myhre@plains.NoDak.edu (Robert Myhre) (12/08/90)
I have a question about drives. I recently bought a used 5 1/4" 1.2 meg drive and I heard yesterday that there is some type of resistor that needs to be removed when a drive is used as a second drive. Since I bought the drive used there were no docs with it... so could someone fill me in on this resistor? Like what is its purpose, what does it look like, where is it located, etc. Thanks for any help. Robert Myhre Computer Engineer in Training... and Training... and Training North Dakota State University ************************************************************************** myhre@plains.nodak.edu (Internet) * "Dad, you have a ham sandwich." myhre@plains (Bitnet) * "Yeah, eat Miss Piggy instead!" uunet!plains!myhre (UUCP) * Lethal Weapon II **************************************************************************
KDM101@psuvm.psu.edu (Kevin Maher) (12/09/90)
The resistor in question is called a terminating resistor. Now, as far as
I know, the resistor was only used on the original PC/XT systems. The
cable that connected the floppies was a simple straight ribbon. The resistor
was used to "tell" the controller that the drive with the resistor was the
A drive. (at least on a true IBM PC . the B drive is the first drive on the
cable and the A is the last) On the AT computer however, the floppy cable
was designed with a few wires twisted. This twist is what determined which
drive was A or B.
If you're using this drive in a computer with a simple straight ribbon cable,
you will need to find a group of jumpers near the connector. Put a jumper
over the pins for DS1 for drive A or DS2 for drive B. On an AT, I guess all
you need to do is put it on DS1 and the cable does the rest.
If somebody out there knows that this is wrong, please tell me.
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Why should I want to disclaim anything??? It only makes me look guilty!amichiel@rodan.acs.syr.edu (Allen J Michielsen) (12/12/90)
In article <90343.010956KDM101@psuvm.psu.edu> KDM101@psuvm.psu.edu >The resistor in question is called a terminating resistor. Now, as far as >I know, the resistor was only used on the original PC/XT systems. The >cable that connected the floppies was a simple straight ribbon. The resistor >was used to "tell" the controller that the drive with the resistor was the >A drive. (at least on a true IBM PC . the B drive is the first drive on the >cable and the A is the last) On the AT computer however, the floppy cable >was designed with a few wires twisted. This twist is what determined which >drive was A or B. I only quoted the entire section because virtually none of it was correct. Having missed the original posted article (I assume). Every pre-ps/2 floppy drive in a desktop class machine that came from IBM did have a terminating resistor pack. The whole function of this resistor is to properly terminate (end/kill) the signal at the end of the floppy controller cable. Whatever drive was at the end of the cable should have had the termination resistor. It is needed because of the physical limitations on the cables and input output gates of the electronics. Due to capacitive coupling, signals echo off the end of the cable and reflect back down the cable to the controller. This echo increases the noise on the line, causing data errors, decreased throughput, and lower reliability. The resistor pack is used for several reasons, first there are several methods of properly terminating the signals, and the correct size depends on the electronics involved, hence the drive manufacturers used a easily modifiable part. Selecting the drive number on the standard floppy system depends on several things. First, there is a built in cabling technique which allows 4 identically configured floppies (possibly except for termination technique, but independantly...) to be selected with ONLY a difference on the drive selection lines. The second technique would be to use a hardware address on the actual floppy drive to tell it, what drive number it actually it. Logically, you should configure a drive for drive 0 and then use the drive select lines to select the drives, else, use the drive X jumper and use the drive select 0 lines. However, IBM in their quest for new and better standards mixed both of these up. ALL IBM computers made by IBM use a twisted cable, and a drive selection of 2. Many clones (especially name clones, which generally are less true copies due to fear of law suits...) used a straight cable and then the drive selection jumpers of either 2 & 3 for a & b, or 2 & 1 for a & b, else 0 & 0 for a & b (implementing the intendard proposed floppy standard....), or even possibly 1 & 2 for a & b. The only way I can tell it to try it. There is a generally serious problem with improper termination... Virtually all new mixed &/or high density floppy controllers, and designed for soft termination on the floppy system. The older drives use hard termination. Most 3.5 drives now use soft termination, which is integral, and cannot be easily removed. Most dealers are not well educated (or informed or even care) enough to know that mixing hard termination with soft termination is a real problem. What happens, is that the bus drivers on the floppy controller are overheated and begin to fail when they get older. The result of this failure is that high density drives flip over into low density mode while writing high density disks, ruining the disk. They also may just begin writing garbage into a file, which creates a often unrecoverable data loss. Several things can be done, each indecreasing order... 1. dig into the soft terminated drives & rip out the terminating resistor pack. 2. move the drives so that the soft terminated drive is at the end of the physical cable -- making a new 'custom' cable if desired for keeping drive designations the same if desired... 3. remove the hard termination pack from the properly terminated drive, and either buy the proper size soft termination resistor pack, else forget the termination pack entirely. al -- Al. Michielsen, Mechanical & Aerospace Engineering, Syracuse University InterNet: amichiel@rodan.acs.syr.edu amichiel@sunrise.acs.syr.edu Bitnet: AMICHIEL@SUNRISE
usenet@linus.mitre.org (Linus's Usenet) (12/12/90)
>The resistor in question is called a terminating resistor. Now, as far as >I know, the resistor was only used on the original PC/XT systems. The >cable that connected the floppies was a simple straight ribbon. The resistor >was used to "tell" the controller that the drive with the resistor was the >A drive. (at least on a true IBM PC . the B drive is the first drive on the >cable and the A is the last) On the AT computer however, the floppy cable >was designed with a few wires twisted. This twist is what determined which >drive was A or B. >If you're using this drive in a computer with a simple straight ribbon cable, >you will need to find a group of jumpers near the connector. Put a jumper >over the pins for DS1 for drive A or DS2 for drive B. On an AT, I guess all >you need to do is put it on DS1 and the cable does the rest. From: carlson@gateway (Bruce Carlson) Path: gateway!carlson > If somebody out there knows that this is wrong, please tell me. I think you are a little off track with your directions. The DS1/DS2 (or it may be DS0/DS1 in some cases) pin selection is related to the twist in the cable. With a properly twisted cable you put the A drive at the end of the cable and the B drive in the middle and you don't have to worry about the DS1/DS2 pin (it is left on DS1). However, this is not related to the terminating resistor. I have seen explanations of why the terminating resistor is needed and I can provide some info from memory (with about 75% confidence). The resistor always goes on the drive on the end of the cable (A: drive) and its function is to absorb or damp signals on the cable and avoid reflections that will produce errors. I have used PCs where neither drive had a resistor and it still seemed to work, but I don't know if I was living dangerously. I had a TEAC 5 1/4 inch drive in my AST (AT compatible) computer and it had a terminating resistor. However, the drive broke and the replacement TEAC drive was constructed differently and did not have the same type of DIP resistor. I don't know if the new drive uses jumpers to select whether it has a resistor connected, or if there have been some other changes to the drives. I got no documentation with the new drive and haven't tried to call TEAC. The other configuration item to look for between PCs and ATs is that one of the cable pins (34 or 35) must be connected with a PC, but must not be connected with the AT. Most drives have a jumper setting to connect or not connect the pin. This pin is used to detect a disk change and if its configured wrong the drive may not work or you may scramble the FAT because the machine doesn't know you have changed disks in the drive. PC disk drives cannot sense when a disk has been changed so programs must recheck the FAT; AT HD drives can respond to a status request and tell that the disk has been changed. Bruce Carlson
koch@motcid.UUCP (Clifton Koch) (12/12/90)
From article <90343.010956KDM101@psuvm.psu.edu>, by KDM101@psuvm.psu.edu (Kevin Maher): > The resistor in question is called a terminating resistor. Now, as far as > I know, the resistor was only used on the original PC/XT systems. The > cable that connected the floppies was a simple straight ribbon. The resistor > was used to "tell" the controller that the drive with the resistor was the > A drive. (at least on a true IBM PC . the B drive is the first drive on the > cable and the A is the last) On the AT computer however, the floppy cable > was designed with a few wires twisted. This twist is what determined which > drive was A or B. > > If you're using this drive in a computer with a simple straight ribbon cable, > you will need to find a group of jumpers near the connector. Put a jumper > over the pins for DS1 for drive A or DS2 for drive B. On an AT, I guess all > you need to do is put it on DS1 and the cable does the rest. The terminating resistors are used to pull control and data lines to a logic 1 level for the floppy cables. The electronics use an open-collector device to talk to the drives, meaning that the driver can only actively pull a line down to a logical 0, and not up to a 1. This is done so that several drives can be daisy chained without having to use tri-statable devices. The correct termination scheme is to have the terminating resistors in the last drive conntected to the cable only. This insures that the drivers will not be overloaded. In practice, I've never had any problems with extra termination resistors in place. On cables: If you have a straight cable, then the drives should be jumpered as DS1 and DS2 for the A: and B: drives respectively. With a twist cable, the drive before the twist is usually B: and the drive after the twist is A:. The norm is to jumper both drives as DS2, but this is controller dependant. I've seen some controllers which require both drives to be jumpered as DS1. Note: By DS1 and DS2, I mean the first two drive selects. Some drives have them marked as DS0 and DS1. -- ----------------------------------------------------------------------------- ... [uunet | mcdchg | gatech]!motcid!koch