VTI Instruments
20
EMX-75XX Index
reference before data can be read or written; this voltage is used for pulling up open drain data
lines and setting reference voltage for threshold detection in input path. No voltage should be
applied on data lines before voltage selection and during voltage configuration process. User can
select a voltage from 3.3 V, 5 V, 12 V or 24 V provided on the module or supply a voltage
through front panel connector, to be used as V
CLAMP
. By default, no voltage is selected and data
lines are floating. There is also an option of configuring a port’s voltage setting as either TTL or
Low Voltage emulation. In TTL emulation setting, open drain lines are pulled up to 5 V and have
a sourcing capability of 4 mA, whereas in Low Voltage (LV) option the pull-up voltage is 3.3 V
with 4 mA drive strength.
When doing read and writes, data written to a port set for output updates data on that port, data
written to a port set for input does not have any effect. Data read from a port always reads current
status of data pins (whether driven by card or by user).
When a port’s direction is output, data is driven by the DIO on the corresponding channels and no
user voltage should be applied on those channels. When direction is input, data lines are pulled up
to V
CLAMP
using 100 k
Ω resistor and user can drive their data on the channels with a V
IN
range of
zero to V
CLAMP
.
TTL and LV emulation voltage selection is only applicable when port’s direction is output. Input
mode will not be allowed for emulation voltage.
Data in output register(s) of concerned port is reset (set to all F’s) after reset and any operation that
involves voltage configuration, as listed below:
1)
When card is reset. All ports are affected.
2)
A port’s direction is changed when its voltage is set for TTL/LV emulation. Only concerned
port is affected.
3)
Port voltage configuration is done. Only concerned port is affected.
Data in output registers is not retained if direction is changed. So for example, when an output port
that is driving AA on data lines is made input, all its channels are pulled up so that user data can
be driven but when the direction is changed back to output, FF gets driven, given that polarity and
voltage setting didn’t change.
Polarity settings are applicable irrespective of direction. On output channels, changing the polarity
flips the state of data lines and further writes are done according to polarity selection (for example,
if polarity is configured as reversed then writing 1 to a channel writes a 0 to it and vice versa).
Read back of data, whether driven by card when direction is output or driven by user when
direction is input, also checks for polarity and if polarity is reversed, 1 is read as 0, otherwise as 1
and 0 is read as 1, otherwise as 0.
Configuration Section
This section of circuit is used to select reference voltage (V
CLAMP
) for ports. Each port has a set of
programmatically controlled switches that let the user select a voltage from 3.3 V, 5 V, 12 V, 24 V
and User supplied voltage, which can be anywhere from 3.3 V to 60 V. In addition, LV and TTL
emulation modes can be selected which use 3.3 V and 5 V as V
CLAMP
respectively, and insert a low
impedance pull-up resistor in the path to source 4 mA.
Output Section
The output section of each channel consists of an N-channel MOSFET, pull-up resistors and over
current sense circuitry. When a 1 is driven on channel, the MOSFET is turned off and data line is
pulled-up to V
CLAMP
using appropriate pull-up resistor (330
Ω for LV emulation, 500
Ω for TTL
emulation and 100
kΩ otherwise
). The rising voltage level at data channel exhibits RC charging
with a rise time of approximately 70
μs with 100 kΩ pull
-up resistor and about 600 ns with low
impedance pull-up in TTL and LV voltage configurations. When a 0 is driven on channel, the
MOSFET is turned on and data line is connected to ground via a current sense resistor of 0.3
Ω.
