AM3270/TR3271 Reference Manual

Details of the PTP Protocol
AM3270 Subdevice Protocols
4–8 086705 Tandem Computers Incorporated
Figure 4-2. Pass-Through Protocol Example
013
t1
t2
t3
(PTP)
(PTP)
(CRT)
Comm
Line
Comm
Line
User
Program
AM3270
Host
x1
x2
TR3271
(PTP)
(PTP)
3270 Devices
and Controller
Cluster
Controller
t1
t2
t3
As illustrated in Figure 4-2, AM3270 terminals t1 and t2 are connected to
corresponding emulated subdevices x1 and x2 on the TR3271 side and have their data
“passed through” to the host. Terminal t3 is directly accessed by the user program
using the CRT protocol.
Joining Subdevices To join the subdevices together to perform the pass-through function, use the SCF
commands ADD and CONNECT. The CONNECT message is sent to either of the line
handlers. The receiving (local) line handler validates parameters pertinent to itself,
including the existence and availability of the subdevice. The line handler then sends
a message to the other line handler that performs a similar set of actions and completes
the CONNECT.
AM3270 treats a successful SCF CONNECT as a combination of OPEN and READ.
File-system requests such as OPEN, READ, and WRITE are not accepted by AM3270
for pass-through subdevices because all communication between an AM3270
subdevice and a TR3271 subdevice goes through the message system.
Data Transfer When data is received from the host for a TR3271 pass-through subdevice, the data is
sent to the corresponding AM3270 subdevice. AM3270 selects the device, writes the
data, completes the request from TR3271, and posts a READ to the AM3270 subdevice.
AM3270 treats its end of the pair in a manner similar (but not identical) to TR3271.
The major difference is that a WRITE request from TR3271 causes the outstanding
implicit READ at the AM3270 end to be canceled. This is necessary because AM3270
has only a single request queue per subdevice.
Formatting No formatting or unformatting of the buffer is performed by either AM3270 or TR3271
in PTP mode. All text is passed on a whole-message basis. This means that AM3270
gathers all parts of a multiblock message before sending it to TR3271.