nichomax@cattell.psych.upenn.edu (Nicholas Maxwell) (04/16/91)
Four years ago I bought a Zenith 180. This is a laptop with a port for an external hard-drive. At the time Zenith was selling a hard drive to be plugged into this port, but they no longer do. In their literature, they say that "any regular PC hard drive" will work when plugged into the port. They also imply that the hard drive needs it's own controller. I gather that there are a variety of interfaces for hard drives (SCSI, ESDI, MFM, RLL, etc.). Does anyone have any idea what interface Zenith would have been referring to? Thanks in advance. Nicholas Maxwell nichomax@cattell.psych.upenn.edu
u855203@probitas.cs.utas.edu.au (Michael Harlow) (04/16/91)
In <41226@netnews.upenn.edu> nichomax@cattell.psych.upenn.edu (Nicholas Maxwell) writes: >Four years ago I bought a Zenith 180. This is a laptop with a port for an >external hard-drive. At the time Zenith was selling a hard drive to be >plugged into this port, but they no longer do. In their literature, they say >that "any regular PC hard drive" will work when plugged into the port. They >also imply that the hard drive needs it's own controller. I gather that there >are a variety of interfaces for hard drives (SCSI, ESDI, MFM, RLL, etc.). Does THIS IS NOT A FLAME I thought I might point out the following observations about many persons discussion of HD interfaces. ST506, ESDI SCSI IDE are interfaces MFM RLL NRZ are recording techniques. MFM (Modified Frequency Modulation) is a 5Mbit/s system, giving 17 x 512byte sectors per track on a standard speed drive (3600 rpm ?) RLL (2,7 Run Length Limited) is a 7.5Mbit/s system, giving 26 x 512 byte sectors ... NRZ (Non-Return to Zero) is a 10Mbit/s system, giving 34 x 512 byte sectors etc.. ST506/412 is a standard interface developed by Shutgard (sp?) or Seagate ? It usually uses MFM recording, and is attached to the drive on up to 3 feet ? of ribbon cable, but uses a serial bit stream to the drive. This was the usual type of interface/controller found in PC's in days gone by. Later, as the Disk Media improved, the higher density of RLL was tried instead of MFM. Thus was born "RLL controllers". These appear to DOS as an ST506 interface, except the drive has 26 sectors instead of 17. Many people try this with older drivers hoping that the media is good enough. ESDI (Enhanced Small Device interface) uses the NRZ recording technique and balanced serial data lines for noise immunity on the way to the drive. The drive thus has a different 'input' electronics. SO an MFM or RLL drive cannot be used plugged onto an ESDI controller. (or vice versa) These drives are about twice as fast as MFM (10 vs 5 Mbit/s). A lot of BIOS's cannot handle strange size drives, so a lot of the larger ESDI drives do dynamic reconfiguration with the help of the controller, to appearing to DOS as having 64 sectors and 750 tracks instead of 1500 tracks. (less than 1024). The ESDI controller again appears as an ST506 to the PC, with its register organisation and control codes. It can be hard to find a CMOS setting for some drives. We had a 300Mb ESDI system, which we set up as type 1 !! (ie a 10Mb drive with 4x308x17 configuration). And it worked fine. IDE (Integrated Drive Electronics ?) is simply a case where the controller is mounted under the drive instead of plugged into the Motherboard. A very simple card is plugged into the motherboard slots where a normal controller would go, and connects to the drive with a 40 pin (44 as well I believe) ribbon cable which contains a 16 bit wide data bus. The advantage of this is that the controller can be optimized exactly for that drive, rather than having to cope with a wide range of drive specification. The controller also can have a cache, do automatic bad block remapping, high speed seeks, look ahead reading, 1:1 interleave, tuned exactly for that drive. The recording technique that is used is up to the manufacture. Most use MFM or RLL. Mine uses NRZ, as I have 34 sectors per track. (33 actually, one is reserved for bad block mapping by the controller). The same problem with CMOS settings can occur with IDE interfaces. My BIOS has a configurable drive type to handle my 33 sectors. Some controllers get around BIOS problems somehow. Again the interface can appear as an ST506 type for compatiblity. It can be very hard to find an XT version of an IDE system, as the cable and the little adapter are designed to be 16 bit. There is also another interface called SMD (Small ? Devices ) They are usually used in the Mini type machines (I think, and am therefore quite likely to be wrong) SCSI interface. This is different again. SCSI is an intelligent bus system that uses an eight (8) bit wide data path to up to 7 devices. It is supposed to be drive and device independant as possible. Drives are supposed to appear as a sequence of block numbers, from 0 to x, rather than head/side/sector coordinates. This allows the drive manufactures to do anything they like, with number of sides and tracks and recording technique and data density. As long as the drive appears as a sequence of linearly numbered blocks of a set size, Every is fine. Internal SCSI connectors are 50 pin ribbon. External connectors are 25 pin D (its hard to mount 7 devices inside a machine) The data travels along the SCSI cable at speeds from 1 to 4Mb/s (bytes not bits !). But ultimatly it depends on the drive for the peak sustained speed, and the amount of cache the drive manufacture has added. I have little experience with SCSI controllers for the PC. I don't know if they appear as ST506 or not. I assume they must, or else the PC wouldn't be able to boot from the SCSI HD. Maybe the ROM on the controller does the trick.. Speaking of ROM's. Most controller will contain a ROM containing a HD control code and low level formatting software. On XT's this is detected and used. Low level formatting is done using debug. On AT class and above, I belive the supplied BIOS on the motherboard does the HD control. This is why most lowlevel formatting is done with the setup program or a disk that came with the machine, not the controller. Formatting. There is two types of formatting. Low level and high level. Low level is where the controller goes and marks on the virgin media where it will save each individual sector, and does a scan for disk defect and marks that area as unusable. The disk is now usable by any operating system. Many drives (SCSI and IDE in particular) come from the factory allready low level formatted, as the controllers are allready attached. Care must be taken when swapping controllers and drives, as differnt brands of controllers use different marks on the disk to define sectors and bad areas To make the disk usable, the Operating system must then put its specific data down onto the disk onto the sectors mark out by the LL format. These include partition table, Boot Blocks, root directory etc. This is done using FDISK and FORMAT on MSDOD Machines. The answer given above are to the best of my knowledge, and could be wrong or inaccurate in some areas (especially spelling and names). I am certainly open to critisism or more details, or discussion on the above statements. ------------ Sorry if that was a bit long. I'd just recently gained posting ability, and was sick of watching incorrect or slightly confusing answer flowing around. Any way, as for the Zenith. As you can see, it does not fit into any of the above catagories. I'm sure its not SCSI with its external 25 pin. There might have been an optional external HD back in the old days, before my time, that used a 37 pin D connector, but this could have been for external floppy drives. Most of the above interfaces need about 50 pins anyway. I believe that some Amstrad portables have provision for an external HD. Talk to an owner. I think the plug would have been a Zenith special anyway with a strange pinout. Maybe you can find an old one second hand ? If the Zenith people say its a standard PC HD interface, ask them which one ! If it needds a controller, there is a problem there, as most controllers need access to the PC's bus for I/O etc, which I cannot see being extended out the back of the machine. My guess is that it is Custom. Or needs a special Zenith controller, and then a standard MFM drive. Hope this helps a bit. (It is but my humble opinion, no warranty given) Mike. -- | Michael Harlow | u855203@probitas.cs.utas.edu.au | GPO Box 1201, | | Hobart, 7001 | Computer Science Department | Tasmania, Australia | University Of Tasmania
bruceh@servprod.inel.gov (Bruce Hiltbrand) (04/24/91)
This is just a follow up note on SCSI drives. I own an XT with a Seagate 296n (SCSI). It uses the ROM BIOS on the controller on the drive to boot. In other words it is independent of the system ROM. It appears to the system as a SCSI device on boot-up. I don't remember what the system sees afterward but I do know that the recording scheme is seen as RLL. I don't if this helps or hinders, but it's my $.02 worth. --Bruce