Radio Shack TRS-80 Service Manual page 7

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2-4. Pin description
(1) XLO, XL1
Crystal oscillator external connection pins (XLO: In, XLJ: Out)
(2) ADO - AD15
16 bits address bus (AOO: least significant address bit, ADIS: most significut address bit ). Tums to high impedance by the
BRQ
signal.
(3) D0-07
Bidirectional data bus tJSed to write and read data of/from the external memory (DO: least significant bit, D7: most
significant bit).
(4) MEO, MEl
Memory enable signals for direct accessing to an external memory (the maximum
I 28KB (64KB
x
2)).
(5) R/W
Read/write signal to perform reading operation when R(W=l and write operation when R(W=O .
(6)
RESET
.<d
0
s
ADI
MEO _.:,..._,/
MEI
0 1
IN
Oata re&d
cycle
OUT
D:ita
wfi1e
cycle
The MPU returns to its initial state with a high input level to this pin.
(7) BRO
Bus request. A high level causes the MPU to respond with the high level BAK signal after exeuted pending command.
(8) BAK
Bus .acknowledge appears in response with a high BRQ indicating that address bus, data bus, R/W, MEO and ME I are in
high
impedance.
(9) OPF
Operation code fetch appears when the MPU fetches an .operation (instruction) code. OPF is outputted only during the
fetch of an instruction code and is not outputted when address data, irnrn<Xliatc data, or the: second byte of a two step
instruction is fetched.
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-"'os
ADO-ADI 5
X
x:
-----"~'-----
M EO
OPF
- - - . . J
Last
1;ycle
-4-
Opefation
fetch
cy~Jc