INMC 80 News


May–September 1981, Issue 4

Page 27 of 71

Line 5, CLOCK.

This line is important as all the bus timing is derived from it. It should spend at least 46% of its time below VOL (0-4V) and at least 46% of is time above VOH (2.4V), it has the other 8% spare to go up and down. The clock on the bus should be 20nS (+/  10nS) ahead of the clock on pin 6 of the Z80.

Line 6, /NMI SW.

Provision has been made on the bus for an NMI switch and this line is to be held high by the bus master. Grounding it will initiate a short pulse on line 21 and the Z80 /NMI input. Users are cautioned that switch bounce may cause more than one NMI.

Line 7, RSFU.

This line is reserved for allocation at a later date, please do not use.

Line 8, AUX CLK.

This line is a new allocation. Many boards (eg disk controllers) require a 1, 2 or 4 MHz signal. This was easily provided when the CPU clock was 2 or 4 MHz, however the advent of the 6 MHz Z80 changes the situation. Any bus master not running at either 2 or 4 MHz must provide a 4 MHz clock on this line. Designers of expansion cards should take note of this and provide a link to allow the board to use this line instead of line 5.

Line 9, /RAM DIS.

This signal is intended to prioritise memory. Normally this signal would be generated by memory on the bus master, an EPROM card or any other high priority memory when a memory read took place. A RAM card would normally gate /RAM DIS with the output buffer, so that in the event of /RAM DIS being asserted the output buffer would fail to be enabled, this would have the effect of “overlaying” RAM with EPROM/​ROM. /RAM DIS should not inhibit a write cycle; it should also remain high for any cycle apart from a memory read.

Line 10, /RESET SW.

A high to low on this line will initiate a reset cycle. It is intended that a switch be connected between this line and ground. The actual RESET line is line 14.

Line 11, /NAS MEM.

This signal is only used by Nascom 1 and is asserted when a Nascom memory address is detected. It would normally be provided by a memory board and would typically be 0000H to 0FFFH or F000H to FFFFH. This is an obsolete signal and no new boards that require it should be designed. This line used to be called MEMEXT.

Line 12, /NAS IO.

This line used to be called IOEXT and many people persist in still calling it that, even though the two are different. In its original form (note that it was active high) it would be taken high to indicate an I/O address external to the Nascom, in its current form it is taken low to indicate a Nascom I/O address, that is to say that it looks for a Nascom I/O address as opposed to looking for an external address. /NAS IO should be taken low within 50nS of a Nascom I/O address and /IORQ, (referenced to the bus). In its original form the onboard ports on the Nascom would remain enabled for a short fraction of an external I/O cycle (the time taken to detect an external address and assert IOEXT) and this was the cause of many obscure problems. If you have problems a good test is to write a short machine code routine to continuously write 80H to port 08H. If the breakpoint register display comes up you have a problem, if not you don’t. /NAS IO is a obsolete signal utilised by Nascom 1 and 2 and all new designs should incorporate full I/O decoding.

Page 27 of 71