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3 Serial interface considerations
The Modbus protocol communicates with the instrumentation by means of an industry
standard serial interface. This interface may be RS-232, RS-422 or RS-485. Some systems
may also support the protocol over other busses or networks, such as Ethernet.
An RS-232 interface allows only two devices to be connected together.
RS-422 supports 1 driver and up to 10 receivers on a single network.
For bi-directional communications, special tri-state circuitry is provided. RS-485
supports up to 32 driver/receiver pairs. With special hardware, the RS-422 and RS-485
limits can be expanded to allow as many as 248 devices on a single network. Each device
on a network must have a unique address, which may be soft configured. Address zero
is reserved for broadcast messages from the host to all slaves.
All devices on a network must also be configured with the same parameters, such as
baud rate and parity. In designing the communication architecture, one should consider
communications performance when deciding how many devices to connect to a host
port. Generally, nearly twice the performance can be achieved by splitting the devices
from one port, onto two ports.
3.1 Communication mode
MODBUS protocol uses half-duplex communications, regardless of the hardware.
Half-duplex hardware shares the same lines for transmit and receive, whereas, full-
duplex hardware has dedicated transmit and receive lines.
3.2 SINGLE ENDED versus DIFFERENTIAL data transmission
Single-ended transmission is performed on one signal line, and the logical state is
interpreted with respect to ground. The main disadvantage of the single-ended solution
is its poor noise immunity.
For differential transmission, a pair of signal lines is necessary for each channel. On one
line, a true signal is transmitted, while on the second one, the inverted signal is
transmitted. The receiver detects voltage difference between the inputs and switches
the output depending on which input line is more positive. Differential data
transmission schemes are less susceptible to common-mode noise than single-ended
schemes.
RS485 ç Serial interface considerations 5
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