The present invention relates to a control unit for a plan printer.

Architectural and engineering drawings are normally originally produced on transparent sheets of drafting film or tracing paper. Copies of the drawings are produced on machines known as plan printers. A print, or copy, of a drawing is generated by placing the original drawing on printing paper adjacent a chemically treated surface thereof and inserting the two together into a plan printer. The printer receives the two sheets and first exposes the chemically treated surface of the printing paper to a particular form of light, such as uv light. The plan or drawing depicted on the drafting film or paper, however, acts as a mask for the printing paper during the exposure process and thereafter the

printing paper has developing fluid applied thereto which if applied in a particular manner only develops the unexposed portions of the printing paper. A plan printer includes a series of rotating cylinders or rollers which receive and guide the drafting film or paper and the printing paper and thereby control the speed at which the paper travels past the exposing light and an area where developing fluid is applied. The speed with which the printing paper passes the developing fluid area determines the amount of fluid applied to the paper and thereby governs the degree to which the unexposed areas of the printing paper are developed. Plan printers are normally provided with a speed control which is adjustable by a user so that the user may select a desired degree of "darkness" for a print or copy. If the speed of a plan printer is selected to be below a predetermined level the amount of developing fluid applied to a sheet of printing paper will be excessive and thereby cause the paper to be overdeveloped. The printing paper may be considered overdeveloped when not only the unexposed portions are darkened by action of the fluid but also the exposed portions are darkened so that the entire sheet appears black. Plan printers of the type described above include known diaro printers.

Recently it has proved necessary for economic reasons to develop accounting and security systems for copying machines which only enable authorised users to operate the machine and record . the numbers of copies which a given user generates using the machine. The problem associated with such security and accounting systems is that they are

unable to be used with plan printers. Known security and accounting systems, when coupled to a copying machine, output an enable signal to the copying machine when a user enters a sequence of predetermined inputs which the system recognizes as valid. The enable signal is used to apply power to circuitry of the copying machine which is essential for operation of the machine. The machine is held in a disabled state until the enable signal is received. Once the enable signal is received the security and accounting system receives count pulses from count pulse generating circuitry of the copying machine and records the number of prints produced by a user according to the number of pulses received.

For plan printers, however, whilst there exists circuitry which is critical to the operation of the printers, this circuitry cannot be enabled by an enable signal such that the printer is operable immediately thereafter. Power must be supplied to the circuits for a predetermined period of time before the printers may be used to generate prints. Any operating costs which are saved by coupling a security and accounting system of the type described above to a plan printer would be negated by time users would need to spend waiting for the printer to be fully enabled after having gained access to the printer via the system. Also most of the presently available plan printers do not include count pulse generating circuitry which generates a count pulse in response to a print being produced.

Accordingly, the present invention provides a control unit which enables a security and

accounting system to be used with a plan printer.

More specifically, the present invention provides a control unit for a plan printer comprising:

means for receiving an enable signal from a security and accounting system; means responsive to said receiving means for inhibiting speed control of said plan printer and setting the speed of said printer such that said printer is disabled, and, when said enable signal is received by said receiving means, enabling speed control of said plan printer.

Preferably said control unit further includes means for outputting a count signal to said security and accounting system when said printer generates a print.

In accordance with the present invention there is also provided a combination of said control unit and said security and accounting system, which generates said enable signal upon receiving a

sequence of valid inputs and records the number of prints taken by a user in response to said count signals.

The present invention will now be described, by way of example only, with reference to the accompanying drawing wherein Figure 1 is a circuit diagram of a control unit according to the present invention.

A control unit 2 is shown connected in Figure 1 to plan printer 4 and a security and accounting system 6. The control unit 2 comprises a relay 8, a potentiometer 10, a power supply circuit 12 and a low pass filter circuit 14.

The relay 8 includes a relay coil 16 and a bank 18 of three parallel switches 20. The switches 20 are simultaneously switched between first and second output terminals, 22 and 24 respectively, in response to signals applied across the coil 16. The first output terminals 22 are connected respectively to the three terminals of the potentiometer 10. The second output terminals 24, when the control unit 2 is connected to a plan printer 4, are connected respectively to the three terminals of a speed control potentiometer 26 of the plan printer 4. The bank 18 includes three input terminals 28 to the switches 20 and the input terminals 28, in use, are connected to speed control circuitry 30 of the plan printer 4. Normally the speed control circuitry 30. is connected directly to the speed control potentiometer 26 of the plan

printer 4. The speed control potentiometer 26 enables a user of the plan printer 4 to select the speed at which printing paper passes through the printer 4. With the control unit 2, connected as shown in Figure 1, to the plan printer 4, the speed control potentiometer 26 is directly connected to the circuitry 30 when an enable signal is applied across the coil 16 so that the switches 20 are moved to connect the input terminals 28 to the second output terminals 24. This enables the plan printer 4 to be operated in a normal manner. When the coil 16, however, is not so energised the switches 20 are positioned so that the input terminals 28 are connected to the first output terminals 22 and the speed control circuitry 30 is connected to the potentiometer 10 of the control unit 2. The potentiometer 10 is chosen so as to be of a similar type to the potentiometer 26 of the plan printer 4, e.g. lOkΩ, lOOkΩ, and has its wiper set so that the plan printer 4 can only be run at a relatively low speed which ensures every print generated by the printer 4 is overdeveloped. When the speed control circuitry 30 is connected to the potentiometer 10 the plan printer 4 is disabled.

The relay coil 16 is connected across input terminals 32 and 34 which, in use, are connected to the security and accounting system 6, as shown in Figure 1. When a user wishes to use the plan printer 4, the security and accounting system 6 asks the user to enter a sequence of inputs which the system analyses to determine if they are valid, thereby indicating that the user is a valid user. Once the inputs are validated the system 6 outputs an enable

signal which is received by the input terminals 32 and 34 and applied across the relay coil 16. This, in turn, causes the switches 20 to be switched so as to connect the speed control circuitry 30 to the speed potentiometer 26. After the user completes a printing operation he informs the system 6 and the enable signal is no longer outputted. This causes the switches to revert to their normal position, thereby connecting the speed control circuitry to the potentiometer 10 of the control unit 2. Normally the input terminal 34 is earthed and the other terminal 32 receives the enable signal which is preferably a pulse signal.

When the security and accounting system 6 ouputs the enable signal an input port of the system 6 is set so as to receive a count signal which indicates that a print has been generated by the plan printer 4. The system 6 records the number of count signals received and thereby generates tallies of prints produced by each valid user for accounting purposes. The control unit 2, in use, outputs count signals to the security and accounting system 6 via output terminals 36 and 38 of the unit 2. The count signals are generated in response to the closing of a switch 40 which is mounted on or within the plan printer 4 so as to close whenever a print is generated by the printer 4. The switch 40 may be considered as part of the control circuit 2. Preferably the switch 40 is a toggle switch provided in the path of travel of a sheet of printing paper inserted into the printer 4 so that as the sheet passes the switch 40, the switch 40 is closed. Voltage is supplied to one terminal of the switch 40

by the power supply circuit 12, via an output terminal 42 of the unit 2. The other terminal of the switch 40 is connected to the low pass filter circuit 14 via an input terminal 44 of the unit 2. The power supply circuit 12 supplies a 12 volt ac voltage to the output terminal 42 and the low pass filter circuit 14 is used to filter out any high frequency transients or spikes generated at the input terminal 44 on closing and opening the switch 40, thereby providing protection for the internal circuitry of the system 6.

The power supply circuit 12 comprises a 240v:12v transformer 46 where the primary winding 48 is connected across two mains input terminals 50 and 52 of the unit 2. A fuse 54 is connected between the primary winding 48 and the output terminal 50 which is provided for connection to the active terminal of a mains supply. A secondary winding 56 of the transformer 46 has one terminal connected to the terminal 34 and its other terminal connected to the output terminal 42 via line filtering circuitry. The line filtering circuitry comprises a capacitor 58 connected in parallel with the secondary winding 56 and a diode 60 connected between the secondary winding 56 and the capacitor 58 such that its cathode is connected to the capacitor 58 and its anode is connected to the secondary winding 56. The low pass filter 14 comprises a resistor 62 and a capacitor 64 both connected in parallel with the output terminals 36 and 38 and a resistor 64 connected in series with the input terminal 44 and the output terminal 36 and between the resistor 62 and the input terminal 44.

The output terminal 38 is connected to the input terminal 34 of the unit 2.

Many modifications will be apparent to those skilled in the art without departing from the scope of the present invention as hereinbefore described with reference to the accompanying drawings.