The three handshake lines (NRFD, NDAC, DAV) control the transfer of message bytes among the devices and form the method for acknowledging the transfer of data. This handshaking process guarantees that the bytes on the data lines are sent and received without any transmission errors and is one of the unique features of the IEEE-488 bus.
The NRFD (Not Ready for Data) handshake line is asserted by a listener to indicate it is not yet ready for the next data or control byte. Note that the controller will not see NRFD released (i.e., ready for data) until all devices have released it.
The NDAC (Not Data Accepted) handshake line is asserted by a listener to indicate it has not yet accepted the data or control byte on the data lines. Note that the controller will not see NDAC released (i.e., data accepted) until all devices have released it.
The DAV (Data Valid) handshake line is asserted by the talker to indicate that a data or control byte has been placed on the data lines and has had the minimum specified stabilizing time. The byte can now be safely accepted by the devices.
The handshaking process is outlined as follows. When the controller or a talker wishes to transmit data on the bus, it sets the DAV line high (data not valid) and checks to see that the NRFD and NDAC lines are both low, then it puts the data on the data lines.
When all the devices that can receive the data are ready, each releases its NRFD (not ready for data) line. When the last receiver releases NRFD and it goes high, the controller or talker takes DAV low indicating that valid data is now on the bus.
In response each receiver takes NRFD low again to indicate it is busy and releases NDAC (not data accepted) when it has received the data. When the last receiver has accepted the data, NDAC will go high and the controller or talker can set DAV high again to transmit the next byte of data.
Note that if after setting the DAV line high, the controller or talker senses that both NRFD and NDAC are high, an error will occur. Also, if any device fails to perform its part of the handshake and releases either NDAC or NRFD, data cannot be transmitted over the bus. Eventually a timeout error will be generated.
The speed of the data transfer is controlled by the response of the slowest device on the bus; for this reason it is difficult to estimate data transfer rates on the IEEE-488 bus as they are device dependent.
