BusWorks® Model 989EN U
ser’s Manual Modbus TCP/IP Digital I/O & Counters
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Acromag, Inc. Tel:248-295-0880 Fax:248-624-9234 Email:
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I/O Configuration Page: Help
–
At the bottom of the I/O Configuration Page
is a help page to provide readily available answers to questions about the
controls. Each control is described in detail as on the next page, but also
obtainable via your internet browser.
Refer to the following definitions and the Modbus Memory Map for
information on optional modes of operation:
Input Functions
Input Wetting Current 2mA/16mA
– Wetting current is used by the inputs
to sense switch contact closure and will attempt to seek a return path.
These currents also act like pull-ups for the tandem open-drain outputs,
which enables the I/O to be tested without actually wiring to the I/O
terminals. If this control is set to 2mA, the wetting current will be 2mA
continuously. If this control is set to 16mA(default), then the wetting current
is initially set to 16mA as the input crosses the threshold, and folds back to
2mA continuous, 20ms after crossing the ON threshold. The purpose of the
higher initial current is to help prevent oxide build-up on external switch
contacts.
Metallic switch contacts can develop higher contact resistance over time due
to corrosion induced by humidity, salt, and other elements that exist in harsh
environments. Use of the 16mA setting provides higher initial wetting current
during contact transfer to reduce or eliminate potential oxide build-up.
However, elastomeric switch contacts are made of carbon material and have
a high contact resistance, with common resistance up to 1K
Ω. With these
types of contacts, the pull-up/wetting current must be reduced to prevent
excessive power dissipation in the contact. High currents through these
switches can produce an IR voltage that can cause the input signal to rise
back above the 4.0V threshold. Thus, you should select 2mA for those
applications where 16mA cannot be tolerated.
If Input Tri-State is ON, this field will have no effect, as the wetting current
generators are disabled in tri-state mode.
Input Tri-State OFF (Default)/ON
– Inputs may be tri-stated and then act as
a high-impedance comparator input with a 4.0V threshold (3.7-4.3V range).
Wetting current generators are disabled with inputs tri-stated, or without
excitation connected. Note that tri-stated inputs will also leave the tandem
open-drain output floating if no load is connected. You may optionally utilize
the open-load detection pull-downs to pull these inputs to the ON state.
IMPORTANT:
If inputs are tri-stated, excitation must be above 8V, or the
output drains will float unless loaded and/or pulled up externally. If the I/O is
not pulled up above the 4.0V input threshold, the output OFF state may not
register properly, or indicate ON.
Input Reset
– Click this button to reset the inputs without invoking a power-
on, output, or system reset of the unit. You do not normally need to use this
control, but it may be useful for trouble-shooting and isolating I/O problems
between input & output circuitry.
WEB BROWSER
I/O Configuration Page
Help
I/O Configuration
The built-in wetting current
generators do not operate
without excitation, or with
inputs tri-stated.
Without a driver, load, or pull-
up connected to the I/O, the
tri-stated I/O is essentially
floating and may not register
the OFF state correctly. In this
state, you will have to use I/O
pull-ups to pull the OFF state
signal above 4.3V in order to
be able to read back the
correct I/O state.
Note that wetting currents are
reduced low excitation
voltages below 8V.
TIP: Unused tri-stated inputs
may utilize the built-in open-
load detection pull-downs to
pull to the ON state.
TIP: You can increase wetting
currents by paralleling inputs,
or connecting to an external
excitation supply and adding
an I/O pull-up to the excitation
voltage rail.
