National Instruments GPIB-PC - Manual (Page 19)

Section One Operation of the GPIB
©National Instruments Corp. 1-3 GPIB-PC User Manual
GPIB Signals and Lines
The interface system consists of 16 signal lines and 8 ground return or
shield drain lines.
The 16 signal lines are divided into the following three groups:
• 8 data lines,
• 3 handshake lines, and
• 5 interface management lines.
Data Lines
The eight data lines, DIO1 through DIO8, carry both data and command
messages. All commands and most data use the 7-bit ASCII or ISO
code set, in which case the 8th bit, DIO8, is unused or used for parity.
Handshake Lines
Three lines asynchronously control the transfer of message bytes
between devices:
• NRFD,
• NDAC, and
• DAV.
The process is called a three-wire interlocked handshake and it
guarantees that message bytes on the data lines are sent and received
without transmission error.
NRFD (not ready for data)
NRFD indicates when a device is ready or not ready to receive a
message byte. The line is driven by all devices when receiving
commands and by Listeners when receiving data messages.