Chapter Three
Operation
Page 47
© April 2007
The configurable Digital I/O status and alarm options are summarized below.
Shared Digital I/O Status
The Digital I/O is a shared pin on the unit. As an output, it is similar to
an open collector transistor. As an input, a short to ground is detected as
ON. When the Digital Output is driven low, the Digital Input will detect
an ON condition. By clicking on the button provided, the Digital Output
may be switched ON and OFF, provided Communications Failure has
previously been set to no action.
Digital Output on
Communications Failure
The digital output can be configured here to perform various actions
when a link is present. Select Set Output to drive the open collector
output when there is no radio link. Select Drop Output to drive the open
collector output when a radio link is present.
Duty Cycle Alarm
This field is visible only on Access Points. Select Set Output to drive the
open collector output when Duty Cycle Alarm Threshold has been
exceeded. Select Drop Output to drive the open collector output when
Duty Cycle Alarm Threshold has NOT been exceeded.
The 805U-E also provides Modbus TCP Client and Modbus TCP Server functionality for I/O
transfer. 5000 x 16bit general purpose registers are provided for Modbus (including the
onboard Digital Input/Output) and are shared for both Client and Server. Modbus TCP Client
(Master) and Modbus TCP Server (Slave) are both supported simultaneously, and when
combined with the built in Modbus TCP to RTU Gateway the 805U-E can transfer I/O
to/from almost any combination of Modbus TCP or RTU devices.
The layout of the I/O registers is summarized in the table below. Each register is internally
saved as a 16 bit value. A Modbus transaction may access the entire 16 bit value of any
register, or alternatively the most significant bit of a register may be accessed as a discrete
value. The main use for the general purpose I/O registers is for intermediate storage, i.e.
when transferring I/O from one Modbus Slave device to another. Also provided is the status
of the onboard digital I/O. The 16 bit status register contain the value FFFF(hex) for ON and
0000(hex) for OFF. Inverted status registers are also provided where the registers contain
0000(hex) for ON and FFFF(hex) for OFF.
Registers
Purpose
1 – 4299
General purpose I/O registers (read/write)
4300
On-board Digital Input value (read only)
4301
Reserved
4320
On-board Digital Output value (read/write)
4370
On-board Digital Input inverted value (read only)
4371-4999
Reserved for future use
Modbus TCP Client (Master) enables the 805U-E to connect to one or more Modbus TCP
Servers (Slaves). All Modbus Master messages are directed either to/from the onboard I/O
registers depending on configuration (described below). The Modbus TCP Client may also
poll Modbus RTU (i.e. serial) devices connected to either the local serial port or a remote
805U-E serial port by enabling the Modbus TCP to RTU gateway at the corresponding serial
