Radio Shack TRS-80 Service Manual page 21

ADDRESS DECODING
The Address Decodlllg p.ovides aU the selects for the system
10 pons, the ROMs, Keyboard, Video,
~nd
RAM memory
controL Refer to the Memory Vlap for exact locations of
devices. The two LSl38 demultiplexors provide the INPUT
and OUTPUT strobes for system 10 pons EO to FF; U41 for
OUTs and U40 for INs. Memory timing consists of U37 and
U61 in a shift register configuration clocked at 10.1376 MHz
to give the correct relationship between RAS", MUX. am,l
CAS'. MUX provides the switch between Row address and
Column address on the address multiplexors U24 and U42
for the memory a..ay. The three CAS signals are generated
by LSl39 (USB!. L$32s (U59, Ul07); one CAS for each row
of RAMs. Note that the first row 01 RAMs is the only one
suitable for 4K RAMs {jumper option U to T, N to P, GG to
FFI. The RO:\.1 strobes (RQMA·, ROMB', and
ROMeO)
are
generated by the remaining section of the LSl39
(usa),
and
part of the LS145 (U60I. The combinational logic below U60
(U75, U73,
un,
U74, U861 deleles two addresses from
ROMC' (37E8 and 37E9) which correspond to the line
printer address. thus ROMCP' is the actual nrobe going to
the 16K bit ROM. The result is that the line printer may be
ac;;cessed for a read as either a memory add.ess /37E81 or an
10 port (F81. See the to Pan Description lor a complete
breakdown of all 10 pons and address spac;;e map. The
remaining strobes KYBO' ilnd VID' are also generated by
the lS145 (U60).
MEMORY MAP OF MODEL III
HEX ADDRESS DESCR IPTION DECIMAL SIZE OK" 1024)
00•• ROMA .K
1FFF
2.00
ROM B
4K
2FFF
3000
ROMC 2K·2
37FF
37EB Printer Status 2
37E9
38•• Keyboard lK
3BFF
3COO
Video lK
3FFF
4000 RAM
16K
7FFF
••00 RAM
16K
BFFF
C."
RAM
16K
FFFF
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