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Peusdotruth table for RTP logic compatible with both OBD0 and OBD1 hardware:
(/PSEN) (OBD1) (DS1230AB)
A15 /WR /OE /CS | /IOWE /WE /OE /CE
----------------------+----------------------
0 a b c | a 1 b c
1 a X X | 1 a 1 0
Inputs on left:
A15 = address line 15 (256k - 512k)
/WR = Write line on MCU
/OE = Output enable (coming from pin 22 of 27c256 / IC Socket)
/CS = Chip select (coming from pin 20 of 27c256 / IC Socket)
Outputs on right:
/IOWE = output to rest of WE peripherals on [[OBD1]] / etc motherboard
/WE = Write enable on DS1230 NVSRAM
/OE = Output enable on DS1230 NVSRAM
/CE = Chip enable on DS1230 NVSRAM
Upon further reflection, it appears that both OBD0 and OBD1 ECUs will
already control /OE as the truth table states above, and as a result,
/CE can simply be tied low. So the truth table reduces to:
A15 /WE | /IOWE /WE /CE
----------+------------------
0 a | a 1 0
1 a | 1 a 0
This can be reduced to four NAND gates (one chip):
/IOWE = /WE | A15 = !(!/WE & !A15) = !/WE NAND !A15
/WE = /WE |!A15 = !(!/WE & A15) = !/WE NAND A15
!/WE = /WE NAND /WE
!A15 = A15 NAND A15
Which is exactly what Doc did originally: http://www.pgmfi.org/phorum/read.php?f=4&i=21&t=21
Doc's design also has a transistor and diode in place to allow insertion of the entire RTP board into a standard EPROM programmer.
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