Measurement Specialties PMD-1024HLS - Manual (Page 19)

Chapter 4
Specifications
Typical for 25 °C unless otherwise specified. Specifications in italic text are guaranteed by design.
Digital input/output
Table 4-1. Digital I/O specifications
Digital input type 74ACT373
Digital output type 74FCT244
Number of I/O 24 (port A0 through port C7)
Configuration 2 banks of 8 and 2 banks of 4 or
3 banks of 8
Pull up/pull-down configuration Internal 47K resistors may be user configured for pull-up or pull-down via
external connection of "Port x Pull-up / Pull-down" to "USB +5 V" or
"GND". Ports A, B, and C are independently configurable.
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)
ƒ
Self -powered hub
ƒ Externally-powered root port hub
Maximum = 15 mA per output
Source current – (Note 2)
ƒ
Bus-powered hub
ƒ Battery-powered root port hub.
Per pin maximum = 40 mA/ [number of active outputs].
15 mA max source current for any single output
Sink current - (Note 3) Current sink max: 440 mA/ [number of outputs].
64 mA max sink current for any single output.
Power up/reset state Input mode (high impedance)
Note 1:
Note 2:
Note 3:
"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 PMD-1024HLS can source their per-pin maximum of 15 mA. This
provides a total requirement of 15 mA*24 = 360 mA. Combining this with the PMD-1024HLS
operating current of 60 mA a fully-loaded current draw of 420 mA is realized.
"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 you have two options when using a PMD-1024HLS. In order to
take advantage of the full drive capability of 15 mA per pin you will need to purchase a
self-powered hub. Otherwise the total output drive must be de-rated. When de-rated, the
total available source current for the digital outputs is the difference between the unloaded
operating current of the PMD-1024HLS (60 mA) and the 100 mA overall maximum.
A low-side resettable fuse protects the PMD-1024HLS. 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 PMD-1024HLS unloaded operating current (60 mA) in your
power budget.
4-1