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April 2002

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The SCSI Workshop - Part 2: Practise

By Eric Baerwaldt © April 2002, Translation: Philhard Ackermann, Christian Hennecke

First some remarks concerning last month's article about SCSI basics:

3. Code of practice

3.1. Configuring SCSI-Systems

In the following sections I'm going to report about the configuration of some PC systems using the experiences I've made by installing or upgrading existing PC infrastructures or migrating machines from EIDE to SCSI for my customers.

I will especially be dealing with the following PC systems: First and second generation IBM PC 350 as well as IBM PC 750, IBM PC 300 PL and IBM PC 300 XL, an IBM PS/2 77s xTx model and an IBM PS/2 77s xNx model. The first three machines mentioned are standard PCs having grown a little bit old already sporting Pentium CPUs and a PCI-/ISA bus, whereas the IBM PC 300 PL and XL machines with their PII and PIII CPUs are more modern systems, and the last two PCs mentioned which feature 80486 CPUs and MCA architecture are quite outdated, but since they have been marketed very heavily back then, they reached high sales figures, and because of their relative high cost and robustness they can still be found in professional environments (especially at banks) where they are being used for special purposes. Since the two PS/2 machines are the only ones in the list that already came with built-in SCSI systems, they in particular offer some interesting technical options.

3.1.1. IBM, Microchannel and SCSI subsystems

With the introduction of the first generation of microchannel systems in 1987 by IBM it showed that the formerly used mass storage subsystems, which were mainly based on the old ST412/506 standard, were simply too slow and too error-prone for the new premium PS/2 models. The performance of the newly developed Microchannel was beyond everything the ISA architecture could ever have offered, so the old MFM harddisks quickly turned out to be a bottleneck. In the smaller ISA models PS/2 25, 30, 35SX, 40SX and L40SX IBM experimented with some 8 bit XT-IDE interfaces, but this proprietary technology showed up to be a dead end and thus was not developed any further since the PS/2 30 286 model. The smaller first generation MCA models 50 and 60 still contained MFM controllers (in the PS/2 50 also IDE) and appropriate harddisks - the PS/2 60 041 model, for instance, came with a full sized 5.25'' 41 MB harddisk. But those harddisks, which were extremely slow from today's point of view, slowed down the whole system dramatically, so IBM consequently furnished their high grade PS/2 systems 70 and 80 with IDE, ESDI and also some of the first SCSI harddisks right from the start.

With the introduction of the PS/2 models 56/57 in 1992 and 76/77 in 1994 IBM threw away all the old stuff and shipped these systems with SCSI controllers and harddisks only, with these controllers partially being onboard solutions (PS/2 56/76 and older PS/2 57/77). In the last PS/2's 77s of the xTx series they also used controller cards, some of them from OEM manufacturers. The server class machines of the 9595, PC Server 320, 500, 520 and 720 series were exclusively based on SCSI subsystems.

Because of the variety of controllers with different standards that has evolved throughout those years and the special technical characteristics of the MCA bus there are some peculiarities to be taken into account if you consider upgrading a machine based on the Microchannel architecture with a new harddisk or a new controller.

Basically any newer MCA machine can be upgraded with an up-to-date harddisk. Caution is advised with newer SCSI harddisks featuring a capacity of more than 1 GB. Some older MCA systems, though hardly still being used nowadays, will refuse to work when equipped with harddisks of more than 1 GB size due to their outdated SCSI BIOS. This especially includes machines of the following series: 8573, Portable P75, and older systems of the 8556/56SX und 8557/57SX type.

The more up-to-date 2nd and 3rd generation PS/2 machines (PS/2 56/57 and 76/77 as well as most PS/2 85, 90 and 95) are capable of dealing with even larger harddisks. But even here there are some points to be taken into account: In their 2nd generation PS/2 systems IBM introduced so-called IML systems. These systems had a small part of their harddisks reserved for the IML code (Initial Microcode Load), which held some parts of the machine's BIOS and thus could not be erased too easily. Systems containing such an IML partition will only accept harddisks up to a size of 3.94 GB as the primary device. This restriction does not apply for secondary devices. The installation of a secondary SCSI controller where harddisks of arbitrary size (see below) can be attached to doesn't pose any problems either.

With the introduction of the 3rd generation PS/2 series in 1994 the IML partition was replaced by a so-called service partition in all machines except for the 56/76 models. This special partition can smoothly be duplicated to a floppy disk. In fact this service partition contains a copy of the reference disk that is needed by any PS/2 machine regardless of model or age and to be able to create a duplicate of the diagnosis disk which contains some additional utilities. The service partition can be erased from the primary harddisk. If left there, this harddisk can again only host 3.94 GB of storage.

There are still newer SCSI controllers available for IBM-PS/2 systems. This also applies for the successors of the PS/2 range of machines, a high grade server generation of models 500, 520 und 720, some of which are solely based on MCA, others being hybrid machines (PC Server 320 and 720 models always carry an MCA as well as a PCI bus system which can be used simultaneously), where SCSI controllers as well as SCSI RAID controllers are available for the MCA bus. In addition the PC Server 520 and 720 models offer room for up to 18 3.5" SCSI harddisks which can be swapped in mid-operation due to the hot-plug technology integrated in those machines. Still some not so recent MCA-based SCSI controllers pose some restrictions: when you intend to use original IBM SCSI controllers even the oldest models can be upgraded by exchanging two eprom chips which are available from IBM if need be. The AHA-1640 controller by Adaptec and the VT-640 and BT-646 controllers from Bustek also need a BIOS upgrade to be able to boot the system from a harddisk larger than 1 GB. The MCS-700 controller by Future Domain also needs a boot BIOS v.1.1 to be able to address larger harddisks correctly. But with all these obstacles gone IBM machines offer full SCSI functionality.

3.1.2. First Example: Configuration of an MCA system

First we are going to deal with the easiest of possible SCSI configurations, namely one of the later IBM PS/2 machines. The 9577-STG is - like any model of the xTx series - a hybrid machine, i.e. it carries both an onboard EIDE controller and a Fast SCSI-2 controller (or, in case of extended configurations, a Fast/Wide SCSI or a Fast/Wide RAID controller). We won't consider the EIDE controller here as it is a technological backstep in regard to the use of OS/2 Warp or eComStation. IBM has built an EIDE controller into any later machine of the xTx series by default, because all these machines are based upon a single board design, although two versions were distributed, namely that of the more sophisticated SCSI models on one side, and that of the less costly models of the i series without SCSI subsystem on the other.

Configuration of such a machine with a SCSI subsystem is as easy as conceivable: Just insert the controller into a free slot, define an ID for a harddisk using the jumper, connect it to the controller, and copy the respective ADF file that is used to register the controller with the system to the reference disk - ready. The SCSI controllers that are delivered by IBM have automatic termination turned on by default, so one doesn't have to care about activating it. The installation of OS/2 Warp also runs through without any problems - OS/2 Warp detects the controller correctly without difficulties.

Adding another harddisk to the same controller is as uncomplicated. Besides the existing Fast SCSI-2 harddisk IBM DPES-31080 I have added an Ultra-3 Wide harddisk model IBM DDYS-T09170 into the machine. For this highly modern harddisk only a suitable interface adapter has to be used between harddisk and cable as the DDYS harddisk comes with either a 68 pin connector or - in case of the RAID and server version - a 80 pin SCA connector. After assignment of a unique SCSI ID the harddisk is detected by the system without problems and - since the harddisk has automatic termination - also terminated correctly, if it is positioned at the end of the bus. Note that this configuration poses one potential source of error for inexperienced users: According the to 16 bit SCSI standard the harddisk allows to define IDs between 0 and 15. But the 8 bit SCSI standard of SCSI-1 and SCSI-2 systems only allows assigning IDs 0 to 7. Therefore also the modern harddisk must not be assigned an ID higher than 7! As Microchannel machines use SCSI ID 7 for the controller, only SCSI IDs 0 to 6 can be selected from. If you use a Fast/Wide controller for this machine though, these restrictions do not apply. Another obstacle in the process of configuration of the new Ultra-3 Wide harddisk is the mode of operation, which ought to be determined by jumper, too. The new IBM harddisks actually negotiate with the controller which operation mode they are to use, so normally no problems should occur. If you want to be on the safe side however, switch the harddisk's corresponding jumper to Force SE Mode (SE means single ended) before operation starts. An incorrect adjustment, which falsely uses the harddisk as LVD device, can lead to the electronic's destruction because of wrong signal levels! This explicit selection of the operating mode is only necessary, if a SCSI-2 controller is used. If the harddisk is used with an Ultra-3 Wide controller, no manual settings are required.

If an external device is to be employed in our configuration, there have to be no adjustments made to the internal devices as termination is done automatically by the internal devices. In this case I have connected an external IBM 3510 CD-ROM drive of SCSI-2 standard to the machine in addition to the two harddisks. It is mandatory to terminate the external drive for the internal components adapt termination at the next boot operation. Again a unique SCSI ID has to be assigned to the external device via the turning switch.

After the addition of and setting up the listed components the following is striking: The new DDYS-T09710 harddisk doesn't only speed-up the machine notably, but enormously. OS/2 Warp version 4.51 took only about 30 seconds to boot until the machine was fully operational. I have to add that the boot operation is prolonged by several FLT drivers that have been added to CONFIG.SYS by hand and eight applications in the System Startup folder. The old DPES-31080 harddisk needed about 80 seconds for the same boot operation. Generally, one can state that harddisk access is dramatically faster using the new Ultra-3 Wide harddisk than using the old Fast SCSI-2 harddisk.

Another considerable performance increase can be achieved by two software settings: IBM usually includes technical goodies with components, but leaves them deactivated by default for some unknown reasons. In our case that means that the SCSI controller's "as delivered" state is to run in asynchronous mode. Under OS/2 Warp, starting with version 3 it is possible to add a parameter /FS to the SCSI controller's statement BASEDEV=FD16-700.ADD in CONFIG.SYS, which switches the controller to the faster synchronous mode. By this a performance increase of approximately 10% can be achieved, since the controller now does data transfers in synchronous mode provided suitable components are available. Another increase in speed can be achieved by modifying the SCSI mode pages: The SCSI mode pages define the usual parameters for operation. Mode page 8 specifies the caching parameters. As today's SCSI harddisks are usually equipped with a larger cache, this I/O memory ought to be activated, i.e. turned on. Obviously word of this hasn't reached IBM's developers yet, who build their harddisks with wonderfully large caches indeed, but partially deactivate them. I am going to deal with the corresponding setup options follow below.

Our example gets really interesting, if a scanner is to be operated on the same machine. Today's SCSI scanners mostly are only capable of a restricted SCSI command set, i.e. they mostly don't use the disconnect/reconnect command by which each component can register and deregister itself with the bus. The result of this deficit is that the scanner remains registered at the bus throughout the whole time of an operation and thus blocks the other components. As a workaround for this weakness it is recommended to use an extra SCSI controller for such slow devices. In our example, as second controller we add an old CE Infosys cache controller to the system, besides the existing Fast SCSI-2 controller. This second controller must not have its own BIOS (or it has to be deactivated). In our case the BIOS-EPROM is removed manually. As the controller does not offer automatic termination, the terminator for the external connector has to be removed from the socket. After that the controller can be operated in parallel to the "main controller" without problems. For OS/2 Warp it only needs a corresponding BASEDEV-statement in CONFIG.SYS for it to be detected and used correctly. Additionally a suitable ADF-item for our MCA system needs to be copied to the reference disk to correctly register it with the system - finished. All SCSI IDs except for ID 7 are available for the scanner - as it is operated at this interface exclusively - that have to selected with the suitable switch. Furthermore, the scanner needs to be terminated, in this case a passive terminator is highly enough. After these preparations we have ensured that the scanner operates independent from the other devices in the machine and does not slow them down.

Generally you should ensure that only high-quality connectors and cables are used for external devices that are connected to a SCSI machine, so distortion and interference are avoided. This is true even more for the later SCSI standards whose high transfer rates can only be achieved with first-class cables. The quality of the round cables that are used for external devices can be determined from the codes printed on them. On high-quality cables you can find - besides other data - the characters "AWG" (meaning "American Wire Gauge"), followed by two digits. The higher the given number, the better the cable's isolation and hence the quality is. Usually cables of the norm AWG 28 are sufficient for SCSI-2 connections while the norm AWG 30 can be viewed as the standard for SCSI-3 and Ultra/Wide SCSI. As a comparison: High-quality printer cables usually comply to the norm AWG 26.

In part three of the workshop we are going to continue with practical configuration of some models with PCI and ISA bus.

Eric Baerwaldt has composed a CD accompanying his SCSI workshop, which contains technical documents (amongst other things of more than 150 harddisks), drivers etc. This CD is available directly from the author for EUR 13.00 for german customers / EUR 15,00 for foreign customers incl. postal charges and shipment. Interested party's please transfer this amount to his bank account 5711 81 841, Sparkasse Nürnberg, BLZ 760 502 10 and send an e-mail with their postal address to The CD will be shipped instantly after the amount has been received.

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