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A low level output from Comparator 64 means no prob lem but if the level therefrom is high when the Counter 59 provides a high window pulse to AND gate 65, the output of the latter will go high, indicating a printing error. The error sensed by CPU 70 indicates a failure which may be caused, for example, by dry print ribbon, necessitating its replacement. In conclusion, there has been described a printer kit which has particular application in the retro?t of exist ing letter sorting machines. The ability of such ma chines to selectively operate in a no-print mode, as at present, or in an automatic print mode, as provided by the present invention, greatly increases the performance and usefulness of such machines. While there have been disclosed speci?c design de tails applicable to a particular machine, the basic princi ples taught herein may be applied to other similar ma chines which nevertheless differ somewhat in construc tion or operation. Changes and modi?cations of the kit may be required to suit particular requirements. Such variations as are within the skill of the designer, and which do not depart from the true scope and spirit of the invention are intended to be covered by the follow ing claims. We claim: 1. A printer kit for use with a letter sorting machine having an input console which includes an operator controlled keyboard for the entry of destination data for each mail piece and an Expanded ZIP Retro?t (EZR) for converting such data into a sort bin designation, comprising in combination: a printer operatively mounted on said console for printing coded address information on each mail piece; a mode select unit having operator-actuated switch ing means for selectively placing said console in a plurality of print and no-print modes; a console code printer interface coupled between said printer and said EZR, said console code printer interface including a main processor and memory, an I/O data bus controller having an input and an output terminal coupled to said main processor and memory, a data path controller having a plurality of input terminals coupled to said keyboard for receiving said destination data therefrom and a plurality of output terminals coupled to said EZR for providing said last mentioned data thereto, a keyboard I/O interface unit interposed between said data path controller and said I/O data bus controller, said keyboard I/O interface unit having a plurality of input and output terminals for the transmission of data between the last mentioned controllers; said mode select unit being coupled to said data path controller for causing the latter to send said desti nation data directly to said EZR for a no-print mode and to said I/O data bus controller via said keyboard I/O interface unit for a print mode, said mode select unit being further coupled to said I/O data bus controller whereby said main processor and memory are conditioned to execute a predeter mined program stored in the latter in accordance with the mode select unit; said main processor accessing said destination data in said I/O data bus controller, assembling the same and determining 5 15 25 30 35 45 65 8 the code equivalent thereto for printing on said mail piece, said main processor and memory fur ther transmitting said keyboard data to said EZR via said data path controller; said main processor and memory being comprised of a CPU, a PROM, and a RAM: said PROM storing program code and being coupled in common to said CPU, said I/O data bus control ler and said RAM via a bidirectional data bus; said program code comprising modes of console opera tion selectable in accordance with the switch set‘ ting of said mode select unit, program initialization, and table lookup for cross referencing destination data and code to be imprinted on said mail piece; said CPU being further coupled to said I/O data bus controller, said PROM and said RAM via a unidi rectional address bus; said CPU also providing a R/W signal to said PROM, said RAM and said I/O data bus controller, means for applying an interrupt signal to said CPU in response to signals from said console indicative of an operator-paced or ma chine-paced print mode, said CPU having ?rst and second output terminals coupled respectively to said EZR and said console, said CPU providing a lookout pulse on said ?rst output terminal for syn chronizing the destination data transmitted to said EZR via said data path controller, and a clutch signal on said second output terminal for synchro nizing the movement of said mail piece in said console with the keying of said destination data; said RAM storing key code information identifying user-programmable keys present on said keyboard.