16 Keysight E1458A 96-Channel Digital I/O Module User GuideConfiguring the Keysight E1458A Digital I/O Module Enabling the Data Line Pull-up NetworkEnabling the Data Line Pull-up NetworkThe characteristics of the data lines on each port are determined by theSN75ALS160 bus transceiver chip (see Figure 3-3). This chip was designed todrive the IEEE 488 bus, which is a proven high reliability communication bus. Thedata lines of each port contain a terminating network that is always present. Thisterminating network has the characteristics of being high impedance when thepower to the module is off. This ensures that when power is removed from themodule, the data lines do not load what is connected to them. With powerapplied, the terminating network is equivalent to the 3k/6.2k resistive networkthat is shown in Figure 2-3.The resistive terminating network provides a pull-up to a logic high level. Inaddition to the resistive termination, an active pull-up to TTL levels for outputsmay be enabled by the pull-up enable switches.Figure 2-3 shows the location of the pull-up enable switches and a detail of asingle pull-up enable switch setting. The factory-shipped default ispull-up disabled for all ports.The data lines may be either outputs or inputs. When the data lines are outputsand the switch is in the enabled position, the outputs will be forced high by boththe resistive termination and the active pull-up. When the data lines are outputsand the switch is in the disabled position, the outputs will be forced high by onlythe 3k/6.2k resistive termination.When the data lines are inputs, the resistive termination network is thedominating load that must be driven. This resistive termination network, if notdriven, will go to a logic high. This characteristic may be used to sense a contactclosure to ground. When the data lines are inputs, the position of the pull-upenable switches makes no difference, as the pull-up enable is only for outputs.
