RE: [N8VEM-S100:71] Re: S-100 Motherboard

["John Monahan" <mon...@vitasoft.org>]

Rich, just a quick one. The POC &/or Reset line pulses are generally
determined by the value  of a capacitor.  First use two small jumper cables
and in parallel add extra capacitance.  Once you arrive at a working value
replace the cap.  

John


-----Original Message-----
From: n8vem...@googlegroups.com [mailto:n8vem...@googlegroups.com] On
Behalf Of Rich Leary
Sent: Tuesday, February 22, 2011 9:21 AM
To: N8VEM-S100
Subject: [N8VEM-S100:71] Re: S-100 Motherboard

So far I have found no issues with the 6-slot motherboard other than the
header orientation issue previously discussed.

What I found with the CPU-Z is that sometimes it would power up OK with the
floppy controller and sometimes not. Once it started it was OK from then on.
I am convinced that the issue with the CPU-Z and the floppy controller is
that the floppy controller (Suntronics UFDC-1 using WD1795 controller) does
not have a connection to /PRESET (75).
It does connect to /POC (99) that the CPU-Z generates. I think that the /POC
signal is either too short or not of "good" quality. I am going to put the
scope on the circuitry and check my theory.

My frst attempt at a fix will be to do a cut and jumper patch on the
UFDC-1 to have /PRESET be the source of the board reset signal. I'll let all
know how that goes.

One side comment - the /POC does not bring the monitor prompt up by itself
and that should happen regardless of whether the floppy controller responds
to /POC. I have to use my RESET button that grounds /PRESET. /POC clearly
does not do everything it should do even if the floppy controller were to be
eliminated or replaced.

In the meantime I will work more with the CPU-Z. The first observation
regards the /POC generation circuit. The circuit on the CPU-Z that generates
the /POC has the usual resistor-capacitor combination but it connects to a
74LS04 inverter input, not to a 74LS14 with a Schmidt trigger input. The
output of that first inverter goes to a second
74LS04 inverter and then through a 74LS125 buffer to the /POC pin 99.
What is strange about the circuit (other than not using 74LS14s) is that the
output of the second 74LS04 is then passed through a third
74LS04 inverter back to the input of the second 74LS04 with a resistor
network connected to that line and the output of the first inverter - some
positive feedback to latch the /POC at the high level.

One change I will look at is changing the 74LS04 out for a 74LS14 and at the
same time lifing the lead from the output of the third inverter that
provides the feedback. I need to check what the other three inverters are
used for to make sure they are OK with a 74LS14.

I did bring up the 5.25 inch drives with both CP/M and DOS/65 but they are
being a bit flaky. I suspect that the 40 track DSDD drive may have a problem
but have not done enough troubleshooting to determine what the issue may be.
It might be the drive itself, the cable, termination (the 1.2 MB drive is at
the end of the cable and is the one terminated), or a chip on the floppy
controller.

Andrew - as far as your questions about the 6502 CPU board - yes, I have my
schematics - I just need to find them. :-)

Some more info...

a. Board is reatively lightly populated - if I were to redo it now I would
put the full 64 KB of RAM on the board as there is plenty of room and there
would then be no need for a RAM board in the system.
b. It generates or responds to most but not all of the standard S-100
signals. Obviously it works with the CompuproRAM 17 and Interfacer 4 and the
Xebec adapter for the S1410 hard disk controller. One key area is PRDY. Both
my CPU-Z and the 6502 synchronize floppy I/O usind PRDY.
My DOS-65 IEEE Standard 696 Guide.pdf file (see Peter Dassow's site)
discusses what I did and while it talks about a much earlier configuration
of boards, the logic has stayed the same. The UFDC-1 discussed above is
heavily modified to place INTRQ and DRQ on lines optimum for fast testing by
the CPU. One footnote is that the CPU-Z, RAM17, Interfacer IV, and Xebec
host adapter are unmodified.
c. The board has an on-board monitor & boot EPROM - a 2716. The EPROM
actually contains many key I/O routines both for console I/O, floppy I/ O,
and hard disk I/O. These routines are then used by the DOS/65 System
Interface Module (SIM) - the DOS/65 equivalent of the CP/M CBIOS.
d. The first page of the address space for the EPROM is actually where the
S-100 I/O devices are mapped. Said another way - the S-100 I/O space is
mapped into  6502 memory starting at $F800. The useable ROM space runs from
$F900 through $FFFF.
e. Current configuration has the CPU running at 2 MHz. I suspect higher
speed operation with one of the WDC CMOS versions of the 6502 is possible
but when I built the board all that was available was the
1 or 2 MHz NMOS version. Glue logic for higher speed operation may need to
change or be replaced with 74ALS hardware or some simple GALs.
That would be an area of fun experimentation and testing. I doubt if much
higher speed operation of the current wire wrap board is possible simply due
to the lack of a ground plane and the variability of the wire wrap runs.
Clearly a PCB would be better for higher speed operation.

I am sure there are other aspects of the board I am forgetting. Let me know
if there is something I did not address.

I will look for the schematics. When I find them I'll do a quick check and
make copies to send off to you.

Rich