Channel-attached NCPs in a CMC configuration

Several operands on the PCCU definition statement are used to define channel characteristics. If you do not define a channel in a channel-attachment major node, the values on the PCCU definition statement are used because the channel is implicitly defined and activated automatically when the NCP is activated.

However, when you define the channel in a channel-attachment major node, the definition statements in the channel-attachment major node contain operands similar to those on the PCCU definition statements. These operands are:

CHANCON=COND|UNCOND
For a data host, the CHANCON operand on the PCCU definition statement is replaced by the CHANCON operand on the LINE definition statement in the channel-attachment major node.
CUADDR=channel device address
For a data host, the CUADDR operand on the PCCU definition statement is replaced by the ADDRESS operand on the LINE definition statement in the channel-attachment major node.
MAXDATA=size
For a data host, the MAXDATA operand on the PCCU definition statement is replaced by the MAXDATA operand on the PU definition statement in the channel-attachment major node.
DELAY=time
For a data host, the DELAY operand on the PCCU definition statement is replaced by the DELAY operand on the PU definition statement in the channel-attachment major node.

Activating the channel-attachment major node to activate the channel causes VTAM® to use the values defined in the channel-attachment major node. Later, if NCP1 is activated, these same operands, if they appear on NCP1 PCCU definition statements, are ignored. If you are defining the channel in a channel-attachment major node and want to avoid confusion concerning these operands, avoid coding duplicate values on the PCCU definition statement.

The RNAME operand on the PCCU definition statement can name a link station associated with a channel link defined in a channel-attachment major node. When the host activates each NCP, the link station and link defined in the channel-attachment major node representing the channel are activated automatically.

The values defined on the PU definition statement in the channel-attachment major node are used for the CUADDR, CHANCON, DELAY, and MAXDATA operands. The PCCU values for these operands are used only when the channel-link station is not defined in a channel-attachment major node.

A channel-attached NCP requires one HOST definition statement for each host that activates the NCP. The HOST definition statement in the NCP major node has a MAXBFRU operand for defining the number of buffers that the host can use for receiving data from the NCP. If you do not define the channel in a channel-attachment major node, the MAXBFRU value on the HOST definition statement is used because the channel is implicitly defined and activated automatically when the NCP is activated.

However, when you define the channel in a channel-attachment major node, the MAXBFRU operand on the LINE definition statement for that major node defines the number of buffers the host can use for receiving data. Figure 1 shows an example of this. When HOST1 activates its channel-attachment major node, VTAM uses the value of the MAXBFRU operand on the LINE definition statement. Later, if NCP1 is activated by HOST1, the MAXBFRU value on the HOST definition statement is ignored.

When you define the channel in a channel-attachment major node, VTAM also ignores the MAXBFRU value on the HOST definition statement when the link station for the channel is activated automatically. This is because the value is determined by the RNAME operand when the NCP is activated.

Note: The MAXBFRU operands on the LINE definition statement of the channel-attachment major node should contain the same values as the MAXBFRU operand specified on the HOST definition statement of the NCP major node.

For more information about defining a channel-attachment connection between an NCP and a VTAM data host, see Channel-attached NCP to a VTAM data host.