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:
Eight bit devices are alsocalled 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:
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.
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
Example 1 (Mixed External)
Example 2 (External
Narrow Devices Only)
Example 4 (Mixed Internal,
Internal and External Devices
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.
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 suprised. 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...