Introduction
Chapter 1
NI-488.2M UM for Windows NT
1-2
© National Instruments Corp.
GPIB Addressing
All GPIB devices and boards must be assigned a unique GPIB address. A GPIB address
is made up of two parts: a primary address and an optional secondary address.
The primary address is a number in the range 0 to 30. The GPIB Controller uses this
address to form a talk or listen address that is sent over the GPIB when communicating
with a device.
A talk address is formed by setting bit 6, the TA (Talk Active) bit of the GPIB address.
A listen address is formed by setting bit 5, the LA (Listen Active) bit of the GPIB
address. For example, if a device is at address 1, the Controller sends hex 41 (address 1
with bit 6 set) to make the device a Talker. Because the Controller is usually at primary
address 0, it sends hex 20 (address 0 with bit 5 set) to make itself a Listener. Figure 1-1
shows the configuration of the GPIB address bits.
Bit
Position
7
6
5
4
3
2
1
0
Meaning
0
TA
LA
GPIB Primary Address (range 0 to 30)
Figure 1-1. GPIB Address Bits
With some devices, you can use secondary addressing. A secondary address is a number
in the range hex 60 to hex 7E. When secondary addressing is in use, the Controller sends
the primary talk or listen address of the device followed by the secondary address of the
device.
Sending Messages Across the GPIB
Devices on the bus communicate by sending messages. Signals and lines transfer these
messages across the GPIB interface, which consists of 16 signal lines and eight ground
return (shield drain) lines. The 16 signal lines are discussed in the following sections.
Data Lines
Eight data lines, DIO1 through DIO8, carry both data and command messages.
