IBM 6400-I15 Maintenance Manual - Page 152
ERROR: DRAM A, Read Operator Panel Keys
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♦ Test I/O Clock - The VX ASIC has two clocks for internal timing, a processor clock and an I/O clock. The processor clock cannot be checked because the processor will not run without it, but the I/O clock can be checked. The I/O clock is used for sending data to the operator panel and to get the time for DRAM refreshes. If the boot code detects a problem with the I/O clock, the error blink code 4-3-4 is sent to the diagnostic LED on the CMX board. ♦ Read Operator Panel Keys - The operator panel keys are read to determine if the user is requesting that the printer be placed in download mode, diagnostic mode, or debug SIMM disabled. If Cancel and Start are pressed during start-up, the printer is placed in download mode. If Eject/Restore and Stop are pressed during start-up, the printer is placed in diagnostic mode. If any other combination of keys are pressed, the debug SIMM is disabled. Key states are put in an internal register in the VX ASIC. ♦ Detect DRAM - The VX ASIC supports up to four banks of DRAM totalling up to 32 Megabytes. Boot code detects how much DRAM is located in each bank, and relocates the banks to create one contiguous block of DRAM. The size and location of each bank is then displayed on the debug terminal through the debug serial port. ♦ Test DRAM - DRAM is tested by writing a pseudo-random number sequence to all the DRAM, then reading the pattern back to verify it against what was originally written. If an error is detected, the message "ERROR: DRAM AT ADDRESS XXXXXXXX" is displayed on the LCD. When finished, the DRAM test sets all memory to zero. ♦ Inform the EC Processor - The first 8 Kilobytes of DRAM are reserved for the EC processor, which must be notified when DRAM is ready to be used. This is done by putting the message "DCS1" in the ECDC communications register in the VX ASIC. ♦ Detect Ethernet Adapter - An Ethernet adapter it must be detected early in the power up sequence so the printer can tell the adapter it has been detected and continue with its own power up sequence. An Ethernet adapter takes a long time to complete its internal diagnostic tests, so the boot code allows the adapter to power up in parallel with the printer to reduce its effect on overall boot time. ♦ Load Boot Messages - The boot code scans flash memory for OEM boot messages and loads them into DRAM. The manufacturer's boot code is the same for all OEMs, and contains a set of default messages. Boot messages for specific OEMs are stored in the DC code, not the boot code. 152 Troubleshooting