• Please review our updated Terms and Rules here
  • Exhibitor application for VCF West 2022 is now open! If you are interested in exhibiting, please fill out the form here.
  • Here are the results of the VCF East 2022 Post Event Survey: Survey Results

Altair 8800b-T Turnkey board

Frank S

Experienced Member
Joined
Mar 31, 2013
Messages
271
Location
Germany
I own a 8800b-T for a while, but i don't have the original Turnkey board.
Today i get one from an US-seller. It is the 200372 Rev.I board.
For reproducing the board eventually, i remove all parts from the board and put it on the scanner
and scan it with a higher resolution.
Then i solder in new sockets, the original DIP-switches and some other stuff.
This takes me about 6 hours of work.
Have anyone a schematic or manual for this revision of the board?

Frank

88TurnkeyNoRam_s.jpg
DSC_1168.jpg
DSC_1169.jpg
 
Last edited:

Frank S

Experienced Member
Joined
Mar 31, 2013
Messages
271
Location
Germany
Bad news!
When i test the functionality of the TurnKey board, i found a little solder bridge between two thruholes.
But i get the reading of the sensor switch not working. Allways FF shows the monitor.
After investigated the address decoder with a scope, I found there is no chip select ever.
A closer look to the schematic then shows the truth.
MITS uses the upper 8 bit for decoding the I/O address.
This works fine with an 8080 or 8085 processor, but not with mine JADE BigZ board.
A little test program let me finish the board testing sucessfully.

Code:
0001   A000             	.org	0a000h
0002   A000             	
0003   A000 3E FF       start	ld	a,0ffh		;load the accu with FF
0004   A002 DB FF       	in	a,(0ffh)	;in FF, with FF from the accu in the upper 8 bits
0005   A004 4F          	ld	c,a		;some stuff to get the readed value to the monitor display routine
0006   A005 CD 09 E0    	call	0e009h
0007   A008 0E 14       	ld	c,20		;space between each value	
0008   A00A CD 09 E0    	call	0e009h	
0009   A00D C3 00 A0    	jp	(start)		;loop the the routine
0010   A010             	
0011   A010             	.end
0012   A010             		tasm: Number of errors = 0

This works while the Z80 put the content of the accu in the upper 8 bits (A8-A15),
so i use the accu to select the sense switches.

Frank
 

glitch

Veteran Member
Joined
Feb 1, 2010
Messages
4,964
Location
Central VA
Some Z80 boards for the S-100 bus support "address mirror" for I/O transfers, you may check the Big Z manual for it. My Cromemco ZPU supports such an option but you have to install a jumper for it.
 

Frank S

Experienced Member
Joined
Mar 31, 2013
Messages
271
Location
Germany
The NorthStar Z80 board also do it, but not mine JADE BigZ board.
As i get my Atair 8800b-T, there was the frontpanel connected with some wires directly to the backplane (only pReset and pRdy).
Beside the Jade BigZ i have 4 CompuPro 16K memory boards, Tarbell 2022 floppy controller and a CompuPro Interfacer-1.
To connect the further unused LED in the frontpanel, my intension was to do it over the original Turnkey board.
The next step is to realize a working CP/M 2.2 with this board.

Frank
 

Frank S

Experienced Member
Joined
Mar 31, 2013
Messages
271
Location
Germany
I solved my address decoding problem with the TurnKey board.:D
Without destroying my original, i looked for an way to get the lower 8 address bits decoded for the SenseSwitch.
The addresses A0-A7 are available from the EPROM sockets, so i create an adapter to get these.
With an 74LS133 (NAND with 13 inputs) i decode 0FFH from A0-A7 and INP for the SenseSwitch.
Then i route three wires to the original decoder K (74LS30) and feed it there in.
These modifications are fully reversible.

Frank
DSC_1172_.jpg
View attachment SenseSwitch_decoder.pdf
 

Frank S

Experienced Member
Joined
Mar 31, 2013
Messages
271
Location
Germany
The second version of the TurnKey board I/O decoder i realize with an 16V8 GAL, because i want decode two I/O devices.
The sense switch on the board requires the address 0FFH and the ACIA 6850 two addresses (in my case 10H,11H).
The GAL 16V8 gives my free pin assignment and both addresses in one package.
Frank
TK_Gal_decoder.jpg
 
Last edited:

Frank S

Experienced Member
Joined
Mar 31, 2013
Messages
271
Location
Germany
Thanks Glitch,
the next step is to program a 1702A EPROM for the TurnKey board.
At the moment i waiting for the arrival of the kit Martin Eberhard created for programming the 1702A EPROM.

Frank
 
Top