ardai@bass.bu.edu (Michael Ardai) (01/17/91)
In article <2251.AA2251@caleb> jdp@caleb.UUCP (Jim Pritchett) writes:
-I am trying to make a cable to connect a Mac style DB-25 SCSI connector
-(on my A2090 controller) to the standard 50 pin rectangular style connector
-(NOT the Mac 50 pin Centronics style connector) on the bare SCSI drive.
-Most of the connections are obvious, but I have a couple of questions.
-
-1. Is it ok to wire all the extra ground wires together? (The 50 pin
-connector on the drive has lots more pins labelled ground.)
Yes. If the Mac connector has more than 1 ground pin, split the grounds
evenly.
-2. There is a pin on the DB-25 labelled 'TPWR'. What does this mean?
-What should I connect it to? (Or, should I leave that pin open?)
This pin is the 5 volt power supply for the termination sips.. It
connects to pin 26 on the drive. The controller card (and any other
SCSI device) should have an isolation diode from 5v to TERMPWR so
multiple devices can source this power.
/mike
--
\|/ Michael L. Ardai ardai@bu-pub.bu.edu ...!sun!teda!ardai (preferred)
--- -------------------------------------------------------------------------
/|\ Teradyne EDA East (617) 254-3420 jlohmeye@entec.Wichita.NCR.COM (John Lohmeyer) (01/21/91)
In article <2251.AA2251@caleb> jdp@caleb.UUCP (Jim Pritchett) writes: > >I am trying to make a cable to connect a Mac style DB-25 SCSI connector >(on my A2090 controller) to the standard 50 pin rectangular style connector >(NOT the Mac 50 pin Centronics style connector) on the bare SCSI drive. >Most of the connections are obvious, but I have a couple of questions. > >1. Is it ok to wire all the extra ground wires together? (The 50 pin >connector on the drive has lots more pins labelled ground.) Yes. Apple left a lot of grounds out on the Mac, which works okay for short cables and lower transfer rates, but there will likely be problems for high transfer rates over longer cables. >2. There is a pin on the DB-25 labelled 'TPWR'. What does this mean? >What should I connect it to? (Or, should I leave that pin open?) This is likely their shorthand for Terminator Power (or TERMPWR). It _should be_ a diode isolated, fused, 5 volt level for the express purpose of supplying power for the SCSI terminator resistors. If there is only one source of +5 volts on this line, you can get away without the isolation diode. If you use a diode on this line, it must be a low forward drop diode such as a schottky diode. The SCSI standard requires a terminator at each end of the SCSI bus. For single-ended systems (which are most common and you certain have on the DB-25 end), this termination consists of a 220 ohm pullup resistor and a 330 ohm pull down resistor on each signal line (18 total). On very short cables and slow transmission speeds, you can often get away with a single terminator, but I don't recommend this. How valuable is your data to you? There is a much improved terminator defined in the SCSI-2 standard. It uses a single 110 ohm resistor on each signal line attached to the output of a 2.85 volt voltage regulator. This terminator is much less sensitive to sagging TERMPWR voltages and it is a closer match to the characteristic impedance of most cables used for SCSI buses. -- John Lohmeyer John.Lohmeyer@Wichita.NCR.COM NCR Corp. uunet!ncrlnk!ncrwic!entec!jlohmeye 3718 N. Rock Rd. Voice: 316-636-8703 Wichita, KS 67226 SCSI BBS 316-636-8700 300/1200/2400 24 hours