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

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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.

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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').


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