National Instruments GPIB-SCSI-A, Руководство пользователя

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Appendix C Operation of the GPIB
© National Instruments Corp. C-3 GPIB-SCSI-A User Manual
on the bus, the System Controller, can make itself the CIC. The GPIB
interface board is usually the System Controller in S mode and is never the
System Controller in G mode.
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.
• Eight data lines
• Three handshake lines
• Five 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 eighth bit, DIO8*, is unused or used for parity.
Handshake Lines
Three lines asynchronously control the transfer of message bytes among
devices. 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.