what's wrong in this section?

[damodamodamo]

I am a newbie for using GPIB.
WhenI  read some materials for GPIB, I find something which confuses me.

" 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, and 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."

In this section above ,when transmitting data in next cycle,Controller or Talker will find the NRFD low,but NDAC high, and this  will occur  an error ?! How can this process go on transmitting next byte of data?

Who can tell what's wrong?

[GPIB_Guru]

Where did you read this? That is a very confusing explanation and seems wrong to me. The Talker waits for devices to be ready. All devices are ready to listen when they are asserting NDAC (not accepted) and are unasserting NRFD (not, not ready for data = ready for data). Then it puts the byte on the bus and asserts DAV.

The following link shows the IEEE 488.1 interlocked handshake. It is not the most explicit, but it should show enough of the steps to get an idea.
http://zone.ni.com/devzone/conceptd.nsf/webmain/FB001AA16AB22189862568080056C7EE?opendocument&node=1230_us

[damodamodamo]

Hi,GPIB Guru
I get this tutorial from :  http://www.techsoft.de/htbasic/tutgpibm.htm?tutgpib.htm
Now ,I know how the handshaking works.
Thank you very much.

Bo