183 the other input of and gate 183 looks at the priority bit of the message character from opf 100 via line 182 driver 181 and line 167 since the message character from opf 100 is its first message character having a priority of zero of low priority and gate 183 remains disabled so that or gate 185 receives no input by which and gate 178 can become enabled and data capture does not occur therefore no handshake signal is returned from opf 010 to transmitting opf 100 the transmit acknowledge flip flop 42 in opf 100 remains reset and its processor module is informed via port b that data capture did not occur when the transmitter of opf 100 does not receive a handshake during its transmission time slot 100 and is thereby advised that the receiver of opf 010 failed to capture the message character sent to it opf 100 waits until its next transmission time slot 100 and repeats the previous transmission this process continues until a handshake is received or the transmission is aborted by the cpu of opf 100 as was described previously the first message character of each transmission is assigned a low priority while all subsequent characters are assigned high priority therefore when the second and all subsequent message characters are loaded into the registers 47 and 48 of opf 001 the priority bit supplied via port b is set for a high priority mode transmission the first data character defines the length of the message so that at the receiving opf here opf 010 the message buffer into which data characters are loaded may be assigned a corresponding number of addresses with the loading of data into the last assigned address indicating the completion of the message.