The Barr Channel Attach Box (CAB) electronically isolates the computer from the mainframe channel and from any other devices on the channel. This isolation protects the mainframe and other channel devices when you disconnect the computer. The CAB also includes the following features.
The CAB monitors its connection to the computer and operates differently depending on whether or not the Barr software is loaded.
When the Barr software is loaded, the adapter sends a signal to the CAB. The CAB connects the computer to the channel while constantly monitoring the signal from the adapter. If the CAB loses the signal from the adapter, the CAB disables the computer’s channel connection.
When the Barr software is not loaded, the computer-to-channel connection is disabled. However, Channel In signals continue to connect directly through to Channel Out. Even if the CAB has no power, it continues to pass channel signals.
LED indicators tell you the status of the CAB-to-mainframe connection. The CAB has four lights:
POWER – The CAB has power. The CAB must have power before you can enable the computer.
CHANNEL – The channel has been varied on at the mainframe.
ENABLED – The computer connection to the channel is enabled. This indicator lights when the CAB switch is set to ENABLE and the device driver has initialized the adapter.
SELECTED – The computer has been selected by the channel and is actively communicating with the mainframe. This indicator often flashes on and off during communication with the channel.
More than one indicator light can be on at a time. For example, when the computer is actively communicating with the mainframe, all four LED indicators are on.
A switch labeled ENABLE/DISABLE gives you manual control over computer access to the channel. Use the switch to connect the computer to or disconnect the computer from the channel.
When you set the switch to ENABLE, the computer can connect to the channel.
When you set the switch to DISABLE, the computer disconnects from the channel.
Usually you do not need to use the ENABLE/DISABLE switch because the CAB maintains the channel’s integrity. During computer maintenance or extended offline operations, however, you might want to manually disable the computer connection to the channel as a precaution.
The ENABLE/DISABLE switch is inset from the CAB housing to prevent accidental movement. To change the setting, move the switch with a pen or screwdriver.
Device priority determines the order in which devices are serviced by the mainframe. The Barr CAB includes a PRIORITY switch that allows you to set the priority of the Barr channel device to HIGH or LOW.
When you set the PRIORITY switch to HIGH, the Barr channel device responds to the Select Out signal.
When you set the PRIORITY switch to LOW, the Barr channel device responds to the Select In signal.
Typically, you would set the CAB to HIGH priority. However, your system requirements determine how you should set this switch. Consult mainframe personnel to determine the requirements at your site. For more information about channel device priority, see your Barr channel hardware manual.
The PRIORITY switch is inset from the CAB housing to prevent accidental movement. To change the setting, move the switch with a pen or screwdriver.