[ 0030 ] fig5 shows a system in accordance with the invention of a subscriber of an isochronous , cyclical communication system for processing consistent data blocks during a read access . the system in accordance with the invention also possesses a communication memory 7 with a receive zone 8 and a transmit zone 9 , a consistency buffer 10 , a receive buffer 11 and a transmit buffer 12 . the system in accordance with the invention differs significantly from the prior art by linking of the receive buffer 11 and the transmit buffer 12 with the communication memory 7 and the consistency buffer 10 . using the multiplexer 13 a link can be established between both the receive buffer 11 and the consistency buffer 10 and also between the receive buffer 11 and the communication memory 7 . likewise by means of multiplexer 14 an alternative link between the transmit buffer 12 and the communication memory 7 or the consistency buffer 10 can be established . the job interface 15 controls the multiplexers 13 and 14 . in the case shown the application 16 reads data from the address range ab c of the consistency block kb c in the receive zone 8 of the communication memory 7 , while data set ds c is being forwarded from receive buffer 11 to consistency buffer 10 which is actually intended for address range ab c . to ensure the consistency of the data read by the application , the data set ds c will thus be copied into the consistency buffer . the job interface 15 controls the multiplexer 13 so that there is a connection between the receive buffer 11 and the consistency buffer 10 . the read access has no effect on the forwarding of data from the transmit zone 9 to the transmit buffer 12 . the transmit zone 9 is thus connected via the multiplexer 14 to the transmit buffer 12 . the use of the consistency buffer 10 during read access is thus only necessary because the data set ds c is destined for address range ab c to which the application 16 has access or could have access . otherwise the data can be forwarded directly from the receive buffer 11 to the receive zone 8 of the communication memory 7 . the job interface 15 will then establish a connection between at the receive buffer 11 and the receive zone 8 . [ 0033 ] fig6 illustrates the sequence of read access in accordance with the invention . during read access by the application to the receive zone 8 of the communication memory 7 a data set ds c which is destined for the address range ab c of the consistency block kb c is copied from the receive buffer 11 to the consistency buffer 10 . after read access has ended the data set ds c will be copied from the consistency buffer 10 into the receive zone 8 of the communication memory 7 . data is sent and received independently of the read access . [ 0034 ] fig7 shows the system from fig5 during a write access . the address range of the consistency block kb d will be writtenby application 16 directly in the transmit zone 9 of the communication memory 7 . the data set ds d from the address range of the consistency block kb d is located in the consistency buffer 10 . it is advantageous that in transmit buffer 12 a complete set of data 17 is “ in stock ” for transmission . in stock here means that the set includes all data which is to be transmitted during the next transmit procedure . [ 0035 ] fig8 illustrates the sequence of a write access in accordance with the invention in the system of fig7 . before write access by application 16 the data set ds d will be copied from the address range ab d of the consistency block kb d which the application can or could write to during the write access from the transmit zone 9 of the communication memory 7 to the consistency buffer 10 . during write access consistent data can be transmitted from a data set ds d from the consistency buffer 10 to the transmit buffer 12 . the job interface 15 therefore connects the consistency buffer 10 with the transmit buffer 12 . if write access by the application 16 is ended before the ending of the copying process of data set ds d from the consistency buffer 10 to the transmit buffer 12 the copying process will be aborted . to guarantee the transmission of a complete data set from the transmit buffer 12 a data set 17 must therefore be kept in stock in this . after the write access data can be forwarded from the address range ab d again and from the transmit zone 9 of the communication memory 7 to the transmit buffer 12 . data which is not located in address range ab d can also be forwarded during write access from the receive zone 9 of the communication memory 9 to the transmit buffer 12 . data can be received independently of write access at the receive port and sent at the transmit buffer 12 . [ 0038 ] fig9 shows a flowchart of a read access in accordance with the invention . first of all an address range ab c in the receive zone of the communication memory is reserved by a consistency block kb c ( step 18 ). “ reservation by a consistency block ” means in this connection that data can neither be copied from the receive buffer into the address range occupied by a consistency block , nor from an address range occupied by a consistency block into the transmit buffer . the address range c includes addresses to which the application has access or could have access during a read access . in the next step ( step 19 ) the application accesses the consistency block kb c in the communication memory for reading . at the same time the data set ds c which is addressed to addresses in the address range ab c of the consistency block kb c is copied from the receive buffer to the consistency buffer . after the end of read access the consistency block kb c is released ( step 20 ). the address range ab c can now be written again with data from the transmit buffer . data which was written during the read access into the consistency buffer can finally be copied into the address range ab c of the communication memory ( step 21 ). [ 0042 ] fig1 shows a flowchart of a write access in accordance with the invention . a data set ds d in address range ab d of the transmit zone 9 of the communication memory which is written or could be written by the application during a write access will first be copied into the consistency buffer ( step 22 ). the address range ab d will then be occupied by the consistency block kb d ( step 23 ). thus data can no longer be forwarded from the address range ab d to the transmit buffer . during the write access however data of the data set ds d can be forwarded from the consistency buffer to the transmit buffer ( step 24 ). after write access has ended the consistency block kb d will be released ( step 25 ). data can again be forwarded from the address range ab d to the transmit buffer . the copying process of data of data set ds d from the consistency buffer to the transmit buffer will be aborted if it is not completed before the end of write access ( step 26 ) and replaced by the current data from the communication memory . a current data record is then copied from the address range ab d of the consistency block kb d to the transmit buffer ( step 27 ).