Ask Question. Asked 6 years, 11 months ago. Active 1 year, 1 month ago. Viewed 32k times. There is a twist of a few lines in this floppy cable. What is the purpose of it? Improve this question. You can tell apart quite easily by looking at them Yes, after a few searches I found it out, but until now I never realized that this happens only at floppy cables. Still remember working with these. Managed to pull the data off a really old 2.
That old machine's seen quite a few upgrades, including a W PSU, 3. PATA has a 40 pin connector, while floppy has a 34 pin connector. Add a comment. Active Oldest Votes. Improve this answer. Aragorn Aragorn 1, 1 1 gold badge 14 14 silver badges 12 12 bronze badges.
The drives themselves can have a switch or jumper that allows the drive to operate as either drive A or as drive B. At least usually Thanks Olivetti, for wasting half a day of my life. It's only a hack for PC floppy drives. Amigas and other such machines that use shugart drives don't use that twist. You're just making stuff up. Show 4 more comments. Summarizing everything tl;dr The drive before the twist will be drive B while the one on the end will be A. Or quoting sawdust , from this comment : The cable twist allows both floppy drives to be configured identically for drive selection when installed for manufacturing convenience , yet operationally, can be uniquely selected as either the first drive or the second drive based on cable position.
Pins and cabling The twisted pins are from pin 10 to pin Explanation Without the twist, we have to configure the drives and set them to be drive A for one and B for the other, because when the motherboard selects for example drive A, both drives would receive the select signal if they are both configured as a drive A. A drive has to ignore all input and produce no output when it's not selected While hard-wired drives are usually a drive B, there is the chance to be a drive A as said by Tonny here : I once had a whole afternoon of entertainment trying to figure out why a drive which came from a working system wouldn't work in another computer Also, please note what Michael Hampton wrote here : Certain non-PC-compatible systems like the Radio Shack Color Computer did actually use floppies without the cable twist, but required manually setting the jumpers, and could indeed use four drives at once.
Community Bot 1. A picture is worth a thousand words! I'm pretty certain that programs that did a lot of access to both floppies in near proximity could leave both drives running. I'm also unclear how a cable cut would prevent both motors from being enabled simultaneously; that would seem like either a hardware restriction on the controller card or a software restriction in the BIOS.
Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. While this is not the most proud example of clean hardware design, how much of this was originally planned in advance, or was it just improvised on the go? Having the desired pins aligned on the pin out nicely enough to make this happen suggests, that this was intentional at least to some degree.
To assemble a twin-floppy PC and have the drives respond correctly to the drive select wires in the cable, there are four choices:. There is no question that option 4 will give the best results cost, time and quality in the IBM assembly plant.
If the cable supplier can be persuaded to make the twist at a reasonable price and with good quality, it's an excellent solution which also saves time and effort in field service.
There seems to be some confusion about how this worked clouding the categorization of what to call it. Let's compare the interfaces between how Shugart designated signals at the drive and how other makers used them vs how IBM re-designated them at the controller in order to support the twist idea. And being standard Shugart drives or equivalent they expected their motor enable on pin 16 just as Shugart intended.
So you have drives that, regardless of their actual role, expect one signal on pin 12 and one on pin 16, and don't pay any attention to what is going on at pins 10 or Given that this scheme works with standard unmodified Shugart-interface drives, I'd categorize it as a clever "hack" or bit of manufacturing engineering. IBM changed only their controller card which already had to be customized to match their bus design and cable, and just bought the drives off the shelf.
You can see the manual for Shugart's longer-predating-the-IBM-PC original single-sided SA drive which originated this interface as an evolution of the earlier 8-inch drive's interface here. When Shugart extended the scheme to double sided drives such as the SA still before the IBM-PC , they put four more places on the far end of the connector adding a side select and a "spare" and corresponding grounds, but didn't change the part of the interface used for drive selection and motor control.
Regardless if IBM's drives had their logo molded into the front plastic, the technical reality is that they were and remain interchangeable with standard Shugart interface drives like the SA Peel the four wires that Shugart had previously added to create their pin double-sided interface off the cable, and even getting an old pin SA to work would have been merely a matter of software support.
While 'hack' is a rather fuzzy term, used for a variety of assignments - including great ideas one would not have thought of - it is exclusive tied to modifying something after market to enable a new usage, not planned by it's original manufacturer.
Keeping that in mind it's clear that the twisted cable is part of the PC design, as it is based on the floppy connector pinout which the PC engineers did device for theit machine. The Connector used is not a standard one used before, but on purpose made to enable this 'twist'. Standard Shugart drives could not be used with this connector.
And a standard bus connector does not allow this 'trick' as it holds the signals in different order. Definitely a hack. A systems with floppy disk drives had separate selection wires in the connection cable, where one of them was to choose between on of 2 or 4 drives connected in parallel. There was a jumper on the drive to activate one of these wires resulting in an address.
Setting up this jumper was somewhat error prone and also extra work. The trend was to make more and more components of the PC self configuring. With the twist all drives could be jumpered on select 1, and a different wire would connect to that.
It's intended. The outbound cable at the motherboard is for a slave drive B. But it has settings for the master drive A as well. So to make all the drives alike, you have a twist in the cable. They all come like that. If one switches the mouse into the unused chanel, you can plug both keyboard and mouse into a single socket, as is done on Laptops. The hardware arrangement supposes at the front of the box, that drive A: is above drive B:.
So a twist is needed between A: and B: if these are to be identical kinds of drive. There is no twist needed between B: and the motherboard. As time goes by, you can drop various connectors, if you don't plan to support that kind of drive. This is done simply by not providing headers on the cable. Because a direct cable remains between B: and the motherboard, dropping B: means that the connectors below the twist are dropped, and the one above the twist aka A: is set.
It is possible to swap the drives in software, but the balance of things is that one gets better assembly and running if one reduces the amount of user fiddling: ie use cables that set the identical floppies to A: and B:. A is above the twist, B is below. Likewise, swapped cables allow one to use things like Null modems which are a serial or parellel cable, with two 'computer' ends, and a twist in the middle. Sign up to join this community. The best answers are voted up and rise to the top.
Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? And why are blind people building computers? Seems strange. No idea on the floppy twist either, but I think someone said that's the connector that go's on the Primary Floppy drive if you have more than 1, so maybe that's the reason. Finchwizard , Aug 14, Very strange Ya i recall IDE cables before didnt have the hole in them I actually checked everywhere and they ALL had the hole in them Maybe i should email Intel and ask them Found this over at Intel's website!
The twist in the floppy cable is for proper attachment. The twisted end plugs into the drive and the other into the controller. Try it the other way around and tell me if your floppy still works. Ok im sure thats not it Saeid, stop using the code tags, use quote, code stretches the width of the page right out, quote wraps to it. That's twice you've done it and I've had to change.
Finchwizard , Aug 15,
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