Micromod MOD: MODBUS RTU Communications Guide - Manual (Page 6)

MODBUS RTU
COMUNICATIONS GUIDE
2
HARDWARE REQUIREMENTS
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 on the
drivers of some instrumentation, allowing 10 driver/receiver pairs. 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 or set with switches. 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.
CABLE REQUIREMENTS
The type of wire to use is usually specified by the hardware manufacturer and will vary with required length.
Wire with twisted pairs and an overall shield is used most often. The shield is tied to earth ground or chassis,
and typically at one end only. The shield is not to be used as a signal common or ground. The table below
lists typical cable recommendations.
Table 1. Cable Requirements
RS-232
Up to 15m (50ft)
virtually any standard shielded twisted pair with drain
(Beldon 9502 or equivalent)
RS-422 and RS-485
Up to 6m (20ft)
virtually any standard shielded or twisted pair
Up to 300m (1000ft) 24 AWG twisted pair with overall foil shield with drain wire
(Beldon 9841 for 2-wire and 9502 for 4-wire or equiv.)
Up to 1200m (4000ft) 24 AWG twisted pair with foil shield and drain wire on each pair
(Beldon 9841 for 2-wire and 9729 for 4-wire or equiv.)
COMMUNICATIONS MODE
Communications can take place in full or half-duplex mode. The communications hardware must be able to
support whatever mode the software is using. Half-duplex hardware shares the same lines for transmit and
receive, whereas, full-duplex hardware has dedicated transmit and receive lines. MODBUS protocol uses
half-duplex communications, regardless of the hardware. Full-duplex hardware is more widely supported by
software drivers and devices, than half-duplex.