Dual Mode Setting; 5250 Express Data Stream - IBM 5500 Operational, Installation, And Maintenance Manual

Dual Mode Setting

5250 Express Data Stream

46
Every Twinax Workstation Controller (WSC) handles ports one at a time.
Every device must wait until the WSC can handle its port. An older WSC has
8 ports to timeslice. However, the 5500 control unit's WSC is actually two
mini controllers, each responsible for servicing 4 ports.
Setting the 5500 control unit's Dual Mode operation to Enable allows each
device to obtain quicker service. To maximize performance, balance the load
between the two mini controllers by dividing your devices equally between
ports 0-3 and ports 4-7.
Use of Twinax Multiplexers
The use of twinax multiplexers can prevent performance gain. Multiplexers
combine the signals of multiple ports into a single cable for a longer
transmission run. Then, the signals are de-multiplexed into separate ports.
Older multiplexers, like the IBM 6299, were developed before the invention
of a Dual Mode WSC. For an older multiplexer to combine all eight ports of a
Dual Mode WSC, the two mini controllers must be reconfigured to function
as a single controller--an action resulting in a loss of performance. Setting the
5500 Control Unit's Dual Mode operation to Disable reproduces this action
and reduces performance.
Instead of losing this performance, it is recommended that you use a new
multiplexer--the IBM 7299. This multiplexer uses time domain multiplexing
to combine the signals from the two active mini controllers.
The independent modes available on the 5250 Express Data Stream are as
follows:
Optimized Mode A point-to-point communications with an Express device,
this mode is ignored by other devices and works in any valid cabling
configuration.
2 Mbps Mode This mode doubles the clock speed and affects all devices on a
port. The following situations could prevent the WSC from using this mode:
• Non-Express devices on the port
• Long cable runs (the maximum distance is reduced by 20% for 2Mbps)
• Non-Express Hubs/Multiplexers
IBM recommends that when using the 2 Mbps Mode, install your Express
devices on a different port than your legacy devices. Also, ensure that you
have Express-enabled Hubs and Multiplexers (IBM 7299).
Planning, Installation, and Maintenance Guide
Chapter 2: Planning Your Installation