BusWorks® Model 989EN U
ser’s Manual Modbus TCP/IP Digital I/O & Counters
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Input Timer Limit:
The timer limit sets the maximum allowable value
between pulses (0-65535ms). Once the limit is reached, the timer
value is set to a programmable value (0-65535).
Input “Wetting” Current (2mA, 16mA, or OFF):
Input wetting current
sources are used to sense contact closure and act like I/O pull-ups to
excitation. Wetting current is set to 16mA by default. This current is
drawn from the external excitation supply. This current can be set to
provide a higher initial current level during contact transfer to help keep
the external switch contacts from building up oxides that can form on
the contact surface and eventually impede connection. The input
wetting current can be set to 2mA continuous, or 16mA/20ms (default,
switches from 2mA to 16mA for 20ms as the input crosses the 4V
threshold, then returns to 2mA). This current can also be disabled by
tri-stating the inputs. A built-in 20ms timer controls the amount of time
the current stays at 16mA before switching back to 2mA in order to limit
power dissipation. In addition, the use of excitation below 8V will cause
the wetting current levels to be reduced.
Input “Tri-State” (Disables Wetting Current):
Inputs may be set to a high
impedance state via the Input Tri-State register. In this state, the
wetting current sources are disabled and the input acts like a simple
comparator with a 4.0V threshold. Unused inputs in the high
impedance state should not be allowed to float, as they may indicate the
wrong I/O state. External pull-up resistors may be needed for some
applications. You can also enable open-load detection to pull-down
unused tri-stated inputs.
Input Thermal Limiting:
With multiple switch inputs closed, high ambient,
and many wetting currents set to 16mA, there is a small potential for
excessive input port power dissipation. When dissipation is such that
the input port die temperature rises above the thermal limit (155°C to
185°C), a built-in temperature monitor will generate an interrupt, force
any 16mA current sources to 2mA continuous, set the thermal flag bit in
the input state register, and still maintain operation. The input thermal
bit will be cleared when the die temperature has cooled ~10°C below
the thermal limit (with 5-15°C of hysteresis range). The device will then
return to its previous programmed settings.
Input Excitation:
Inputs will not operate without excitation.
Excitation
is normally connected between the port Exc and Return terminals and
must be a DC voltage from 8 to 28V DC (31V Maximum). The tandem
outputs may temporarily shut down for excitation voltages above 28V.
The excitation input includes transient protection and filtering. Note that
the input wetting currents are sourced from the excitation supply and
your input signal maximum is determined by your excitation voltage
level.
Input Scan Time:
Input updates are obtained every 1ms, typical.
All 989EN models include 16, open-drain, mosfet switches with a
common source. Outputs are connected in tandem with buffered inputs
(see above). Outputs include transient voltage suppression and are
intended for low-side, DC voltage switching applications only. Outputs
also include integrated over-voltage, over-temperature, and over-
current protection (see below). An external excitation supply connected
between port Excitation (EXC) and Return (RTN) terminal is required
for output operation. The open-drain outputs can operate without
external pull-ups as the tandem input circui
t includes “wetting current”
generators (integrated pull-ups). You can add pull-ups to increase
output current drive capability beyond 2mA/channel.
SPECIFICATIONS
Digital Inputs
Note: For input-only
operation, the
corresponding tandem
output channel must be
turned OFF to avoid
contention with an
externally driven input
signal.
