Agriculture Reference
In-Depth Information
between two devices, most RS-232C interfaces on computers use an asynchronous
protocol. The EIA has also developed a few more standards on single-ended serial
communications interfaces to replace RS-232C for improved performance. Among
them, RS-423A is the enhanced version of RS-232C with several notable differences,
including the higher data rate (up to 100 vs. 20 Kbps), longer cable length (up to 1220
vs. 15 m), and capacity to support multiple receivers (up to 10 vs. 1) on the same line.
When an even higher data rate is required, a couple of enhanced versions of serial
interface, RS-422A and RS-485, can support the data communication rate up to
10 Mbps over cable lengths up to 1220 m by using a differential data transmission
mode on a twisted pair of wires. The RS-485 interface is basically a superset of the
RS-422A interface for being able to support up to 32 drivers and 32 receivers on the
same line attribute to an embedded high-speed switching function. One example was
the use of an RS-232C serial communication protocol on a handheld measurement
instrument for soil water content, with GPS positioning (Qiu and He, 2004).
The USB interface has quickly become the most common data communication
path between a computer and various peripherals after being introduced in the mid-
1990s. Many computer makers now have added USB ports as a standard design
for their products, and some even have eliminated the old serial and parallel ports.
A USB uses a special four-conductor cable, up to 5 m long, with two data lines to
carry two sets of data signals in an asynchronous mode and two power lines to both
supply an optional +5 V power to the connecting peripheral devices, and provide a
hot-swappable capability for safely connecting or disconnecting USB devices with-
out powering down or rebooting the computer. The data communications protocol
of a USB is based on three types of packets: token, data, and handshake. The host
(normally the computer) starts a transaction by sending out a 7-bit-long token packet
to the address on the demanding device, which allows the USB to connect peripheral
devices using a single USB port. To support higher data transfer speed requirements
for data collection and dissemination, USB 2.0 can provide a peak data transfer rate
of 480 Mbps. Newer versions of USB protocol for supporting a higher rate are also
under development. A couple of representative examples of USB application in AIS
include an embedded microwave moisture meter (Lewis and Trabelsi, 2011), and a
USB-interfaced greenhouse environment monitoring system (Cheng et al., 2006).
Agricultural equipment manufacturers worldwide have agreed on ISOBUS as the
universal protocol for electronic communication between implements, tractors, and
computers (Stone et al., 1999). An ISOBUS is a standardized Lewis communication
method between tractors and implements, and is capable of ensuring full compat-
ibility of data transfer between mobile systems and office computer systems used on
the farm. Using a pair of signal wires, a multisegment ISOBUS could link up to more
than 100 electronic control units (ECUs, often called nodes) with no more than 30
nodes on any one segment. To ensure data communication security without unneces-
sary delay in identifying the communication partner, each node is assigned a unique
identifier. Different from the data communication process in conventional networks,
an ISOBUS broadcasts data to be transferred in the form of a message consisting of
both an identifier and the transferring data to all nodes in the network. The identifier
could identify the type, the communication partner, and the priority of the message,
and function like a data highway by allowing multiple nodes to share the same set
