8
2.4 Serial Communication Interfaces
Two communication interfaces are supported in the iServer: RS232 and RS485. These
standards define the electrical characteristics of a communication network. The RS485
port of the iServer is fully compatible to use with RS485 instruments. The RS485 is an
extended version of the RS422 communication standard which increases the allowable
number of devices from 10 to 32 by improving the electrical characteristics.
• The
RS232
standard (point-to-point) allows a single device to be connected to an
iServer. The iServer operates with full-duplex RS232 using eight wires: Rx-receive,
Tx-transmit, DTR, DSR, DCD, CTS, RTS and common ground wires. RS232 cable
length is limited to 50 feet.
• The
RS485
standard (multi-point) allows one or more devices (multi-dropped) to be
connected to the iServer using a two-wire connection (half-duplex) +Rx/+Tx and
–Rx/-Tx. Use of RS485 communications allows up to 31 devices to connect to the
iServer with cable length up to 4000 feet long.
Although the RS485 is commonly referred to as a "two wire" connection, the
iServer also provides a ground/return shield connection to use as a common
connection for EMI noise protection.
Table 2.2
shows the differences between RS232 and RS485 communication interfaces.
Table 2.2
Data Transmission
RS232
RS485
Characteristics
Transmission Mode
Single ended
Differential
Electrical connections
8 wire (-W); 7 wire (DIN Rail)
2 wire
Drivers per line
1 driver
32 drivers
Receivers per line
1 receiver
32 receiver
Maximum cable length
50 ft (15 meters)
4000 ft (1200 meters)
Changing between RS232 and RS485 interfaces, as well as modifying the other
parameters is possible through the iServer firmware using its home Web page or
Telnet connection (see
Part 4
for details).
