Enabling The Data Line Pull-Up Network - Keysight VXI Bus 75000 C Series User And Programming Manual

Configuring the Keysight E1458A Digital I/O Module

Enabling the Data Line Pull-up Network

The characteristics of the data lines on each port are determined by the
SN75ALS160 bus transceiver chip (see Figure 3-3). This chip was designed to
drive the IEEE 488 bus, which is a proven high reliability communication bus. The
data lines of each port contain a terminating network that is always present. This
terminating network has the characteristics of being high impedance when the
power to the module is off. This ensures that when power is removed from the
module, the data lines do not load what is connected to them. With power
applied, the terminating network is equivalent to the 3k/6.2k resistive network
that is shown in Figure 2-3.
The resistive terminating network provides a pull-up to a logic high level. In
addition to the resistive termination, an active pull-up to TTL levels for outputs
may be enabled by the pull-up enable switches.
Figure 2-3 shows the location of the pull-up enable switches and a detail of a
single pull-up enable switch setting. The factory-shipped default is
pull-up disabled for all ports.
The data lines may be either outputs or inputs. When the data lines are outputs
and the switch is in the enabled position, the outputs will be forced high by both
the resistive termination and the active pull-up. When the data lines are outputs
and the switch is in the disabled position, the outputs will be forced high by only
the 3k/6.2k resistive termination.
When the data lines are inputs, the resistive termination network is the
dominating load that must be driven. This resistive termination network, if not
driven, will go to a logic high. This characteristic may be used to sense a contact
closure to ground. When the data lines are inputs, the position of the pull-up
enable switches makes no difference, as the pull-up enable is only for outputs.
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Enabling the Data Line Pull-up Network
Keysight E1458A 96-Channel Digital I/O Module User Guide