Transparent and Non-Transparent Modes

Transparent (Data Stream) Mode

The —transparent (data stream) mode of SNA IMF allows your application program to obtain the untranslated data stream in either no-wait or wait I/O. In transparent mode, SNA IMF does not interpret any of the control information in the data stream. It does not differentiate between the data contained in a field and the control information that describes the field and locates it on the screen. Therefore, in transparent mode, you cannot call non-transparent mode SNA IMF intrinsics that access individual fields in the internal screen image (such as READFIELD or WRITEFIELD).

In transparent mode, you obtain data from the host by calling RECV3270 and then READSTREAM. To send LU.T2 data to the host, call WRITESTREAM and then TRAN3270. To send LU.T1 data to the host, you need to call only TRAN3270. You cannot call READSTREAM and WRITESTREAM in non-transparent mode, because the original data stream is not preserved after it is translated.

Use transparent mode only for LU.T1 emulation or for communicating with host applications designed specifically for transferring files without translation. You must specify transparent mode for LU.T1 sessions, which communicate with host SCS applications .

To specify transparent mode, you open a device by calling the OPEN3270 intrinsic with bit 14 of the flags parameter set to 1 (the transparent mode option).

An option of the OPEN3270 intrinsic allows a printer to accept both LU.T1 and LU.T3 printer sessions. When you set up a printer that way, you can run your LU.T1 sessions in transparent mode and your LU.T3 sessions in non-transparent mode. See the description of the OPEN3270 intrinsic, in Chapter 3 “Intrinsics Used with Standard MPE I/O” for more information.

Only the following intrinsics can be used in transparent mode:

Binary data may be transmitted in transparent mode because the SDLC protocol allows transmission of transparent data. Because SNA IMF does not attempt to interpret the data, all possible values of data may be transmitted and received in transparent mode.

While in transparent mode, you are responsible for any character code translation. Use the Native Language Support (NLS) tables to perform the conversion. See the Native Language Support Reference Manual for more information.

When a file is transferred from the host system to the HP 3000, the entire file is sent in sequential blocks known as request units (RUs). Each LU.T2 or LU.T3 RU (or each RU chain, if chaining is used from the host contains a 3270 data stream write command. LU.T1 RUs contain SCS control codes instead of 3270 data stream commands.

The received data stream is passed unaltered through the SNA IMF software. The HP 3000 software transfers the data stream directly into the internal screen image untranslated. The LU.T2 and LU.T3 data returned to the user is a 3270 data stream consisting of a write command byte and transparent data. SNA IMF processes and removes both the SNA controls and the SDLC protocol information before passing the received data to the user.

When transmitting data from the HP 3000 to the host system, the internal screen image used by the WRITESTREAM intrinsic need contain only the data to be transferred. The SNA IMF software partitions the data to be transmitted into RUs and adds the necessary SDLC and SNA protocol information. The SDLC protocol and SNA controls provide coordination between the host system and the HP 3000; however, you must establish the interprocess controls between the host application and the HP application. For example, you must identify the start or end of a file.

The AID and cursor address values specified in your intrinsic call are optionally inserted at the beginning of the first frame. An AID (attention identifier) is the numeric code indicating that you have pressed a key. The AID value and cursor address can be suppressed in the TRAN3270 call.

If AID and cursor suppression are not used, and you are sending a CLEAR or PA key during an LU.T2 session, call WRITESTREAM with an outbuflen of zero before calling TRAN3270. In transparent mode, SNA IMF will not check the AID key to determine whether it is a CLEAR or PA key. In non-transparent mode, no data is sent with the CLEAR and PA keys.

It is important to note that the lack of a true screen image invalidates using the host READ BUFFER and READ MODIFIED commands. These commands will not be executed by SNA IMF. If SNA IMF receives a READ BUFFER or READ MODIFIED command from the host, it will reject the command with sense code X'1003' (unsupported function). Thus, for the host to obtain data from a transparent mode device, this device must have a TRAN3270 intrinsic request outstanding. Also, the host must be indicating that it is ready to receive data. All other commands are accepted and passed on to your program along with any associated data. SNA IMF performs no other actions that normally might be associated with a specific command.

The size of the internal screen image determines the amount of data that can be transmitted or received. For LU.T2 and LU.T3 sessions, a screen size of 480, 1920, or 3440 characters will have a buffer limit of 540, 2160, and 3870 bytes, respectively. If data received from the host system is larger than the buffer limit, the buffer is filled to the limit and the rest of the data is lost. When the user issues the next READSTREAM call, a result code of 52 will be returned, indicating that the entire data stream could not be received. For LU.T1 sessions, SNA IMF can support an internal screen image of only 3870 bytes and an RU size of only 3584 bytes. If you exceed this RU limit, the internal screen image may be overwritten if the host application sends a split order. A split order occurs when SCS control codes or parameters span RU boundaries within an RU chain.

If a host writes to a transparent mode device that already has data in its internal screen image, SNA IMF will reject the data with sense code X'082D' (device busy). When you indicate that you are ready for more data by issuing another RECV3270 call, SNA IMF sends an LUSTAT to the host, indicating that the device is now available (X'00010000' for LU.T1 and LU.T3, X'0001D000' for LU.T2). This intervention prevents the host from writing over a data stream not yet obtained by the user.

Non-Transparent Mode

In non-transparent mode, the data stream is translated into the image of a terminal screen. Once the data is translated, SNA IMF can locate the cursor on the screen, locate each field on the screen, and identify the attributes of each field. Data that has been entered into the fields can be differentiated from control information in the data stream that locates and describes the fields on the screen.

In non-transparent mode, the internal screen image is maintained in ASCII character code, and the data is converted to EBCDIC character code only before being transmitted to the host system. Data received in EBCDIC character code is translated to ASCII character code before being placed in the internal screen image.

You cannot call READSTREAM and WRITESTREAM in non-transparent mode, because the original data stream is not preserved after it is translated.