Sharp ER-A750, Service Manual

Page: 61 / 105

1 Output address change
The ER-A750 uses the expansion RAM board at the RAS3 area and
its high-order IMB space (200000h
∼
3FFFFFh).
Basically, the ER-03MB is supposed to be used in the area from
300000h
∼
7FFFFFh; the ER-04MB in the area from 700000h
∼
8FFFFFh. Therefore, it is necessary to convert the addresses as
shown below.
Fig. 12
Table 16
<Address conversion table>
ER-A750
Address A23 A22 A21 A20
2XXXXXh 0 0 1 0
3XXXXXh 0 0 1 1
4XXXXXh 0 1 0 0
5XXXXXh 0 1 0 1
↓
ER-03/04MB
Address RA23 RA22 RA21 RA20
7XXXXXh 0 1 1 1
8XXXXXh 1 0 0 0
9XXXXXh 1 0 0 1
AXXXXXh 1 0 1 0
In the tables, the address 400000h and after at the side of the ER-
A750 are not used by the ER-03MB or ER-04MB, but is kept consid-
ering the commercialization of 4 Mb of expansion RAM in future.
The ER-03MB and ER-04MB use RA21 and RA20, respectively, be-
cause they do not have RA23 or RA22.
The address conversion circuit of RA21 and RA20 is shown below.
Fig. 13
2 Decode base signal change
As decode signals, RAS3E and RAS30 signals are generated for even
and odd addresses, respectively, based on the RAS3 signal
(200000h
∼
3FFFFFh). To allocate even and odd addresses, the ad-
dress line A0 is used.
13-2. Terminal table
The terminal table of the RAM connector is shown below.
Table 17
<Expansion RAM connector terminal table>
Signal name Pin No. Pin No. Signal name
GND 1 2 GND
GND 3 4 GND
RA21 5 6 RA20
PSX (Open) 7 8 PRAS30
+5V 9 10 +5V
VCKDC 11 12 VCKDC
VRAM 13 14 VRAM
A16 15 16 A15
A14 17 18 WR
A12 19 20 A13
A7 21 22 A8
A6 23 24 A9
A5 25 26 A11
A4 27 28 PSREF
A3 29 30 A10
A2 31 32 A17
A1 33 34 A19
A0 35 36 PRAS3E
A18 37 38 D7
D0 39 40 D6
D1 41 42 D5
D2 43 44 D4
RESET 45 46 D3
GND 47 48 GND
GND 49 50 GND
* The signals in ( ) are not used.
200000H
300000H
400000H
700000H
800000H
900000H
ER-A750
ER-03MB
ER-04MB
RA21
RA20
A21
A20
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