Lexicon 200, Service Manual

Page: 23 / 90

Lexicon
5-3
SRAM Test (2)
The SRAM Test performed during the POST is a destructive test, meaning that any data stored in
SRAM will be lost. The entire contents of the SRAM is tested by first writing 00 hex (00000000
binary) to all of the memory locations, and then verified by reading back all of the memory
locations. This write/read sequence is also performed using the following patterns: 55 hex
(01010101 binary), AA hex (10101010 binary) and FF hex (11111111 binary).
Before the test is executed, the following test code will be displayed on the Diagnostic LEDs:
Test # Test Name Diagnostic LED
2 SRAM COMPRESSOR “–10dB”
If a failure occurs, the (Red) LEVEL LEDs will be turned on in addition to the Diagnostic LED, and the CPU
will attempt to continuously loop the test for troubleshooting purposes.
If the Bypass button is pressed, the failure is ignored and the next test will be executed.
Lexichip3 WCS Test (3)
The Lexichip3 WCS test will check the program memory space (the Writeable Control Store) of the
Lexichip3. The RAM (memory space) is first filled with the value 55 hex (01010101 binary), then each
memory location is read to see if it contains 55. If 55 is in the memory location, the location is filled with AA
hex (10101010 binary), and the next location is processed. Once the RAM has been checked for 55's and
filled with AA's, the process is then repeated checking for AA's and storing 0's into memory. Following this
test is an Address test to verify all the address lines are active. Finally, the memory is checked for 0's.
Before the test is executed, the following test code will be displayed on the Diagnostic LEDs:
Test # Test Name Diagnostic LED
3 Lexichip3 WCS COMPRESSOR “–3dB”
If a failure occurs, the (Red) LEVEL LEDs will be turned on in addition to the Diagnostic LED, and the CPU
will attempt to continuously loop the test for troubleshooting purposes.
If the Bypass button is pressed, the failure is ignored and the next test will be executed.
If the Tap/Cancel button is pressed, the CPU will attempt to go into a mode where it can execute the test
continuously.
Lexichip3 INT Test (4)
The Lexichip3 INT test will verify that the Lexichip3 interrupt (INT/) is working and occurring at the proper
intervals. The Lexichip3 will provide MPX 200 with the interrupt (INT/) to the Z80's maskable interrupt line.
Before the test is executed, the following test code will be displayed on the Diagnostic LEDs:
Test # Test Name Diagnostic LED
4 Lexichip3 INT COMPRESSOR “Threshold”
If a failure occurs, the (Red) LEVEL LEDs will be turned on in addition to the Diagnostic LED, and the CPU
will attempt to continuously loop the test for troubleshooting purposes.
If the Bypass button is pressed, the failure is ignored and the next test will be executed.