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6.2.1 Specification
Parameter Specification
RS232 transmitter output low voltage (typical) -5.5V
RS232 transmitter output high voltage (typical) +5.9V
RS232 Input threshold voltage +1.5V
RS485 differential output voltage (minimum with load resistance 120Ω) +2V
RS485 differential input signal threshold +-220mV
Maximum nodes in RS485 mode 256
RS485 termination resistor 120Ω
6.2.2 Settings
Measurements can be scheduled as a multiple of the base-interval. The fastest possible measurement rate
is achieved by setting the interval to 1 in which case measurements will occur on every base interval. To
reduce power consumption, the measurement rate can be turned down by increasing the interval.
In serial capture mode the measurement interval can be used to reduce the number of readings being
provided by a connected sensor or system that may be permanently powered. If for instance, a connected
system is sending a message every second but it is only required to be read and transmitted every minute,
the measurement interval can be set to 1 minute in which case the ORB will wake on the minute interval,
receive a message and return to sleep thereby missing the other 59 messages sent by the attached system.
Since serial packets cannot be interrogated by the ORB without a customised script, it makes sense to set
the measurement interval to the same as the transmit interval in most cases.
The serial port on the ORB can be configured as an RS232 or RS485 hardware interface using the type
option.
If RS485 mode is selected, an optional 120Ω termination resistor can be selected by selecting the Termina-
tion resistor option. The purpose of the termination resistor is to match the impedance of a transmission
line to the hardware impedance of the interface to which it is connected. Termination is generally not
required in lower speed networks (9600 baud or less) and networks shorter than 500m in length. No more
than 2 termination resistors should be used, one at each end of the RS485 transmission line.
A baud rate of 4800, 9600, 19200, 38400, 56800 or 115200 needs to be selected using the baud rate option.
Other settings, including the number of bits, odd or even parity and 1 or 2 stop bits are added in the
settings field. The most common setup is 8 bits, no parity and 1 stop bit or “8N1”.
The serial interface can be configured in serial capture mode or MODBUS mode using the mode option.
Capture mode is typically used where an external sensor sends serial data and a portion of that serial
data is to be captured. MODBUS mode is used to connect to external sensors that are compatible with the
MODBUS standard.
In serial capture mode The ORB listens for periodic data and when received, transmits this data at the next
send interval. The maximum length of a message that can be captured is 512 characters. Once 512 char-
acters have been received, the ORB will terminate the capture and will transmit it on the next transmit
interval.
In capture mode, the max-time setting can be used to set a timeout after which the serial port will return to
sleep. Max-time can be used as a way to end serial measurement in the event that no serial data is
received, or as a mechanism to allow the ORB to sample the serial port for a defined time-period.
Note If the serial port needs to be kept on all the time, set the max-time to longer than the measurement
interval. The contents of the serial buffer is retained as long as the ORB does not return to sleep.
ORB User Guide, Release 1.1
6.2. Serial Interface 59