Patent GB-1515321-A (GB)
Title: Random access line printer
IPC: G06K15/08
Filed: 1975-05-22
Abstract: 1515321 Control of printers CENTRONICS DATA COMPUTER CORP 22 May 1975 [5 June 1974] 22112/75 Heading G4H A bidirectional printer comprises means for determining the end points of a line of data to be printed, for determining which end point is nearer to the print head position at the end of the previous line, and for moving the printhead to that end point for printing in the appropriate direction. Since the printhead need only move along that part of the line where data is to be printed, a saving in printing time is achieved. The printer (Fig. 1, not shown) comprises a 7-wire printhead for printing 7 x 5 or 7 x 9 dot matrix characters. Each character may be printed in expanded form (each column printed twice) or on an enlarged scale (using 7 x 6 matrices). 132 characters may be printed on each line. The position of the printhead is determined by a photocell-LED combination on the print carriage co-operating with a strip 113, Fig. 2a (not shown) containing parallel spaced slits. Two such combinations co-operating with two staggered lines of slits may be used (Figs. 2l-2o, not shown) to generate signals which also indicate the direction of movement of the printhead. Data to be printed is entered in a register 351a, b which is recirculated once under the control of a counter 425, Fig. 3c (not shown) to determine the end points for that line. If the data is to be printed from left to right, the contents of 351 are transferred via a "multiplexer" 799 to a dot matrix character generator 810, Fig. 7 (not shown); if printing is to be in the reverse direction, then register 357 is recirculated 132 times under the control of (425) and a counter (436) to feed the characters into a register 352a,b in the reverse order, from which they are passed to 799. A counter 455 is incremented until the first non-zero character in 351 is detected, when the count is transferred to a latch 463 to give the position of the lefthand edge; the count is 455 when an end of line code is detected (or after a count of 132) represents the position of the right-hand edge. These counts are compared in 464, 476 with the contents of counters 450, 451, which initially represent the printhead position. If the printhead lies to the left (right) of the left (right) hand edge, signal HLL(HGR) causes the printhead to move towards the nearer edge and to start printing when it is reached (HEL, HER). If the printhead lies between the left and right edges (HGL, HLR) counters 450, 451 are respectively in-and decremented until the first match (HEL or HER) is obtained: this indicates which edge is nearer. The printhead is then moved towards and slightly beyond this edge, then reversed to start printing as the edge is passed (the data being fed to 352 where necessary). Logic circuitry for the above operations is described with reference to Figs. 3a, 3c, 4g-4f and 5a-5c (not shown). Each character is expressed in 6-bit ASC11 code with two further bits which indicate whether normal, expanded or enlarged characters are to be printed; circuitry is shown for producing these characters and for producing the necessary extra column selection signals for 7 x 9 matrix printing, e.g. Figs. 6, 7, not shown). Only 66 expanded characters may be printed on one line; if there are more, the printhead is returned to the left-hand margin for further printing. Means are provided for ensuring that the dot columns are printed in registration for both left-right and right-left printing. Malfunction of the LED's operates an audible or visual alarm (Fig. 4f). The data stored in 351 is preceded by a dummy character. Conversion to 80-column printing is possible.