Lexicon 200, Service Manual

Page: 29 / 90

Lexicon
5-9
If the test failed, the (Red) LEVEL LEDs will light.
EEPROM Test (6)
This test will read each byte in the User Register portion of the EEPROM and add them together to
calculate a checksum. This value is compared with the checksum value stored in the EEPROM itself. This
checksum will be recalculated each time a register is stored.
The test will also verify that the EEPROM has been initialized properly. This is done by storing the software
version of the EEPROM in the first five bytes of the EEPROM, and then verifying the stored value is correct
when the test is executed. If the stored value read from the first five bytes of the EEPROM is incorrect, the
EEPROM will be initialized.
Before the test is executed, the following test code will be displayed on the Diagnostic LEDs, along with the
test number on the 7-segment displays:
Test # Test Name Diagnostic LED
7-segment
display
6 EEPROM EFFECTS “Gate“ 6
Pressing the Load button will execute the test.
Pressing the Tap/Cancel button will run the test continuously.
When the test is executed, the leftmost 7-segment display will display an “r” to indicate the test is running.
The pass/fail status of the test is displayed on the 7-segment displays and front panel LEVEL LEDs.
If the test passed, the (Green) LEVEL -30dB LEDs will light.
If the test failed, the (Red) LEVEL LEDs will light.
DRAM Test (7)
The DRAM test is a modified checksum test executed by the Lexichip. Each memory location is written
with a unique value, constructed to exercise all 24 of the DRAM data bits. During this operation, a modified
checksum that alternately adds or subtracts successive values is calculated and stored in the ADF. The
modification is required because the Lexichip does not "lose" most significant bits on overflow, but saturates
instead. This means that, without the modification, the checksum would always be 0xFFFFFF (digital full-
scale, the saturation value). A second pass through each location is run with the checksum calculated from
the memory locations. The test passes if the second checksum equals that stored in the ADF. The test
takes about 12 seconds and interrupts must be disable while it runs, so the display is simplified during that
interval.
During the test, there is limited control of the front panel LEDs due to the interrupts being turned off.
Because of this, the front panel LEDs cannot be updated to indicate the test number on the Diagnostic
LEDs, or the pass/fail status of the test.
Before the test is executed, the following test code will be displayed on the Diagnostic LEDs, along with the
test number on the 7-segment display:
Test # Test Name Diagnostic LED
7-segment
display
7 DRAM EFFECTS “Chamber” 7
Pressing the Load button will execute the test.