Portable
PLUS
Computer
Reference
Information
10·13
Probe A2U17, pin
11
and
look for the voltage to switch between 0
and
5 Vdc. This test checks
that
the
51
msec timer
is
enabled.
If
the voltage does
not
toggle, check
whether
A2U14, pin 5 toggles.
If
this
does
not
toggle, check
that
the
proper
signals are present
on
U14, pins 10
and
11-15
.
Press any key to return to the main menu.
Test 3. Diagnostic module EPROM checksum test.
This test does a checksum test
on
the
code in
the
diagnostic module EPROMs.
Press function key
@)
to
run
this test.
This test takes approximately
60 seconds to run.
If
the EPROMs have the proper checksums, the test
will respond with "Diagnostic
EPROMs OK" in the display.
If
the test fails, one or more of the follow-
ing messages will appear:
error format:
XXX
EPROM
BAD
#
xxx
= HIGH or LOW
# = data bit where error occurred
(00-015)
If
an
EPROM(s) fails, check
that
the EPROM(s) is installed correctly,
and
that the
data
lines indicated
by the test toggle
when
the test
is
run.
Circuit Description. Connector
J1
is
the connection to the computer motherboard PCA. Address buff-
ers
U1
and
U2
are always enabled,
and
gate 17 bits of the latched address to the diagnostic module.
These buffers reduce load capacitance
on
the address lines.
Tristate data transceivers
U4
and
US
control the transfer of data between the diagnostic module
and
the computer. The
DT
/R*
line controls the direction of data transfer; a combination of
DEN*
and
address space decoding enables the
data
transfer.
U6, U7, U12,
and
U13 determine
when
the module
is
enabled, decode the plug-in address range,
and
provide chip select signals to the EPROM. U8 responds to a "read" from
ODCO
(hex.) to tell
the
system
that
a ROM drawer is installed in the left port.
EPROM U15 contains the odd-address memory; EPROM U16 contains the even-address memory.
Data latch
U14
is
enabled at
I/O
address 00C2. A high line
MDO
turns
on
the voltage to EPROM Xl
(a
blank 8K-byte service EPROM). When MD3
and
BWR* are asserted, the "clear"
input
to
multivibrator
U17
is
also asserted. When
MOl
is high,
Q1
turns on. This raises
the
output
voltage
from regulator
U7, causing the Xl programming voltage to increase approximately 5 Vdc to
about
21
Vdc.
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