Measurement Computing USB-DIO24H/37 - Manual (Page 15)

15
Chapter 4
Specifications
Typical for 25°C unless otherwise specified.
Specifications in italic text are guaranteed by design.
Digital input/output
Table 1. Digital I/O specifications
Digital input type 74ACT373
Digital output type 74FCT244
Number of I/O 24 (Port A Bit 0 through Port C Bit 7)
Configuration 2 banks of 8 and 2 banks of 4, or
3 banks of 8
Pull up/pull-down configuration Internal 47K resistors configured for pull-up to +5 V
Input high voltage 2.0 V min, 5.5 V absolute max
Input low voltage 0.8 V max, –0.5 V absolute min
Output high voltage (IOH = -15 mA) 2.4 V min
Output low voltage (IOL=64 mA) 0.55 V max
Source current – (Note 1)
Self -powered hub
Externally-powered root port hub
Maximum = 15 mA per output
Source current – (Note 2)
Bus-powered hub
Battery-powered root port hub.
Not supported
Sink current – (Note 3) Current sink max: 365 mA / [number of outputs].
64 mA max sink current for any single output.
Power up/reset state Input mode (high impedance)
Note 1: "Self-powered hub" refers to a USB hub with an external power supply. Self-powered hubs allow a
connected USB device to draw up to 500 mA. "Root port hubs" reside in the PC’s USB Host
Controller. The USB port(s) on your PC are root port hubs. All externally powered root port hubs (i.e.
desktop PC’s) provide up to 500 mA of current for a USB device. In this configuration, all 24 digital
outputs of the USB-DIO24H/37 can source their per-pin maximum of 15 mA. This provides a total
requirement of 15 mA*24 = 360 mA. Combining this with the USB-DIO24H/37 operating current of
135 mA a fully-loaded current draw of 495 mA is realized.
Note 2: "Bus-powered hub" refers to a USB hub that derives power directly from the USB +5 V and does not
have its own power supply. These hubs allow a connected USB device to draw up to 100 mA. Battery-
powered root port hubs provide 100 mA or 500 mA, depending upon the manufacturer. A laptop PC
that is not connected to an external power adapter is an example of a battery-powered root port hub. If
your laptop is constrained to the 100 mA maximum a USB-DIO24H/37 is not guaranteed to work. In
order to use the product you will need to purchase a self-powered hub.
Note 3: A low-side resettable fuse protects the USB-DIO24H/37. This is designed to protect the host PC or hub
from an over current condition. Assuming all return currents in sinking applications return via the USB
cable ground signal, the maximum allowable return current is 500 mA. Please include the USB-
DIO24H/37 unloaded operating current (135 mA) in your power budget.