fig2 shows in block form a communication data switching apparatus according to the present invention . the communication data switching apparatus is particularly useful for transmitting packets of audio data in a mca system used in digital business radio services . as shown in fig2 the communication data switching apparatus includes a plurality of units 1 connected to a memory bus 2 . each of the units 1 comprises a receiver 100 , a transmitter 200 , a buffer memory 300 , and a memory transfer controller 400 . in each of the units 1 , the receiver 100 is connected directly between an input line and the buffer memory 300 , and the transmitter 200 is connected directly between an output line and the buffer memory 300 . the memory transfer controller 400 is connected directly between the buffer memory 300 and the memory bus 2 . the receiver 100 , the transmitter 200 , the buffer memory 300 , and the memory transfer controller 400 are controlled by respective control units fig2 disposed therein and a cpu ( fig2 ) which is a host device for the control units . the receiver 100 , the transmitter 200 , the buffer memory 300 , and the memory transfer controller 400 are connected to each other through the cpu . serial data supplied from the input line to one of the units 1 are received by the receiver 100 and converted thereby into bytes of parallel data which are then successively written in the buffer memory 300 . immediately after the parallel data have been written in the buffer memory 300 , the written parallel data are converted by the memory transfer controller 400 into serial data which are then transferred through the memory bus 2 simultaneously to all other units 1 . in each of the all other units 1 , the transferred data are received by the memory transfer controller 400 , converted thereby back to parallel data which are then written in the buffer memory 300 . the transmitter 200 adds a new header to the data stored in the buffer memory 300 , and converts the data from the buffer memory 300 into serial data which are then outputted from the output line . fig3 shows in block form a detailed structure of each of the buffer memories 300 shown in fig2 . as shown in fig3 the buffer memory 300 comprises a 5 - port ram which includes a selector 306 , a memory data bus 313 , and a ram 307 which is supplied with an address through the selector 306 and data through the memory data bus 313 . the selector 306 is supplied with an output signal from a memory bus address counter 301 , the sum of a predetermined unit number assigned to each of the units 1 and an output signal from a line reception address counter 302 which is supplied with an output signal from an arithmetic logic unit ( alu ) 303 to which the output signal from the memory bus address counter 301 and a predetermined bit train &# 34 ; 00000001 &# 34 ; are supplied , an output signal from a line transmission address counter 304 which is supplied with control data from the cpu , an output signal from a line transmission address latch 305 which is also supplied with the control data from the cpu , and an address directly from the cpu . the selector 306 selects either one of the supplied signals , and outputs the selected signal to the ram 307 . to the memory data bus 313 , there are connected a memory bus receiving circuit 310 for receiving data from the memory bus 2 , a memory bus transmitting circuit 312 for transmitting data to the memory bus 2 , a line receiving circuit 309 for receiving data from the input line , a line transmitting circuit 311 for transmitting data to the output line , and a cpu interface circuit 308 connected to the cpu which controls the components in the unit 1 . the ram 307 is connected to these circuits through the memory data bus 313 for being accessed thereby . the ram 307 is used as a 5 - port ram configuration in a time - division multiplex mode by these circuits . in this embodiment , the time - division multiplex mode has a basic clock frequency of 4 . 096 mhz . the time - division multiplex mode of each of the buffer memories 300 will be described below with reference to fig4 . in the time - division multiplex mode , one period is composed of 8 clock pulses which start respectively at times t 0 - t 7 . at the time t 0 , one byte of data is read from the ram 307 into the memory bus transmitting circuit 312 and converted into 8 - bit serial data thereby . the 8 - bit serial data will be transmitted to the memory bus 2 during a period from the time t 0 to the time t 7 . during a period from the time t 1 to the time t 4 , the cpu can access the ram 307 . during this period , the selector 306 selects the cpu address , and the cpu interface circuit 308 connects the cpu and the memory data bus 313 to each other . since the cpu directly accesses the ram 307 , signal editing such as for the conversion of a header for packet transmission can be carried out in the buffer memory 300 during this period . the cpu writes high - order 10 bits of addresses of the ram 307 where the initial portion of edited transmission data is stored , into the line transmission address counter 304 , and writes low - order 6 bits thereof into the line transmission address latch 305 . the line transmitting circuit 311 reads one byte of data from the ram 307 with an address indicated by the line transmission address counter 304 and the line transmission address latch 305 , converts the read byte of data into serial data , and transmits the serial data to the output line . after 8 bits corresponding to one byte of data have been transmitted , the cpu updates the line transmission address counter 304 , accesses the ram 307 at the time t 5 , reads the next one byte of data from the ram 307 , and sets the read byte of data in the line transmitting circuit 311 . as described later , transmission data in the buffer memory are written at intervals of 64 bytes . therefore , low - order 6 bits of addresses indicative of transmission data are fixed , and only high - order 10 bits thereof are updated . the line receiving circuit 309 converts each byte of serial data received from the input line into parallel data , and writes the parallel data in the ram 307 at the time t 6 . the memory bus receiving circuit 310 converts 8 - bit serial data transmitted from the time t 0 to the time t 7 into parallel data , and writes the parallel data in the ram 307 at the time t 7 . the data are put into and out of the memory bus 2 using addresses indicated by the memory bus address counter 301 . the memory bus address counter 301 is employed to establish synchronism between the units 1 . the memory bus address counter 301 comprises a 16 - bit counter which counts up every 8 bits at the basic clock frequency of 4 . 096 mhz , i . e ., at a rate of 512 khz . fig5 shows the relationship between the count of the memory bus address counter 301 and time . the count of the memory bus address counter 301 is 0 at a time 0 . the count of the memory bus address counter 301 periodically varies such that it increases with time and returns to 0 upon elapse of 128 ms . the communication data switching apparatus in the illustrated embodiment has a basic clock delivery circuit ( not shown ). the memory bus address counters 301 in the respective units 1 are arranged to have their counts fully synchronized between the units 1 by a clock signal outputted from the basic clock delivery circuit . when reception data start being received by the receiver 100 , the line reception address counter 302 loads the output signal from the alu 303 as an address in the timed relation to the start of reception of the data . the loaded address is the sum of the count of the memory bus address counter 301 and the bit train &# 34 ; 00000001 &# 34 ; which are added by the alu 303 . in fig5 the reception data start being received upon elapse of 20 ms , and their reception ends upon elapse of 50 ms . since the address at which the reception data are written in the buffer memory 300 is the sum of the transfer address of the memory bus 2 and &# 34 ; 1 &# 34 ;, the reception data are delivered to the memory data bus 313 without delay immediately after the reception data have been written in the ram 307 in the buffer memory 300 , transmitted through the memory bus transmitting circuit 312 to the memory bus 2 , and transferred to all other units 1 . the time required until the reception data are delivered to the memory bus 2 is determined by adjusting the number of bits to be added by the alu 303 . consequently , the time required until the reception data are delivered to the memory bus 2 can easily be adjusted depending on the operation of the communication data switching apparatus . fig6 shows a timing sequence of data on the lines of each unit and a timing sequence of the memory bus 2 . each of the units 1 is associated with 64 lines which transmit data at a rate of 64 kbps . there are 64 units , and each unit is connected to each of the other units through one line . all data from the units are multiplexed on the memory bus 2 and transferred at a rate of 4 . 096 mbps . each byte of data on the lines is multiplexed on the memory bus 2 , and 64 bytes of data of the lines 1 through 64 are multiplexed in a time - division multiplex mode during 125 μs . fig7 shows the allocation of addresses in the buffer memory 300 . the addresses are divided every 64 bytes , and the data of the lines 1 through 64 are written in bytes . the buffer memory 300 has a storage capacity of 64 kbytes . 64 - byte data are written in the buffer memory 300 in 125 μs in synchronism with the transfer of data in the memory bus 2 . after elapse of 125 μs , the address returns to the address 0 , and the buffer memory 300 writes data over the existing data . because the buffer memory 300 of each unit stores the data received by all the units for the past 125 μs , the transmitter 200 can search the data stored in the buffer memory 300 for data to be relayed , rewrite the header code into a header code for transmission , and transmit the data . fig8 shows in block form an arrangement of the memory transfer controller 400 for quickly searching for data to be relayed from the data stored in the buffer memory 300 . the memory transfer controller 400 comprises a header detector 401 and a 2 - port ram 402 . reception data sent from the memory bus 2 are converted by the memory bus receiving circuit 310 into parallel data , which are simultaneously written in the ram 307 and checked by the head detector 401 . when the head detector 401 detects the header of data to be transmitted , the head detector 401 uses low - order 6 bits of 16 - bit data of the memory bus address counter 301 as a portion &# 34 ; a &# 34 ; of an address of the 2 - port ram 402 , and also uses a destination code contained in the header code as the remainder of the address of the 2 - port ram 402 . in the 2 - port ram 402 , high - order 10 bits of the 16 - bit data of the memory bus address counter 301 are written as data &# 34 ; b &# 34 ; at the address thus determined . therefore , the address at which the data are written in the 2 - port ram 402 includes low - order 6 bits of the 16 - bit data of the memory bus address counter 301 at the time the header of the reception data is written in the ram 307 , the low - order 6 bits being indicative of the number of the line over which the data have been received . the address of the 2 - port ram 402 also includes the destination code of the reception data . by accessing the 2 - port ram 402 , the cpu for the transmitter 200 can read the data of the address from the ram 307 where the header of the received signal is stored from the line number and the destination code . reading data of this address allows a relaying process to be performed for quickly detecting and delivering data to be transmitted . according to the present invention , inasmuch as data received by one unit are immediately transferred to all other units , i . e ., the buffer memories associated with the transmitters connected to all the other lines , the received signal can be distributed without delay to a plurality of output lines and simultaneously transmitted . furthermore , the communication data switching apparatus can easily be designed because the time required after data have been received until the received data are delivered to the memory bus can be adjusted by the number of bits to be added . the communication data switching apparatus can be used to establish a communication data switching system . although certain preferred embodiments of the present invention have been shown and described in detail , it should be understood that various changes and modifications may be made therein without departing from the scope of the appended claims .