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 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:
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
An important consideration should be kept in mind when using an external
* 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
* 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
[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.]
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).
Example 3 (Mixed Internal,
In this configuration, set "Wide SCSI messages
- Internal" to Enabled. In this example,
the last wide device has termination enabled.
Example 4 (Mixed Internal,
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
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.
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 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...
This maintains the point-to-point requirement of the SCSI bus.