BASIC
II
ROMs
The early TRS-80s
that
have
BASIC II ROMs
are
easily identified. There
are
no
ROMs plugged
into Z33
or
Z34. Instead, there
is
a 4-conductor
flat
ribbon cable connecting the
CPU
Board
to
a
small 4-chip ROM Board. This ROM Board
is
attached
with
double-sided tape to the etched
side
of
the
CPU
Board. The three ROMs on this
Board contain BASIC
II.
These
ROMs may
be
supplied by various vendors. Figure 25
is
the
Schematic
for
Level II BASIC.
In later TRS-80s
that
have
BASIC II
ROMs,
you
may
have
a small 4-socket ROM Board
with
just
two
ROM chips
or
a
CPU
Board
with
ROMs
that
are
plugged directly into Z33 and Z34. The
two
chips on the small ROM Board
are
used
on
Rev-
isions D
and
E
and
the chips
that
mount
directly
onto the
CPU
Board
are
used
on Revision G
Boards.
These
revision letters
are
stamped on
one side
of
the
CPU
Board. Refer
to
Changes
to the TRS-80 Schematic
Diagram
(Figure 28)
for
the
changes
noted above.
104
The
second
and
third
versions
of
the small
ROM
Board
use
a 6-conductor ribbon cable instead
of
a 4-conductor ribbon cable:
In the second,
or
alternate
Level
II
ROM
version, the +5 volts
is
routed through Z4, pin
16 instead
of
through
J1
(pin 24
is
blank).
Ground
is
routed through Z3, pin 12 instead
of
through
J1
(pin
12
is
blank). (Refer to
Figure 26).
In the
third,
or
current
Level
II ROM version
(which
has
just two chips), ground
is
routed
through
Z2,
pin
12
and
the
+5
volts
is
routed
through Z4, pin 16. Also, signal A 12
goes
directly
to
pin
21
of
Z1
and
A13
goes
to
pin
15
of
Z4 instead
of
pin 14. Pins 20 and
21
of
Z2
are
tied together and pins 13 and 14
of
Z4
are
tied together. (Refer
to
Figure 27.)
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