HP 3000 Manuals HP 3000 Manuals
Understanding Host Screen Formats When you write an HP 3000 program designed to communicate with a host application that will not be modified, you need to know what the host screen formatsare and how the host program works. You must know when the host program requires input, when it sends data, and how to determine when the host has finished sending a particular internal screen image. You should also know if a single screen of data will be transmitted in more than one data transmission. You can use the automatic print feature in Pass Thruto show when and how the host program writes to your screen. Start Pass Thru specifying format = 3 in the info string of the RUN command, and manually access your host program. Your internal screen image will be printed after every call to either the TRAN3270 or the RECV3270 intrinsic. The printed screen images show you the screens sent by the host, with attribute and null characters differentiated from blank characters. If the host sends a single screen in more than one transmission, you will be able to determine that from the printed images. See Using SNA IMF Pass Thru for more information on Pass Thru and its automatic print feature. Because your HP program emulates a 3278 display station or 3287 printer, you must design your program to respond to the host program whenever it modifies your internal screen image. Your program must know what the host system will do in a particular situation. If the host may do one of several things at a particular time, your program must be written to handle variable situations. When you can modify existing host applications, or when you can write local and host applications in tandem, you have the opportunity to write simpler programs. Screens do not need operator instructions or even much structure. In fact, you can use an unformatted screenwith no fields at all. Unformatted screens are often more efficient for new applications. You can write the programs to suit your local needs, possibly with the HP 3000 application controlling the host program. IBM 3278 Screen Layout On an IBM 3278 display station, data fields extend horizontally across the screen and are separated from one another by an attribute character. Data fields may occupy one or more contiguous lines on the screen. SNA IMF intrinsics read a screen one row at a time, beginning at the upper left corner and moving along each row from left to right, looking for a field attribute character. The first field attribute character found causes the following field to be labeled field number one; the field following the next attribute character is labeled field number two, and so on as SNA IMF scans down the screen. If an attribute character is not in the very first character position on the screen, then the first characters of data belong to the last field on the screen; that is, the last field wraps the screen. SNA IMF intrinsics number rows in the screen 0 through 23 and columns 0 through 79. The IBM screen layout numbers rows 1 through 24 and columns 1 through 80.Figure 2-1 compares the way SNA IMF and IBM number the rows and columns on a screen.Figure 2-1. Comparing SNA IMF and IBM Screen Layouts An attribute is not considered part of the data field. The field-oriented intrinsics never return the attribute when they return field data. The data field begins with the first position following the attribute character. This position is defined as the location of the first data character in the field; here, offset = 0. When the READFIELD intrinsic returns data from a field, embedded and leading null characters are returned with data characters, but trailing null characters are not returned. This means that the actual field length parameter count does not include trailing null characters. Intrinsic Calling Sequences The order in which you call SNA IMF intrinsics depends on whether your device or the host system will be the first to send data after a communication path is opened. If the host has a screen of data waiting for you, the host will want to send the screen before allowing you to send any data. If the host system requires you to log on or start the host application, your device must send the first screen of data to the host system. You can simplify your program by developing routines that consist of commonly used sequences of intrinsics. Be sure these routines check the value returned in the result parameter of each intrinsic. Appendix F contains complete sample programs. Following are suggested intrinsic sequences, presented to help you get started writing your programs. You may want to run Pass Thru with the format parameter in the info string set to three or four (the automatic print option), to verify the correct intrinsic calling sequence. See Using SNA IMF Pass Thru for more information. Host Transmits Data First If you expect the host to send data to your device first, use intrinsics in a sequence like this: Establishing communications with the host: OPEN3270 Establish communications with the host system. RECV3270 Check to see if the data has been received. RECV3270 Accept an initial screen of data from the host. SCREENATTR Determine the number of fields in the screen and update the cursor variables containing the coordinates of the cursor position. Reading data, from a specified field: FIELDATTR Determine the attribute values for a specified field. READFIELD Read data from a specified field in the screen. (Repeat the FIELDATTR/READFIELD sequence as many times as necessary.) Reading data, from part or all of the screen: ATTRLIST Return the locations of the attribute characters. READSCREEN Read data from part or all of the screen. Writing data, into one field at a time: WRITEFIELD Write data into an unprotected field. Writing data, into several fields at a time: SCREENATTR Update the cursor (row/column) variables again. STREAM3270 Write data into several fields at one time. TRAN3270 Send a data screen to the host system. Receiving another screen of data from the host: RECV3270 Accept a new data screen from the host. SCREENATTR Determine the number of fields in the screen and update the cursor variables containing the coordinates of the cursor position. FIELDATTR Determine the attributes of the fields specified. Closing a session with the host: CLOSE3270 Disassociate your terminal from an SSCP-LU session. Terminal Transmits Data First If you expect to send data to the host system first, use the intrinsics in a sequence like this: Sending data to the host first: OPEN3270 Establish communications with the host. RECV3270 Accept the SNA IMF banner message. TRAN3270 Send the SYS REQ key, which puts you into the LU-SSCP session and allows you to send data to the host. RECV3270 Receive a blank SSCP-LU screen. WRITEFIELD Write to the LU-SSCP screen. TRAN3270 Send the data screen to the host. RECV3270 Accept a new screen of data from the host. SCREENATTR Determine the number of fields in the screen and update the variables containing the coordinates of the cursor position. Reading data, from a specified field: FIELDATTR Determine the field attribute values for a specified field. READFIELD Read data from a specified field in the screen. (Repeat the FIELDATTR/READFIELD sequence as many times as necessary.) Reading data, from part or all of the screen: ATTRLIST Return the locations of the attribute characters. READSCREEN Read data from part or all of the screen. Closing a session with the host: CLOSE3270 End communications with the host.
NOTE Be sure to log off from the host application you are using before you issue the CLOSE3270 intrinsic. The CLOSE3270 intrinsic terminates your session with the host system, but it does not terminate the host application.If you call CLOSE3270 before you log off from the host application, the host application is left running, and you have no way of terminating it, because your session has been closed.
For complete example programs, in non-transparent and transparent mode, see appendix F, "Sample Programs."
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