+acadiel Posted April 7, 2020 Share Posted April 7, 2020 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. Quote Link to comment Share on other sites More sharing options...
+mizapf Posted April 7, 2020 Share Posted April 7, 2020 Did you check the schematics of the 99/2? http://ftp.whtech.com/datasheets and manuals/99-2 Computer/TI992_Tech_Diagrams.pdf The LS139 is a 2/4 demultiplexer (two-digit binary number to four outputs), with two of them in one chip package. If only one demux is needed, you'll find some pins unconnected. 1 Quote Link to comment Share on other sites More sharing options...
+acadiel Posted April 8, 2020 Author Share Posted April 8, 2020 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'). Quote Link to comment Share on other sites More sharing options...
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