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Servo To Go, Inc.
8117 Groton Lane
Indianapolis, IN

Ph: 317 257-1655


Timer (U14, 82C54 or 71054C) Lock Up on Model II

Due to noise on the clock signal going to the input of timer 1 and possibly timer 2 (jumper configuration), the timer can lock up. This can be seen as interrupts not being generated after a short time or a period of  even 2-3 hours. The increased noise may be due to one or more of several factors, but not limited to them. Some possible reasons are: changes in part manufacturers, changes in production of chips, changes to the programming contained inside the PLD, or changes to the socket containing the PLD. The fix is simple to implement. Solder a 220 ohm 1/4 watt resistor between pins 12 and 15 of the timer chip (U14). This increases the current draw enough that the PLD cannot provide the current required at full voltage (~5V), so the output high voltage is reduced to around 3.50-3.75V. This inturn reduces the signal bounce to a functional level and the lock ups do not occur.
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