Jump to content
Sign in to follow this  
acadiel

TI-99/2 74LS139N "dead bug" IC

Recommended Posts

74LS139N Pinout

Pin		Connected to				Function	Notes
1		Cut off from CRUCLKB (origin)		Ea		Hooked before the CF40058 chip to what was supposed to go to pin #20 (CRUCLKB)
2		139 Pin 14 (H)				A0a		Forced high by Vcc
3		139 Pin 15				A1a		S0 Keyboard Scan (CF40058 pin 18) via Pin15 on the '139
4		n/c
5		CF40058 #20	CRUCLKB			O1a		Hooked to CF40058 CRUCLKB (Pin #20)
6		n/c
7		4B 2564 EPROM #22 /CE			03a		Turns EPROM 4B on or off
8		n/c
9		Cut off from S0 keybd scan (origin)	03b		Hooked before the CF40058 chip to what was supposed to go to pin #18 (KEYBD S0)
10		n/c
11		n/c
12		n/c
13		n/c
14		139 Pin 16 (H)				A0b		Forced high by Vcc
15		CF40058 #18 KEYBD S0			Eb		Hooked to CF40058 S0 Keyboard Scan (pin #18)
16		Vcc (H)					Vcc		Vcc

Notes:  A0a, A0b are high because they are hooked to VCC.  A1a and Eb are hooked together for state.

Truth Table (A0 being low taken out):
-------------------------------------
Inputs				Outputs
Ea	A0	A1		O1	O3
H	X	X		H	H	
L	H	L		L	H
L	H	H		H	L

 

Above is the pinout of a dead bug (seen here and other places:  http://www.ti99.eu/?attachment_id=3164 and http://www.ti99.eu/?attachment_id=3165 and http://www.ti99.eu/?attachment_id=3166 and here https://atariage.com/forums/uploads/monthly_2020_03/IMG-1044.JPG.3788dcca167e2aec344af26576fd2c38.JPG) that is attached to different pins on the 99/2 - with, I believe, the express purpose of turning pin #22 off on EPROM #4B to do bank switching in the >4000 address space.

 

Those with a 99/2 - can you please verify this pinout?  

 

Those who can - can you figure out what's going on here logic-wise?  The Keyboard Scan S0 and CRUCLKB pins were cut - they originally went directly through to the KEYBD S0 and CRUCLKB pins on the I/O controller.  Now, they're being diverted through this 74LS139, which only had 9 pins connected:

- Three pins were what was originally going to pins #18 and #20 on the CF40058 I/O controller

- Two pins going from pins #18 and #20 on the CF40058 I/O controller TO the 74LS139

- One pin going to Vcc

- One pin going to the CE pin (22) of the 2564 4B EPROM, to turn it on/off

- Two pins forced high (A0a and A0b) on the two sides of the 74LS139 to probably impact the truth table

 

I'd love to see an explanation of this - would explain how the 99/2 did bank switching.

Share this post


Link to post
Share on other sites

I have... but I'm not sure if this mod is on the schematics, since it appears to be done to production boards (Farmer Potato and Klaus').

 

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
Sign in to follow this  

  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...