[Coco] PT68K-4

[Dave Philipsen]

This is a fairly lengthy explanation/question about the PT68K-4 so if 
you don't know what that is or you're not interested feel free to stop 
here and go on to the next message.

I have an old PT68K-4 motherboard that I bought somewhere around 25 
years ago. At one time I had it up and running OSK V2.4 with a hard 
drive, floppy, ET4000 VGA card, and a modem.  Well, along the way 
everything but the motherboard and the original manual has disappeared.  
It's been so long since I used this machine that I don't even remember 
what the boot screen looked like.  So I found an old XT power supply, a 
new HD floppy drive, a power LED, and a HALT LED and hooked them all 
up.  The 'BIOS' EPROMs on this board are labeled 'MONK' which is the 
monitor that was used for booting OSK. I believe 'HUMBUG' EPROMs were 
used for other operating systems. The board has the full 4MB of memory 
and I went ahead and 'wiggled' pretty much all of the chips of the 
motherboard in their sockets including the RAM chips.  Right now I do 
not have a VGA card but I'm waiting on receipt of an ISA bus ET4000 VGA 
card from a gracious doner in Australia.

When I turn on the power supply the POWER LED lights and the HALT LED 
lights momentarily then goes out (which I'm pretty sure is normal).  I 
also get a 'beep' out of the speaker which I believe is a very good 
sign.  So I put a 'scope on the *CS line on the first DUART which 
controls COM1 and COM2 and I see regular negative going strobe pulse 
there indicating that the monitor is looking for input from a serial 
port (presumably COM1).  Also, the schematic shows me that the 'beep' I 
hear when I power up the motherboard comes from a single-bit output on 
the DUART which to me means that the CPU is talking to the DUART.  I 
also 'scoped the pin on the DUART where the recv/xmt clock comes in from 
a can oscillator.  Rough calculations from my 'scope say the frequency 
is about 3.7 MHz which is corrrect (oscillator is 3.6864 MHz).

Here's the question:  Right after reset and the speaker beeps there is a 
quick little burst of data that comes out of the TxD on the DUART.  I 
can see the burst on my 'scope (it might be just one or two characters) 
on the TxD pin of the DUART and I can see it on the TxD pin of the COM1 
connector at RS232 levels.  When I hook this up to a COM port on my PC 
with PuTTY running in serial port mode I get nothing intelligible coming 
in to the serial port on the PC.  I've tried 200, 600, 1200, 2400, 9600, 
19200, and 38400 bps on PuTTY but I can't see anything other than some 
garbage characters.  Can anyone tell what I should be seeing and what 
the monitor program (MONK) expects to be seeing from me?

Dave Philipsen



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