Channel commands for channel-attached type 2 and type 4 physical units

Table 1 describes the channel commands used by the VTAM program to write data to and read data from channel-attached type 2 and type 4 physical units.

Table 1. VTAM channel commands for type 2 and type 4 physical units
Command code (hex)	Command	Description	Contents of address field
01	Write	Transfers data from storage in the host processor to the I/O device.	Output area
02	Read	Transfers data from the I/O device to storage in the host processor.	Input area
03	No-op	Causes the channel to respond with a channel end and device end. It is the last CCW in a read or write chain.	Zero
04	Sense	Transfers 1 or 2 bytes of sense data to storage in the host processor.	Address of sense data area
08	TIC (Transfer in Channel)	Causes the channel to fetch an instruction that is not the next sequential instruction within the channel program sequence.	Address of next CCW to be executed
09	Write Break	Transfers data from storage in the host processor to the I/O device and indicates that it is the last or only Write command in a chain of Write CCWs.	Output area
31	Write Start 0	Begins a Write sequence. Alternates with Write Start 1.	Zero
32	Read Start 0	Begins a Read sequence. Alternates with Read Start 1.	Zero
51	Write Start 1	Begins a Write sequence. Alternates with Write Start 0.	Zero
52	Read Start 1	Begins a Read sequence. Alternates with Read Start 0.	Zero
Note: Data transfer does not occur on Read-Start or Write-Start commands.
61	Write XID	The host sends the Write XID command to signal the NCP that a channel contact sequence is beginning and to prepare to receive the host’s XID.	Zero
62	Read XID	The host sends the Read XID command to signal the NCP that the host expects to read the NCP’s XID.	Zero
A3	Discontact	Indicates that the channel is no longer contacted and the attachment to the transmission group should be broken. Releases the PIUs on the channel hold and intermediate queues.	Zero
C3	Contact	Establishes contact between the host and the NCP. Tells the NCP to use XID information for operations with the host.	Zero
93	Restart	Causes the controller to reset its switches to indicate that the last Write-Start and Read-Start commands were Write-Start-1 and Read-Start-1 commands.	Zero