Parity checking system with reduced usage of I/O pins

A data integrity system comprising:

• a plurality of units connected together where data is transferred during a transfer period between at least three units of said plurality of units whore a unit may send and receive data to and from one or more units of said plurality of units dung a transfer period and each unit of said plurality of units has a plurality of input/output lines where only one line of said input/output lines is used as a first output line for a PARITY OUT signal indicating the parity of all data transmitted during a transfer period from said unit to all other units and for communicating said PARITY OUT signal to a parity checking means and where only one line of said input/output lines is used as a second output line for a PARITY IN signal indicating the parity of all data received during a transfer period by said unit from all other units and for communicating said PARITY IN signal to a parity checking means and where the use of only said first and second output lines by each of said units reduces the number of input/output lines used in communicating the parity of said data being transmitted or received by said unit;
• a data unit within each said unit of said plurality of units, each said data unit comprising:
• data means during a transfer period either receiving data from one or more of said other data units or transmitting data to one or more of said other data units or receiving data from one or more of said other data units and transmitting data to one or more of said other data units;
• first means, if said data unit is to transmit data to said other data units, for generating on said first output line said PARITY OUT signal indicating parity of all data transmitted during a transfer period by said data unit to said other data units;
• and
• second means, if said data unit is to receive data from said other data units, for generating on said second output line said PARITY IN signal indicating parity of all data received during a transfer period by said data unit from said other data units;
• and
• said parity checking means, located in one of s aid plurality of units, for receiving all said PARITY OUT signals and all said PARITY IN signals generated by said plurality of data units during a transfer period and for detecting from all said received PARITY IN signals and all said PARITY OUT signals whether an error had occurred during the transfer of data between said plurality of units during a transfer period.