embodiments of the present invention are described below using the accompanying drawings . fig1 is a block diagram showing a first embodiment of a discharge lamp lighting device which applies the present invention . the discharge lamp lighting device is applied to , for example , a projection - type display shown in fig2 . referring to fig2 , a reflector 77 and a high - pressure discharge lamp 78 constitute a light source that irradiates light from the rear of an image display device 76 . the light , after being passed through the image display device 76 , is projected onto a screen 74 through optics 75 . the image display device 76 is , for example , a liquid - crystal display element , and is driven by an image display device driver 79 and thus displays an image , whereby a large - screen image can be obtained on the screen 74 . a discharge lamp lighting device 80 controls starting up and lighting up the high - pressure discharge lamp 78 . referring back to fig1 , symbol 1 denotes a power supply input terminal ; 2 , an mos - fet ; 3 , a diode ; 4 , a choke coil ; 5 , a capacitor ; 6 , 7 , resistors ; 8 , 9 , 10 , 11 , mos - fets ; 12 , a resistor ; 13 , a discharge lamp ; 14 , an igniter circuit ; 15 , an arithmetic processing circuit ; 16 , 17 , low - pass filters ( lpfs ); 18 , a pwm controller ; 19 , an on / off signal input terminal of the pwm controller 18 ; 20 , a control voltage input terminal of the pwm controller 18 ; 21 , a driver of the mos - fet 2 ; 22 , a driver of the mos - fets 8 , 9 , 10 , 11 ; 23 , an on / off signal input terminal of the driver 22 ; 24 , 25 , input terminals of the driver 22 ; 26 , a lamp - on signal input terminal ; 27 , a low - power mode signal input / serial data receiving terminal ( hereinafter , referred to as rxd ); and 28 , a serial data transmitting terminal ( hereinafter , referred to as txd ). the mos - fet 2 , the diode 3 , the choke coil 4 , the capacitor 5 , the driver 21 , and the pwm controller 18 constitute a power control circuit 30 . the mos - fets 8 , 9 , 10 , 11 , and the driver 22 constitute an alternating - current ( ac ) conversion circuit 31 . the igniter circuit 14 generates high - voltage pulses and starts the high - pressure discharge lamp 13 . the arithmetic processing circuit 15 is constructed of , for example , a microcomputer . the arithmetic processing circuit 15 includes a bi - directional communication unit which conducts bi - directional communications with an exterior of the discharge lamp lighting device 80 , and is adapted to control the discharge lamp lighting device 80 in accordance with a required command received via the bi - directional communication unit . one embodiment of a bi - directional communication unit is a unit using uart communication . the circuit 15 detects an output voltage from a voltage divided in the resistors 6 , 7 , and further detects an output current from a voltage generated in the resistor 12 . in accordance with detection results on the above - mentioned output voltage and output current , the arithmetic processing circuit 15 also computes the output voltage and then controls this voltage by applying a limiting voltage to the control voltage input terminal 20 of the pwm controller 18 to ensure a constant output voltage . additionally , the arithmetic processing circuit 15 compares the above - described detection results with limit values lv 1 and lv 2 determined inside the processing circuit 15 . here , lv 1 signifies an output voltage limit value and lv 2 signifies an output current limit value . if the above - detected output voltage is in excess of lv 1 , a signal is transmitted to both the on / off signal input terminal 19 of the pwm controller 18 and the on / off signal input terminal 23 of the driver 22 to stop the discharge lamp lighting device . if the above - detected output current is in excess of lv 2 , a control voltage is applied to the control voltage input terminal 20 of the pwm controller 18 so that the output current will be limited by a current value determined by lv 2 . in both cases , the pwm controller 18 is thus controlled . next , the basic operation of a typical discharge lamp lighting device is described below . first , the way the high - pressure discharge lamp 13 is started up is described referring to fig3 . fig3 is a timing chart explaining how an output voltage changes from the time the discharge lamp lighting device receives an input from the lamp - on input terminal 26 , to the time the discharge lamp enters a stable lighting state . in fig3 , “ lamp - on signal ” denotes a change in a lamp - on signal received from the lamp - on input terminal 26 . at a time “ t 0 ”, when the lamp - on signal is received and enters an active hi ( high ) state ( see fig3 ), a maximum voltage v 3 is output as an output voltage of the power control circuit 30 since the lamp 13 is not on . when a high - voltage pulse from the igniter circuit 14 is further superimposed on the above - mentioned voltage v 3 , a voltage v 4 is applied to the high - pressure discharge lamp 13 , thus starting up the lamp . next , at a time “ t 1 ”, high - voltage small - current glow discharge is started , and this state further changes to high - voltage small - current arc discharge at a time “ t 2 ”. the lamp voltage increases with increases in a temperature of the lamp . at a time “ t 3 ”, the ac conversion circuit 31 starts operating and the high - pressure discharge lamp 13 changes to an ac lighting mode . after this , when a stationary voltage v 4 is reached at a time “ t 4 ”, the power control circuit 30 supplies constant electric power to the high - pressure discharge lamp 13 by activating constant - power control . the frequency of a rectangular wave from “ t 3 ” onward is generally called the inverter frequency . operation modes of the discharge lamp after it has been lit up ( i . e ., after “ t 4 ” in fig3 ) are described next . there are typically four operation modes of the discharge lamp : ( 1 ) an “ off ” mode in which the lamp is off , ( 2 ) a stationary power mode in which the lamp is normally on , ( 3 ) a low - power mode in which the lamp is lit up with power suppressed below that of the stationary power mode , and ( 4 ) an extremely - low - power mode in which , when the stationary power mode or the low - power mode is changed to the “ off ” mode , the lamp is lit up with the power reduced to , for example , about 30 % of its original level and this state is maintained . in the low - power mode , effects such as noise reduction can be obtained since it is possible , by lighting up the lamp with the power suppressed to , for example , about 80 % of the power level used in the stationary power mode , to suppress power consumption and thus extend lamp life and to reduce a rotating speed of a lamp fan . it is understood that in the extremely - low - power mode , when the lamp changes from its “ on ” state to an “ off ” state , power is temporarily maintained at a very low level , not immediately changed to a power level of 0 , for reduced electrode deterioration and hence for longer lamp life . a timing chart of the above operation modes is shown in fig4 . in fig4 , operation starts from the “ off ” mode , and then changes to the stationary power mode on lighting , and after temporarily changing to the low - power mode , returns to the stationary power mode . finally , the operation mode changes to the “ off ” mode . the four modes of the lamp are each identified by a combination of two bits , one for a lamp - on signal entering the input terminal 26 of the arithmetic processing circuit 15 , and the other for a low - power mode signal entering the input terminal 27 . ( hereinafter , for the sake of convenience in description , these signals are referred to as the signals 26 , 27 .) more specifically , as listed in fig4 , when the combination of the lamp - on signal 26 and the low - power mode signal 27 is ( low , hi ), this denotes the “ off ” mode . likewise , ( hi , hi ) denotes the stationary power mode , ( hi , low ) the low - power mode , and ( low , low ) the extremely - low - power mode . when operation changes from the stationary power mode or the low - power mode to the extremely - low - power mode , the power level momentarily changes , for example , from 100 % ( or 80 %) to 30 %, and this change is likely to cause electrode deterioration . therefore , as indicated by the dotted - line arrow in the lamp power level transition diagram of fig4 , a change period of about several seconds may be provided for power to be reduced gently when operation changes from the stationary power mode or the low - power mode to the extremely - low - power mode . a further life - extending effect can be obtained as a result . hereinafter , the mode during such a change period is referred to as a slow extremely - low - power mode . the basic operation of the discharge lamp lighting device has been described heretofore . next , description is given of the uart communication control featuring the present embodiment . uart communication is full - duplex communication during which data can be transmitted and received simultaneously . it is an asynchronous communication scheme in which data is transmitted with a start bit and a stop bit appended to the front and rear , respectively , of the data . the rs - 232c communication using a personal computer is a typical example . fig5 shows an example of a uart communication command format , in which rxd denotes command data sending and txd denotes command data receiving . in both cases , one command is constituted of 1 start bit , 1 stop bit , 8 data bits , and 1 parity bit . the rxd and txd here are equivalent to the low - power mode signal rxd 27 and txd 28 shown in fig1 . the use of rxd requires care since it is also used as a low - power mode signal . for uart communication , when a command is not yet transmitted , both rxd and txd need to be at a “ hi ” level as in fig5 . therefore , although uart communication is possible in the stationary power mode and “ off ” mode where the low - power mode signal rxd 27 becomes “ hi ”, the uart communication is not possible in the low - power mode and extremely - low - power mode where the low - power mode signal rxd 27 becomes “ low ”. next , such control functions as listed in table 1 below are assigned to different types of command data . commands 30 h to 33 h , where h stands for hexadecimal notation , set the inverter frequency to predefined values . the command 30 h , for example , activates the arithmetic processing circuit 15 to control the ac conversion circuit 31 so that the inverter frequency is 150 hz . since the inverter frequency can be arbitrarily changed in this manner , a life - extending effect can be obtained by , for example , optimizing the inverter frequency according to a particular usage time of the lamp . for a command 34 h , the arithmetic processing circuit 15 controls power so that before operation changes to the extremely - low - power mode mentioned above , the operation enters a slow extremely - low - power transition mode . next , the on / off operation of external synchronization using commands 36 h and 37 h is described . external synchronization means causing the inverter frequency and power superimposition to be synchronized with respect to a trigger signal received from an exterior of the discharge lamp lighting device . fig6 shows how the external synchronization is established . in general , the external trigger signal is superimposed on the lamp - on signal and input to the discharge lamp lighting device . when the lamp is on ( i . e ., in the stationary power mode or low - power mode of fig4 ), the lamp - on signal is “ hi ”, and when the synchronization is established , the lamp changes to “ low ” ( i . e ., a lamp - on signal a in fig6 is generated ). the arithmetic processing circuit 15 controls the ac conversion circuit 31 so that an ac driving function operates at the falling edge of the lamp - on signal a . however , malfunction results if the lamp - on signal a in fig6 is used intact to identify the operation mode . more specifically , during a superimposing period of the external trigger , the lamp - on signal is maintained at a “ low ” level and the “ off ” mode persists as the operation mode . to avoid the inconvenience , the lpf 17 is inserted on a route of the lamp - on signal and the results obtained by filtering with the lpf are integrated , whereby a signal of a substantially “ hi ” level , such as a lamp - on signal b of fig6 , can be obtained . thus , malfunction can be avoided by using this lamp - on signal b for mode identification . the same also applies to the low - power mode signal rxd 27 . using the low - power mode signal rxd 27 intact for mode identification causes malfunction since , when a command is transmitted , there exists a period during which the signal becomes “ low ”. to avoid this , the lpf 16 is inserted on a route of the low - power mode signal rxd 27 and the results obtained by filtering with the lpf are integrated . as described above , according to the present embodiment , inverter frequency setting , slow extremely - low - power control , external synchronization control , and the like can be performed by conducting uart communication control of the discharge lamp lighting device . next , an example of circuit composition according to a second embodiment of the present invention is shown in fig7 . the present embodiment is characterized in that multiple lamps can be lit up with one discharge lamp lighting device by providing an involatile memory such as an eeprom , storing multiple sets of setup data in the memory , and modifying desired sets of setup data according to a difference in the types of lamps to be connected . additionally , it is possible to accommodate sudden changes in design and to improve development efficiency , by making the internal setup data of the eeprom modifiable . in fig7 that shows the circuit composition according to the second embodiment of the present invention , the same symbol is assigned to each of sections equivalent to those of fig1 which shows an example of the circuit composition according to the first embodiment . the composition in fig7 differs in that an eeprom 32 and a dip switch 33 that allows “ hi ”/“ low ” output selection are provided . description of all other sections is omitted since each is the same as in the first embodiment . the eeprom 32 is connected to an arithmetic processing circuit 15 by a three - wire serial bus or the like , and is capable of reading out and writing in data . further , various sets of setup data likely to require modification according to lamp types or during a development and design phase are saved in a split form in multiple internal regions of the eeprom 32 . fig8 shows one such example , in which two types of setup data regions , 32 a and 32 b , are provided . for example , when a lamp manufactured by company a is to be used as a lamp 13 , data is read in from the setup data region 32 a , and when a lamp manufactured by company b is to be used , data is read in from the setup data region 32 b . the dip switch 33 is used to select either of the setup data regions . when an output of the dip switch 33 is “ hi ”, data is read in from setup data region 32 a , and when the output of the dip switch 33 is “ low ”, data is read in from setup data region 32 b . if three or more setup data regions are to be set , the number of bits in the output of the dip switch 33 can be increased according to the number of setup data regions desired . next , a specific example of setup data is shown in table 2 below . the setup data in table 2 is a specific example of data settings in one setup data region . the settings are : ( 1 ) a load current limit value , ( 2 ) a slow extremely - low - power duration , ( 3 ) an inverter frequency , ( 4 ) an extremely - low - power level value , ( 5 ) an overvoltage limit value , ( 6 ) a low - voltage limit value , ( 7 ) an overpower limit value , ( 8 ) a temperature limit value , ( 9 ) an input voltage limit value , ( 10 ) a pulse - superimposing height ratio , and ( 11 ) a pulse - superimposing width . details of these settings are as shown in table 2 , and further detailed description of the settings is omitted . in the present embodiment , setup data within the eeprom can be read / written from an exterior of the discharge lamp lighting device via uart communication . table 3 below an exemplifies uart commands associated with eeprom data reading / writing . fig9 to 12 each show an example of a uart communication protocol fig9 shows an example of a protocol for 1 - byte data writing into the eeprom . first , a command 50 h is transmitted from an external device to the discharge lamp lighting device . the arithmetic processing circuit 15 of the discharge lamp lighting device receives the command and returns the same command 50 h to the external device . next , the arithmetic processing circuit 15 receives an address and data , and similarly to the above , returns the same address and the same data . after this , the arithmetic processing circuit 15 writes the data into a specified address of the eeprom 32 , thus completing the operation . fig1 shows an example of a protocol for 1 - byte data reading from the eeprom . first , a command b 0 h is transmitted from the external device to the discharge lamp lighting device . the arithmetic processing circuit 15 of the discharge lamp lighting device receives the command and returns the same command b 0 h to the external device . next , the arithmetic processing circuit 15 receives an address and similarly to the above , returns the same address . after this , the arithmetic processing circuit 15 reads data from a specified address of the eeprom 32 and stores the data . finally , the arithmetic processing circuit 15 receives a data request command 00 h and returns the stored data . fig1 and 12 show examples of protocols for respectively writing and reading multiple bytes of data . the operation in these figures is substantially the same as that of fig9 and 10 , except that a command specifying the number of sets of data to be read / written is transmitted after an address has been transmitted and received . data as much as there actually are bytes in the above command is transmitted and received . the transmitted address is a starting address of the data . the address is incremented by 1 with each additional set of data . the dip switch 33 may be a slide switch or a rotary switch or may be merely set by means of resistor wiring . next , an example of circuit composition according to the third embodiment of the present invention is shown in fig1 . the present embodiment is characterized in that an operating state of a discharge lamp lighting device can be inquired about via uart communication . in fig1 that shows the circuit composition according to the third embodiment of the present invention , the same symbol is assigned to each of sections equivalent to those of fig1 which shows an example of the circuit composition according to the first embodiment . the composition in fig1 differs in that a frequency - measuring circuit 35 is provided . description of all other sections is omitted since each is the same as in the first embodiment . table 4 below exemplifies a command associated with inquiry from an external device . for example , when a command a 0 h is transmitted from the external device to the discharge lamp lighting device , an arithmetic processing circuit 15 returns an inverter frequency currently being used . when a command a 1 h is transmitted , the frequency - measuring circuit 35 measures an output , so - called chopper frequency , of a pwm controller 18 provided in a power control circuit 30 , and the arithmetic processing circuit 15 receives frequency measurement results and returns the results to the external device . the frequency - measuring circuit 35 is constructed of , for example , a counter circuit , and when the number of pulses during a period of one second is counted , this count denotes the frequency . when a command 82 h is transmitted , the arithmetic processing circuit 15 returns a present state of the discharge lamp lighting device . if an error is not occurring , a command 00 h is returned . if an error is occurring , a command associated with the error is returned . for example , even after an “ off ” mode has been set as an operation mode , if the power control circuit 30 generates an output voltage , a command 0 eh is returned since a lamp voltage error is judged to have occurred . when the operation mode is a stationary power mode or a low - power mode , if lamp power exceeding a limit value is supplied , a command 0 fh is returned since a lamp overpower is judged to have occurred . the above inquiry command is only an example , and the command may be extended when any other state of the discharge lamp lighting device is to be examined . while the second and third embodiments have heretofore been described assuming the use of the eeprom 32 as an involatile memory , the present invention is not limited by these embodiments and a flash rom or the like may be used instead . further , although the uart scheme has been used for communication , three - wire serial communication or other communication schemes may be used instead . as described above , the discharge lamp lighting device of the present invention can be improved in added value by , during operation , modifying various data settings , and confirming states of the discharge lamp lighting device , by means of uart communication control . in addition , multiple lamps can be lit up with one discharge lamp lighting device by providing an involatile memory such as an eeprom , saving multiple sets of setup data in the memory , and modifying desired sets of setup data according to a difference in the types of lamps to be connected .