Mixing Narrow and Wide SCSI Devices

Using 50 and 68 Pin Internal Ports Simultaneously


This information was derived from the Adaptec, Inc.in their ASK topic "Procedure for connecting both narrow and wide devices to a wide Adaptec SCSI card." "Can all three connectors on an AHA-2940UW be used at the same time?" I have edited it to address the IBM PS/2 Fast/Wide and Differential Fast/Wide SCSI controllers. People looking for configuration of Adaptec PCI SCSI adapters should go to Adaptec, not use this information.

What is the proper device sequence when connecting both 50 pin and 68 pin devices to either the internal or external 68 pin connector on the IBM F/W or DF/W card?

Connecting both 50 pin (i.e., 8-bit) and 68 pin (i.e., 16-bit) devices to an IBM wide host adapter is a straight-forward process once the following basics are understood:

  • Signal requirements of narrow devices
  • Signal requirements of wide devices
  • External converters and how they deal with termination
  • Internal converters and how they deal with termination

Eight bit devices are also called narrow devices and 16-bit devices are also called wide devices.

Note: Do not try to convert from 68 pin to 50 pin on the differential connections. This article only applies to Single Ended SCSI devices.

Signal Requirements of Narrow Devices

Narrow devices require 50-pin inputs. This "Low Byte" or the Low Data Byte.looks as follows:

Signal Requirements of Wide Devices

Wide devices require 68-pin inputs. This "High Byte" or the High Data Byte looks as follows:

68 to 50 Pin External Converter

In order to go from a 68-pin external connection to a 50-pin external connection, all that is required is termination of the High Byte.

An important consideration should be kept in mind when using an external converter.

* As drawn above, only 8-bit devices should be used to the right of the converter.

Warning! 68-pin to 50-pin external converters are available from other manufacturers, but only converters that actively terminate the High Byte should be used. 68-pin to 50-pin cables are also available through other manufacturers, but most of these cables do not terminate the High Byte, and should therefore not be used.

Ed. The availability of C68 to HPDB68 cables makes this a "nice to know".

68 to 50 Pin Internal Converter

An internal converter converts a 68-pin ribbon cable connector to a 50-pin connector. They block the High Byte, but does not terminate it, because one of the rules of SCSI states that termination may only occur at the end of the bus1. The High Byte is blocked and only the Low Byte and Control signals are fed through the converter.

Data Flow for 68 to 50 Pin Converter

The following considerations should be kept in mind when using an internal converter:

  • Internal converters can be used anywhere along the ribbon cable except on the last connector.
  • When an internal converter is used, the last device along the ribbon cable must be a wide device that is terminated (Example 3 below) or you must use a ribbon cable that has terminator built into its end (ACK-W2W-5IT) (Example 4 below).
  • Several internal converters can be used along a ribbon cable.
  • Other connectors along the ribbon cable that are not connected to an internal converter should be used only with wide devices or remain unused.

Warning! Internal converters that terminate the High Byte are available from other manufacturers and are useful if the last device must be a narrow device. This type of converter can not be used in the middle of the bus.

The following are examples of how to connect a SCSI bus with wide and narrow devices. These examples refer to both the Corvette and Corvette Turbo. They MIGHT possibly apply to the Cheetah and Passplay adapters. The SCSI adapter automatically terminates it's end of the bus, but you must manually set termination of the "High Data Byte" - IF - a Wide device is attached to the 50 pin edgecard - OR - for some odd reason, you have a wide device on a 50 pin cable attached to the Wide internal conductor.
Ed. though I have a vivid imagination, the only plausible way is using a 68 to 50 pin adapter from a 9585-K/N, then a narrow cable to a Wide device.

External Devices

Example 1 (Mixed External)

In this configuration, set "Wide SCSI messages - External" to Enabled. Set "Fast SCSI - External"
to Enabled BUT keep in mind:

  • One IBM SCSI device enclosure model 3511.
  • Up to three external SCSI device enclosures model 3510.
  • Any external configuration where the SCSI cable length does not exceed 3 meters.
  • Ed. Some older SCSI devices may not support "Fast" (10MHz rate). If there are lots of errors, or it takes longer than normal to use the device, you may fix the problem by setting "Fast SCSI External" to Disabled.This will force all devices on the external port to 5MHz, INCLUDING the wide device.

Example 2 (External Narrow Devices Only)

In this configuration, "Wide SCSI messages - External" can be set to Enabled or Disabled. You can choose to enable or disable Wide Messages, because the device and the adapter can properly negotiate and support narrow transfers. "Fast SCSI - External" should be enabled if the devices can support the Fast SCSI (10MHz rate).

Internal Devices

Example 3 (Mixed Internal, Wide Last)

In this configuration, set "Wide SCSI messages - Internal" to Enabled. In this example, the last wide device has termination enabled.

Example 4 (Mixed Internal, Narrow Last)

In this configuration, set "Wide SCSI messages - Internal" to Enabled. The 68 pin cable allows the wide device and the adapter to properly negotiate and support wide transfers. In this example, there is a wide (16-bit) terminator on the cable after the narrow device.

Internal and External Devices

Example 5 (Mixed External and Mixed Internal)

In this configuration, you have to consider the data width (Wide - Narrow) and the speed of the external bus (Fast Enabled/Disabled).

Connecting All Three Ports of Corvette/Turbo Corvette

Caution! Using both the 50 and 68 pin ports of the Corvette or Corvette Turbo is not recommended.

The interior 68 pin SCSI bus on the Corvette can be pictured as two parallel horizontal lines, the 'upper' eight data bits, and the 'lower' eight data bits. The lower bus is tapped and routed to the internal 50 pin SCSI edgecard connector as well.

Using 50 and 68 Pin Internal Ports Simultaneously

Think of the Low Byte (Green) and High Byte (Red) transmission characteristics. If both connectors are used, the Low Byte transmission path is not the same length as the High Byte. The narrow device may use the old passive termination, which is not as precise as the newer active termination. The narrow cable may have a different impedance as well.

What keeps this bus from hurtling off the edge is that the SCSI devices negotiate for control of the bus. I have no O scope to view the waveforms, but if there isn't some reflectance on the low byte, I'd be surprised. My suspicion is the extra cable length creates "noise" on the SCSI bus that impedes the negotiation and the data transmission.

Prove me wrong, I look forward to your data.

From other readings, it appears that having Wide messages enabled for a narrow device on a narrow cable results in no harm no foul, as the adapter and device negotiate transmission width.

If a wide device on an 8 bit cable is queried by a wide adapter, they will negotiate for a wide transfer. So in this case, the operator MUST force the transfer to 8 bit by using Wide Messages - Disabled.

The autotermination pays no mind to the wide enabled or disabled settings. It responds only to the presence or absence of a device... This maintains the point-to-point requirement of the SCSI bus.

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