Service manual

Chapter 5 E01S Fault finding
FileStore Service Manual 69
read from the EPROM when the /OE and /CE are both low.
4.4.4 INTON/INTOFF
Non-maskable interrupts (NMIs) are generated from two sources - the disc drive interface or
the Econet network interface.
Interrupts from the floppy disc controller function in the following manner:
Upon start-up IC 19 pin 9 is cleared by the reset, causing pin 11 IC23 to be high, hence the
NMI is high (inactive). If either the Data Request (DRQ) pin 38 or the Interrupt Request
(IRQ) pin 39 of IC 26 of the Floppy Disc Controller go high, an NMI will be produced. For
information on the cause of these and how to clear them refer to the manufacturer‘s data
sheet. When pins 38 and 39 are cleared low, the low on pin 12 IC19 will be clocked through
by φ2 and the NMI will go high, provided that pin 9 or pin 10 of IC23 is low.
When an interrupt is generated from the Econet network, pin 1 of the Econet module will go
low which will take pin 6 IC22 high. IC19 pin 6 determines whether this causes an active
NMI to the processor. Pin 3 IC19 is decoded by IC17 to be &FC28, which is used as the
Econet Interrupt Enable, hence when this goes low pin 9 IC23 will go high and provided that
TR0 (pin l Econet Module) is low the NMI on the processor will be low. Pin 5 IC17 decodes
to &FC24 the Econet Interrupt Disable which will set pin 9 IC23 low, hence causing changes
on pin 10 IC23 to have no effect on the processor NMI pin.