Patent GB-1368507-A (GB)
Title: Selective printing apparatus
IPC: B41J9/14
Filed: 1972-04-19
Abstract: 1368507 Printers INTERNATIONAL BUSINESS MACHINES CORP 19 April 1972 [26 April 1971] 18068/72 Heading G4H  In a line printer with sub-scan operation, a control system controls the length of time for which each hammer actuator is energized dependent on the length of a print scan by arranging for the energization of each hammer actuator to be initiated at a preselected instant in a subscan and to be terminated at another preselected instant in a subscan. A type chain moves along a row of hammers spaced along the print line. During a first subscan, hammers numbers 1, 4, 7 ... get type elements aligned with them in turn, then during a second subscan this happens for hammers numbers 2, 5, 8 ..., then during a third subscan it happens for hammers numbers 3, 6, 9 .... Three subscans form a scan. Thus each hammer has the option of printing one character during a given scan. Further scans follow so that each type element is presented to each hammer eventually. A slotted drum rotating with the typechain is sensed magnetically by an emitter 146 to drive a subscan counter 194, which updates a shift register 198 which modifies the contents of a shift register 200 (itself loaded from a binary counter 196) using adder 204 so that shift register 200 indicates the ordinal number of the type element currently aligned with a hammer. In a computer 154, a register 170A addresses storage 160 A to obtain a character to print and this is supplied via B register 168, and ALU 164 to data register 2 178. The information in shift register 200 is supplied via assembler 210, A register 166, ALU 164, and register 170 B to obtain from storage 160 B the code of the character currently aligned with a hammer, this being placed in B register 168. The character to be printed is concurrently moved from data register 2 178 via assembler 210 to A register 166. The two characters are compared by subtraction in ALU 164, a zero result in data register 1 180 ("COMPARE") supplying a "hammer set' pulse via AND 184 to hammer drivers 158. Information corresponding to that in register 170 A reaches a hammer address register 192, via data register 2 178, and is decoded by a set decoder 214 and a reset decoder 216, the output of the set decoder being passed by gates 218 to select the hammer driver to be set by the "hammer set'' pulse, if present, and to seleet a hammer status bi-stable at 234 which is set via AND 250 if "COMPARE" is present. Before the next comparison is performed in the computer (i.e. for the next hammer coming into alignment in the subscan), the output of the reset decoder 216 is passed by gates 220 to select a hammer status bi-stable at 234 which is reset via AND 252 if "COMPARE" is absent, and to select a hammer driver to be reset by a clock pulse on line 244 passed to line 242 via AND 240. However the hammer driver resetting is inhibited at AND 240 if the selected hammer status bi-stable at 234 is set. The difference between the set and reset decoders 214, 216 produces a delay of almost 2/3 of a scan time between setting and resetting times for a given hammer driver, but because of the inhibition at AND 240, the time between setting and actual resetting is almost 1 2/3 scan times. Alternatively, the time could be less than one scan or more than two. As a modification, the hammer may move along the line so that each serves four adjacent positions.