Linemode

In some cases, the client or the application does not support full-screen presentation, or the user needs to work in a linemode environment. For these reasons, most emulators support linemode. Linemode supports a go-ahead function to simulate a half-duplex format. With go-ahead negotiated, the partner cannot send data until it receives a go-ahead from the current sender of data. In most cases, sessions are naturally half-duplex and the go-ahead adds unneeded transmissions. Therefore, the Telnet default is to Suppress Go Ahead (SGA). If go-ahead is needed to maintain a half-duplex format, use the NOSGA parameter. SGA or NOSGA can be coded at all three parameter block levels for different levels of granularity.

Telnet supports the following types of Linemode connections:

Standard
Binary
Transform

Standard Linemode is assumed if neither DBCS transform nor BINARY linemode parameters are specified, or if the device type is not supported by transform. Standard Linemode is the only connection mode that requires translation by Telnet. Telnet provides multicultural support for standard Linemode connections. ASCII and EBCDIC code pages are the basis for translation. Telnet uses the ICONV services available in the C runtime library. For custom code page information, see the ICONV services in z/OS XL C/C++ Programming Guide. When ASCII and EBCDIC code pages are specified, a conversion descriptor will be given to Telnet. Telnet creates ASCII-EBCDIC and EBCDIC-ASCII translation tables based on the conversion descriptor. The CODEPAGE parameter is used to specify the code page names. For example:

CODEPAGE ISO8859-1 IBM1047

The possible results from CODEPAGE processing are:

If a conversion descriptor is not returned, CODEPAGE is not coded, or there is an error in the syntax, a default code page of ISO8859-1 will be used for ASCII, and the language environment code page taken from locale information will be used as the EBCDIC code page.
If a conversion descriptor is not returned again, a default code page of IBM-1047 will be used for EBCDIC.
If a conversion descriptor is not returned again, predefined translation tables within Telnet will be used. These tables are similar, but not exactly the same as the tables which would have been generated if ISO8859-1 and IBM-1047 had worked. Some of the differences are noted below:
```
 EBCDIC           ASCII                             
 x'0D25' -----> x'0D0085' using ISO8859-1/IBM-1047  
 x'0D25' -----> x'0D0A'   using internal tables  
 x'15'   <----  x'0A'     using ISO8859-1/IBM-1047 
 x'25'   <----  x'0A'     using internal tables  
```

No message is issued to the console if the first conversion succeeds. If there is any conversion failure a message is issued. If one of the later conversions succeeds, a message is issued indicating success.

If your Linemode connection does not perform correctly, the default translation tables may be causing the problem. Try the internal Telnet translation tables by specifying TNSTD for both ASCII and EBCDIC choices. For example:

CodePage TNSTD TNSTD

The internal code pages must be used together. If only one of the two internal tables is specified, then the other internal table will also be used.

CODEPAGE can be coded at all three parameter block levels for different levels of granularity.

Binary Linemode is set using the BINARYLINEMODE parameter. It indicates that Telnet should not do translation. The ASCII data from the client should be passed as-is to the VTAM® application. BINARYLINEMODE or NOBINARYLINEMODE can be coded at all three parameter block levels for different levels of granularity.

Transform Linemode is set using the DBCSTRANSFORM parameter. When coded, all data that passes through Telnet will be transformed from DBCS or SBCS ASCII full screen to 3270 full screen for all supported device types. If the device type is not supported, Standard or Binary Linemode is used. DBCSTRANSFORM can be coded in TELNETPARMS or PARMSGROUP for different levels of granularity. It cannot be coded in TELNETGLOBALS. A unique logmode for transform can be set using TELNETDEVICE with a device type of TRANSFORM. Any logmode used must not support extended graphics.

Note: Transform can be used by only one port when multiple ports are active on one TCP/IP stack. DBCSTRANSFORM supports a maximum of 250 concurrent connections.

DBCSTRANSFORM can be used for either the VT100 single-byte character set (SBCS) or VT282 double-byte character set (DBCS) transform mode. When DBCSTRANSFORM is specified and the TCP/IP procedure JCL has been modified as shown below, ASCII-based terminal emulators (VT100 or VT282) will appear as full-screen 3270 terminals. Telnet receives ASCII data from the client and transforms it into SBCS or DBCS EBCDIC data, depending on the terminal type. Telnet adds appropriate SNA control bytes to give the appearance that the data is coming from a 3270 terminal. Telnet receives EBCDIC data from the host application and transforms the SNA control bytes and data into appropriate ASCII control bytes and data. The data is sent to the ASCII-based terminal where it is displayed in 3270 full-screen emulation. DBCSTRANSFORM requires additional special Data Definition (DD) statements in the TCP/IP procedure.

You must add the following three DD statements to the TCP/IP procedure JCL to support Transform:

//TNDBCSCN DD DSN=TCPIP.SEZAINST(TNDBCSCN),DISP=SHR
//TNDBCSXL DD DSN=TCPIP.SEZAXLD2,DISP=SHR
//TNDBCSER DD SYSOUT=*

The TNDBCSCN DD statement must point to the configuration data set for 3270 DBCS transform mode. This configuration data set specifies the default DBCS conversion mode that will take effect at initialization time. Specify the CODEKIND and CHARMODE parameters according to the required DBCS code page. If CODEKIND and CHARMODE are not specified, or if the TNDBCSCN DD statement is not added, by default, CODEKIND is SJISKANJI and CHARMODE is ALPHABET. A sample can be found in SEZAINST(TNDBCSCN).
The TNDBCSXL DD statement must point to the data set containing binary translation table code files for 3270 DBCS transform mode. The installation data set, SEZAXLD2, contains the default binary translation table code files. The binary translation table code files for 3270 Transform can be customized by using the CONVXLAT command. See z/OS Communications Server: IP Configuration Reference for more information about customizing translation table code files. If the TNDBCSXL DD statement is not added, the following message will appear and an abend will occur:
```
IEC130I PASCAL01 DD STATEMENT MISSING
```
The TNDBCSER DD statement defines where Transform-specific error messages are recorded. This DD statement can specify an output data set or SYSOUT=*. If the TNDBCSER DD statement is not added, transform initialization will fail.

Specifying the DBCSTRACE parameter sends detailed trace output from 3270 Transform to the location specified in the SYSPRINT output DD statement. Additional detailed trace output is also sent to TNDBCSER. Both data sets will contain detailed trace data. DBCSTRACE or NODBCSTRACE can be coded in TELNETPARMS or PARMSGROUP for different levels of granularity. They cannot be coded in TELNETGLOBALS.