ASCI u68 (SystemX)

Brad unclefalter at
Fri Oct 16 23:59:24 CDT 2015

Thanks so much Brent!  Your insight on the 6821 proved to make this thing
run way better!  Swapping 29 and 30, the system 'boots up' consistently and
properly.  Before, you'd hit reset and then any key and it might come up or
it might not, or it might generate weird messages and then go dark.  Now it
comes up every time.

I connected the RS232 again and tried bauds between 110 and 9600 but no
dice.  I'm assuming bits, parity etc are all standard.. hrmmm.  Curious now.
Wish a manual still existed somewhere.

-----Original Message-----
From: cctalk [mailto:cctalk-bounces at] On Behalf Of Brent
Sent: Friday, October 16, 2015 9:46 PM
To: General Discussion: On-Topic and Off-Topic Posts <cctalk at>
Subject: Re: ASCI u68 (SystemX)

On 2015-Oct-16, at 7:30 PM, Brad wrote:

> Here's some detail shots of the unit (including two of the front and 
> back of the mainboard).  Note the 6821 in the middle that has 'BAD' 
> scratched into it.
> Front:
> scaec0dg8.jpg.html 
> smagz2dcc.jpg.html
> Back:
> sdgtagrl1.jpg.html
> Keypad:
> sioikuzdu.jpg.html

>From what can be readily discerned, with some presumption, in functional not
numeric order:

IC26 6875		clock generator

IC22 68B00		processor
	. . obviously

IC7 TBP18S22	PROM 256*8
	probably for address mapping of the RAM / ROM / IO.

IC19 2716		 EPROM 2K*8	 (or 2708 1K*8, note jumpers)
	firmware/monitor ROM

IC18 2716		 EPROM 2K*8	 (or 2708 1K*8, note jumpers)
	optional ROM

IC16 6514		SRAM 1024*4
IC17 6514		SRAM 1024*4
	1 KBytes of RAM

IC30 6821		IO 2*8+4
IC31 6821		IO 2*8+4
	IO to the keypad & LED display.
	It looks like the hex-to-7-seg decoding is done in software,
	over on the kbd/display PCB is a CA3082 7*NPN-tran, presumably
	used as LED drivers. One of the chips is masked by the photoflash.

IC29 6821		IO 2*8+4
	Goes to connector P1, just GPIO pins to play with.

IC15 6850		UART
IC13 1489		RS232 receiver
IC28 75150		line driver
IC25 4702		bit-rate generator
	Comprise a serial line interface.
	Not clear how the bit-rate is set, I'm guessing the rate selection
pins of the 4702
	are fed by IC14 74175 latch, fed in turn from IO pins on IC30, so
the baud rate would be
	be under programmatic control.

IC27 555		timer
	Not clear what that's for, might be an oscillator/timer for
kbd/display scanning,
	feeding into IC30 or 31.

The remaining chips are bus drivers and a little glue logic.


It's pretty straightforward for what it is.
Date codes of 1981, but the support for 2708 and 2716 EPROMs suggest the
design is a few years earlier.

As IC30, the bad 6821, is involved in the keyboard/display you might try
pulling it and replacing it with IC29.
If you want to be particular, pull it and check for bad voltages and current
limits on the IO pins before plugging in a sub.

The expectation if it's running properly is that you'd press a key for a
command followed by hex address or data, e.g. EXAMine, enter the address
XXXX, displayed as entered, memory contents displayed on entering 4th
address digit.

No virus found in this message.
Checked by AVG -
Version: 2015.0.6172 / Virus Database: 4447/10805 - Release Date: 10/12/15

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