-16 decoder.)
KSO
Key strobe
Key strobe
Mode switch strobe
Key (Mode switch) read (return)
Key (Mode switch) read (return)
Key read strobe
(enabling signal for 3-state buffer)
Display digit designate (ist digit)
Designate: High
Display digit designate (13th digit)
Display segment select (a).
Select: High
Display segment select (g)_
Display decimal: point sélect (DP)
Display arrow segmnént sélect ( W )
Display comma segment select ( ,
)
Display segment data strobe
Display segment (DP,:COM, W re and buzzer |
data strobe
|
'Buzzer
Low:
Buzzer on.
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