514 Digital Troubleshooting

At power-up, if the display does not light or lights up and fails to report errors or begin operation, use the following troubleshooting procedures.

First check the state of SWR1 (A1U6-21). If this status line is less than 0.8 V, basic processor operation is intact. Examining SWR2 through SWR5 (A1U6-22 through -25, respectively) should indicate how far the software progressed before finding an error. If the state of SWR1 is not less than 0.8 V, the problem may be in the 6303Y Main Processor (A1U6), the ROM or RAM decode circuitry (A1U9), the ROM (A1U8) or RAM (A1U10), or the address/data lines among these parts.

Note

The functions of SWR1 through SWR5 as power-up status lines only persist for 2 to 3 seconds. These functions end when the keyboard scanner begins operation (if it can). Extremely difficult cases may require the use of an oscilloscope triggered on the falling edge ofSWRl to examine the states of SWR2 through SWR5.

To determine the relative health of the 6303Y Main Processor (A1U6), first check for a valid E clock at pin 68. The default for the E clock after reset is a rectangular wave with a period of 1.628 us and a duty cycle of about 67%.

If the processor is able to fetch instructions from the ROM, the software initializes the processor, and the E clock becomes a square wave with a period of 1.085 us. Since this initialization occurs almost immediately with a working meter, the resulting square wave on the E clock line is a good indication that the software has begun to execute.

If the E clock remains a 1.628 us rectangular wave, the SWR2 (A1U6-22) keyboard scan line may be shorted to ground. This condition would cause the Main Processor to HALT after reset. Check whether the 6303Y Main Processor is attempting to access ROM; LIR* (pin 64) should transition for a short period of time after reset. If it does, the 6303Y Main Processor is probably operational, and the problem is external to the processor.

The processor can execute an instruction that stops both itself and the E clock. Therefore, the absence of any activity on pin 68 does not necessarily mean that A1U6 or A1Y2 is bad. If some other failure prevents proper ROM access, the processor may have just "gone to sleep". This can be verified by checking for a rectangular wave occurring at pin 68 for a short time after RESET* transitions high on pin 7. A1U6 and A1Y2 are probably operational if this rectangular wave is present.

t cyc t cyc

Figure 5-3. Main Processor Timing

To check the ROM decode circuitry, verify that A1U9-6 is transitioning low and that these transitions correspond roughly to the low-going transitions of LIR*. Pin 6 must be low when LIR* is low; see Figure 5-3. Verify that this signal also appears at the ROM Chip Enable, A1U8-20. If the ROM Chip Enable is present, the problem is with the ROM itself or there is a fault in the address/data lines among the 6303Y Main Processor, ROM, RAM, and IEEE option connector.

If SWR1 (A1U6-21) and SWR2 (A1U6-22) transition low, but SWR3 (A1U6-23) remains high, the problem is with the RAM decode circuitry (A1U9), the external RAM (A1U10), or the address/data/control lines between the RAM and the 6303Y Main Processor.

To check the RAM decode circuitry, verify that A1U9-8 is transitioning low and that these transitions correspond approximately to the low-going transitions of WR* (A1U6-66). It may be necessary to continually reset (power on) the meter to check these lines, since the activity probably halts quickly when the meter software goes awry. Verify that the signal on A1U9-8 also appears at the RAM Chip Enable, A1U10-20. If the RAM

Chip Enable is present, the problem is with either the RAM itself or the address, data, RD*, or WR* lines between the 6303Y Main Processor and the external RAM.

Figure 5-3 shows the timing relationships of the 6303Y Main Processor lines LIR* and WR* to the system clock (E) and the address lines A0..A15. The ROM and RAM Chip Enables correspond to the active (low) region shown for the address lines.

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