Wiring a 68 pin 'P' connector to 50 pin adapter (either Centronics50 or HD50) By Gary Field Dec. 16, 1998 I have noticed that some commercially made adapters cause a short circuit between TERMPWR and ground when connecting SCSI-1 devices to a SCSI-2 bus (which should be OK to do). I have shown below an interconnection table which will NOT have this problem. It seems that the manufacturers of many of these adapters (even ones with a good reputation) have designed their adapters visually rather than by signal description/function. I say this because I have taken a couple apart and I can see where they went wrong. If you look at the layout of a circuit board which makes the connections between a HD 68 pin and a HD 50 pin you see a nice symetrical fan-like pattern, and for the most part following this pattern gives you the correct wiring. HOWEVER; there are 3 signals that must NOT follow the obvious pattern or TERMPWR can end up shorted to GROUND. This is NOT a good thing. The pins in question are: 17,18 and 51 on the HD 68 connector. As you can see below these are TERMPWR. If you follow the obvious pattern: - HD68 pin 17 connects to HD50 pin 12 (which is RESERVED in SCSI-2) - HD68 pin 18 connects to HD50 pin 13 (which should be OPEN) - HD68 pin 51 connects to HD50 pin 37 (which is RESERVED in SCSI-2) To make things worse HD50 pins 12 and 37 were originally defined as GROUND in SCSI-1. Also, the Pioneer DVD-U02 DVD drive neglected to leave pin 25 (which turns into HD50 pin 13) open. Which also causes the shorted condition. Unfortunately, most of these adapters are molded in plastic so that you can't easily open it up and cut those connections. In order to fix them you need to break off the pins in question on the HD68 connector. Grouped by pin function Centronics or HD 68 pin 'P' connector pin # Signal Name 50 pin connector pin # 40 DB(0) 26 41 DB(1) 27 42 DB(2) 28 43 DB(3) 29 44 DB(4) 30 45 DB(5) 31 46 DB(6) 32 47 DB(7) 33 48 DB(P) 34 55 ATN 41 57 BSY 43 58 ACK 44 59 RST 45 60 MSG 46 61 SEL 47 62 C/D 48 63 REQ 49 64 I/O 50 17,18 TERMPWR 38 51,52 49 GND 35 50 GND 36 54 GND 40 56 GND 42 15 GND 10 16 DIFFSENS / GND 11 20 GND 15 22 GND 17 6 Return for DB(0) 1 7 Return for DB(1) 2 8 Return for DB(2) 3 9 Return for DB(3) 4 10 Return for DB(4) 5 11 Return for DB(5) 6 12 Return for DB(6) 7 13 Return for DB(7) 8 14 Return for DB(P) 9 21 Return for ATN 16 23 Return for BSY 18 24 Return for ACK 19 25 Return for RST 20 26 Return for MSG 21 27 Return for SEL 22 28 Return for C/D 23 29 Return for REQ 24 30 Return for I/O 25 Not connected: Not connected: 1,2,3,4,5,19 12,13,14 31,32,33,34, 53 37, 39 Terminated or Not connected: 35,36,37,38,39 65,66,67,68 --------------------------------------------------------------------------- Sorted by 50 pin connector pin number Centronics or HD 68 pin 'P' connector pin # Signal Name 50 pin connector pin # 6 Return for DB(0) 1 7 Return for DB(1) 2 8 Return for DB(2) 3 9 Return for DB(3) 4 10 Return for DB(4) 5 11 Return for DB(5) 6 12 Return for DB(6) 7 13 Return for DB(7) 8 14 Return for DB(P) 9 15 GND 10 16 DIFFSENS / GND 11 20 GND 15 21 Return for ATN 16 22 GND 17 23 Return for BSY 18 24 Return for ACK 19 25 Return for RST 20 26 Return for MSG 21 27 Return for SEL 22 28 Return for C/D 23 29 Return for REQ 24 30 Return for I/O 25 40 DB(0) 26 41 DB(1) 27 42 DB(2) 28 43 DB(3) 29 44 DB(4) 30 45 DB(5) 31 46 DB(6) 32 47 DB(7) 33 48 DB(P) 34 49 GND 35 50 GND 36 17,18 TERMPWR 38 51,52 54 GND 40 55 ATN 41 56 GND 42 57 BSY 43 58 ACK 44 59 RST 45 60 MSG 46 61 SEL 47 62 C/D 48 63 REQ 49 64 I/O 50 Not connected: Not connected: 1,2,3,4,5,19 12,13,14 31,32,33,34, 53 37, 39 Terminated or Not connected: 35,36,37,38,39 65,66,67,68 ---------------------------------------------------------------------------