Sep 21, 2004
By Al Cooper
Over the last series of columns, I discussed how spacing data down the page is controlled through the use of ANSI or Machine Code carriage control. This time I'll start a discussion of how to control font selection in line mode.
Impact printers in the '60s and early '70s were severely limited in their ability to "select fonts!" For example, a 1403 printer came with a race track shaped "print train" that could be removed and replaced with a different print train in order to change the look of the type, size of type or number of repetitions of individual characters. This ability to change print trains was useful when operators had to switch between print jobs for internal use to those for external use when they wanted a different typeface. However it did not allow for the mixing of typefaces or size of type within a print job.
One font attribute that was commonly faked with these early printers was the ability to print "bold" characters by using the "print with no space" in either the ANSI or Machine Code carriage control and reprinting the same data over top of the first printing. Because these were impact printers, the impression left on the paper was darker and thicker than a single imprint therefore it looked "bold".
With the advent of non-impact printers such as the IBM 3800 Mod 1, IBM introduced a new JCL parameter called CHARS. The CHARS parameter allows a person setting up print job JCL to select a specific "Character Set" for each print file. With this option a job that produces multiple print files can select different typefaces, sizes or weights for each print file. This carried forward the concept of the interchangeable print train, but without the hassles of physically changing it or the possibility that the wrong one would be installed.
In conjunction with the 3800-1 printer, IBM took font control one step further with the addition of a control character known as a Table Reference Character (TRC). The optional TRC value in IBM line mode printing is always the second byte of a print file immediately following the carriage control byte and followed by the data to be printed using that font. Because of the memory limitations of the 3800-1, a single print job could use only up to 4 character set names and these were represented in the TRC byte by the values "0", "1", "2" and "3". In order to specify the specific character set to be used for each TRC value, the CHARS parameter had an optional format that allowed for a list of up to 4 character set names. These names were positional in nature so that the first name included represented font "0", the second name represented font "1" and so on.
When using TRC control all data for the entire print line is printed with the character set selected by the TRC value. This worked well when printing headings with one character set and variable data with another character set, but could not isolate individual fields within the print record for printing with different fonts. In order to get around this limitation, the "write no space" option was used often so that one line of data containing only the fields to be included with one character set would be printed and then the second line containing only the data to be represented by a second character set would be printed over the first using a different TRC value resulting in a single print line with some fields in one character set and the remainder in another.
Because non-impact printers don't create a physical impression on the paper, printing the same line twice with no spacing no longer works to create bold type. As such, the use of the CHARS parameter in conjunction with TRC values (one medium and one bold) was needed in order to highlight specific fields with bold type.
Due to the "non-impact" nature of the 3800-1 laser printer, IBM introduced a significant number of character sets that could be used with these printers. The most widely used family of character sets included in the SYS1.IMAGELIB library supplied by IBM was Gothic Text. The names GT10, GT12 & GT15 are widely known in the mainframe printing industry because they continue to be used today. GT10 is a 10 pitch (10 characters per inch horizontally) while GT12 is 12 pitch and GT15 is 15 pitch. A number of other character sets were also provided at that time but these 3 are probably the most well known of all of the "System Fonts."
The 3800-1 also introduced the option of using either standard machine resident character sets or of customizing your own character sets and storing them in a library on the mainframe that was accessible to JES. Some clients used the utility IEBIMAGE extensively to build complex custom character sets involving a mix of 10, 12 & 15 pitch characters. Some even used this as a simplified version of proportional fonts even though it wasn't as truly variable as it should have been, but it was better looking than using fixed pitch fonts.
The use of TRC values with a list of character sets has been used in line mode printing since the early 80s and continues to be used heavily in AFP applications today.
The next column will touch on other methods of controlling font selection in AFP datastreams. Questions? Contact Al Cooper at ac@888999copi.com.