Section Four
Register Bit Descriptions
© National Instruments Corporation
4-7
GPIB-1014P User Manual
Data In Register (DIR)
VMEbus Address:
Base A 1 (hex)
Attributes:
Read Only
7
6
5
4
3
2
1
0
DI7
DI6
DI5
DI4
DI3
DI2
DI1
DI0
R
The Data In Register (DIR) is used to move data from the GPIB to the VMEbus when the interface
is a Listener. Incoming information is separately latched by this register and is not destroyed by a
write to the Command/Data Out Register (CDOR) which locates at the same address. The GPIB
Ready For Data (RFD) message is held false until the byte is removed from the DIR by an I/O read
from a VMEbus master. The Acceptor Handshake (AH) completes automatically after the byte has
been read. In RFD Holdoff mode (refer to Auxiliary Register A, later in this section) the GPIB
Handshake is not finished until the Finish Handshake (FH) auxiliary command is issued telling the
TLC to release the Holdoff. By using the RFD Holdoff mode, the same byte can be read several
times, or a GPIB Talker that is ready to provide more data can be held off until the program is
ready to proceed.
DI0 is the least significant bit of the data byte and corresponds to GPIB DIO1. DI7 is the most
significant bit of the data byte and corresponds to GPIB DIO8.
Bit
Mnemonic
Description
7-0r
DIR[7-0]
Data In Bits 7 through 0