INMC 80 News


May–September 1981 · Issue 4

Page 26 of 71
58RSFUReserved for future use
59INT 0Interrupt
60INT 1request
61INT 2lines
62INT 3
63/PWREPowerfail warning
64AUXPWR Backup power
65NDEF 1Not to
66NDEF 2be defined
67GNDGround to seperate power and signal lines.


1) * is an open collector line.
2) IEI to be linked to IEO on cards not using the interrupt daisy chain.
3) /BAI to be linked to /BAO on cards not using the DMA daisy chain.
4) Bus drivers must be able to drive 75/15 U.L.
5) Bus receivers must not load the bus past I/O.25 U.L.
6) Bus master to pull up all open collector lines with 2k2.
7) Bus master to pull up the following lines with 10k, /HALT, /MREQ, /IORQ, /RD, /WR, /M1, /RFSH.
8) Bus timing reference point is pin 6 of the Z80. As the bus is in essence a buffered Z80, the timing of bus signals is as the Zilog/​Mostek Z80 data book. All Z80 signals are buffered onto the bus with 20nS +/  10nS buffers, the sole exception being the bus clock which should be 20nS (+/  10nS) ahead of the Z80 clock (pin6). The timing of other signals is detailed in the description of the particular signal. All expansion card timing must, however be referenced to the bus.
9) Cards using /BAI, /BAO, IEI & IEO should pull them up with 2k2.
10) Bus termination. Long buses may require termination. 220R on each line to a 2.6V low impedance source should solve 99% of problems.
11) Grounding. The ground line to the PSU should be as short as possible and as thick as possible.
12) The names of the various bus signals are as detailed above, please do not change them or abbreviate them, ie AUX CLK not AUX CLOCK or A CLOCK etc.


The following is a line by line description of the bus and should help resolve any ambiguities.

Lines 1-4, GND.

The quality of the system ground cannot be overemphasised. Ground noise problems were at the root of the now infamous Nascom “Memory plague”. The faster that systems go the more critical the noise problem will become. Noise problems will manifest themselves as a generally unreliable system with a predilection to do “odd” things.

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