RXBROWN@UALR.BITNET ("MR.FANTASTIC") (08/22/90)
Ray Rankins writes.... >Does anyone have a schematic or some sort of diagram or instructions for >building an Apple compatable SCSI Terminator? This is for an Apple IIGS >with Apple's SCSI card. I can't see paying $15 to $30 for something >that probably costs about $5 in parts. >Anyone have any they want to sell cheap? I thought Apple 2s did not need a terminator. I have an 80 Meg drive from Tulin and it does not have a terminator. Maybe its built in, but I don't know. I just plugged it in and it worked. :) >Ray Rankins |(518) 387-7340 | INTERNET: rankins@argentina.crd.ge.com >2 Moonglow Rd. |(518) 583-3320 | COMPUSERVE: 71131,3236 >Gansevoort, NY 12831 | | AmericaOnline: RayRankins ><insert standard disclaimer here> Robert Brown RXBROWN@UALR Apple // GS the power to be your best!
gwyn@smoke.BRL.MIL (Doug Gwyn) (08/23/90)
In article <47740433B95FE00E21@UALR.BITNET> RXBROWN@UALR.BITNET ("MR.FANTASTIC") writes: >I thought Apple 2s did not need a terminator. I have an 80 Meg drive from >Tulin and it does not have a terminator. Maybe its built in, but I don't know. >I just plugged it in and it worked. :) You were extremely lucky. All SCSI busses should have termination on both ends. Improperly terminated SCSI busses may or may not be usable, but in any event the bus signal lines would be noisier than intended, due to pulse echoes off the unterminated end of the bus. Operating with reduced margins like this is just asking for trouble. The original Apple II (non-DMA) SCSI card did not include on-board termination, but rather relied on a terminator being provided externally as near to the card as possible (i.e. at the end of the "SCSI System Cable"). A second terminator would then be needed at the other end of the bus, unless there was no further bus (i.e. only one device were attached). The situation is simpler with the new Apple II (DMA) SCSI card; since it includes termination, all bus configurations now call for just one terminator at the far (non-computer) end of the bus. Most bare SCSI devices are shipped with termination installed; this should be removed for all devices other than those at the ends of the bus. Apple recommends that ALL external SCSI devices not contain internal termination, so that it is not necessary to open up the device to toggle its termination. If no external device has its own termination, then external terminators such as the ones that Apple sells are necesary (just one external terminator, except when using the older SCSI card with multiple external devices).
lhaider@pro-beagle.cts.com (Laer Haider) (08/25/90)
>I thought Apple 2s did not need a terminator. I have an 80 Meg drive from >Tulin and it does not have a terminator. Maybe its built in, but I don't >know. I just plugged it in and it worked. :) Most third party SCSI HD manufacturers terminate the drive internally. This is done to make the drive a "plug -n- play" device for most users who are new to SCSI devices. It begins to be a pain in the a$$ if you wish to hook up more than one SCSI device in daisy chain fashion. If you ever need to add another SCSI device to an internally terminated SCSI HD (for those of you that can afford it), you need to open the HD's case, find 2 or 3 rectangular plastic looking pieces that have prongs sticking out of one side. You probably can't see the prongs, they're stuck into a socket. Anyway, CAREFULLY pull them straight out using a chip puller or pliers. Now your SCSI interface can pass through to the next device. Save them terminators in case you ever need to terminate the HD again. They're kinda fragile, so be careful. Laer Haider lhaider@pro-beagle.cts.com