SC502, v1.0, User Guide

SC502 is a power supply, reset, and status card designed for Z50Bus.

This card provides power, rest, and status indicators for a Z50Bus system.

A 5 volt supply at up to 1 ampere is generated from an input of 8 to 15 volts D.C. at up to 1 ampere. The input is connected via SK1 or ST1. Power out (5 volts) is supplied to the bus and is also available for external devices connected via ST2 or P2.

A nice clean reset signal is generated at power-up and when the user presses one of the reset buttons. An external reset button can be connected via P3.

LED indicators show the state of the power, reset, interrupt, halt, and wait signals. LED indicators can be mounted off the PCB via P4.


JP1, usually a wire link, enables the on/off toggle switch polarity to be set. Different parts of the world have different standards for on and off. Some use UP for ON, while others use DOWN for ON.

JP2 enables the voltage supervisor and reset device. Having this enabled with a jumper shunt can be useful when diagnosing problems. The jumper shunt can also be removed if you have an alternative active reset device connected.

JP3 connects the reset from this card to the bus reset signal. A jumper shunt should normally be fitted here but can be removed if your system has an alternative reset circuit.

Indicator LEDs

LED1 indicates the presence of 5 volts. It should light when the on/off toggle switch is in the on position.

LED2 indicates the RESET signal is active (low). This LED should light for a short while (about 0.5 seconds) following switch on as the voltage supervisor and reset device (U2) generates a nice clean reset signal. If the reset LED stays on it usually indicates the 5 volt supply is less than about 4.75 volts.

LED3 indicates the INTERRUPT signal is active (low). Interrupts are normally handled extremely quickly so you are unlikely to see this LED light in normal use. If the LED’s light is visible at all it probably means interrupts are being disabled for a significant amount of time. If the LED stays on, it is likely interrupts have been left turned off and pending interrupts are not being processed.

LED4 indicates the HALT signal is active (low). In normal use, you will be unlikely to see this LED light, for similar reasons to the INTERRUPT LED. If the LED stays on it is likely the processor is waiting for an interrupt that never occurs.

LED5 indicates the WAIT signal is active (low). Again, you are unlikely to see this LED light in normal use. However, if your hardware makes extensive use of the WAIT feature then you might see it light.

Hardware faults, or even software faults, could cause any of the signal indicators to light when they are not supposed to.


The regulator is a simple linear regulator and can get quite hot. It will get hottest when the input voltage is near the high end of the permitted range (15 volts) and/or the current drawn is near the high end of the permitted range (1 ampere). The PCB acts as a heat sink but to run at the extreme end of the range an additional heatsink is desirable.

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