Serial 0 Configured As An Rs485 Port - Cisco IR829 Hardware Installation Manual

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Serial 0 configured as an RS485 Port

The RS232 pin out follows the EIA-561 standard, however, there is no standard for an RJ-45 connector used

for RS485. In the IR809 and IR829, the pin-pairs 1,2 and 3,6 were chosen for RS485 so that an Ethernet

CAT-5 with the standard twisted-pair pin assignments can be used.

Note: RS485 usually requires 120 Ohm terminators at each end of the bus but there may be variations that

depend upon the cable type.

The following are some of the common RS485 characteristics:

• Electrical

• Implications

2 Wire RS485 (Half Duplex)

Characteristics of a 2 wire, or Half Duplex RS485 connection include:

• Bus cable with a max. length of 1200 m (4000 ft) depending upon cable quality and speed of the data

• Stub cable with a max. of 5 meters

• Multi-master capability similar to coaxial cable based Ethernet

• Inverted line is generally indicated by the index "A" or "-" (TX/RX-)

• Non-inverted line designated as "B" or "+" (TX/RX+)

• Except for trivially short point to point connections, terminating resistors are generally required.

• A ground wire is also normally used in addition to what is shown in

Figure 14: 2 Wire RS485 (Half Duplex)

Cisco IR829 Industrial Integrated Services Router Hardware Installation Guide

• Same signal levels as RS-422 except the topology is implemented as a bus rather than point-to-point

• A – B (differential signal) < -0.3V = MARK = OFF = logical 1

• A – B (differential signal) > +0.3V = SPACE = ON = logical 0

• When RS485 is implemented as a bus, effectively, this is a form of a LAN and requires a higher

level protocol. As a bus technology, you must have a protocol that controls access to the media vs.

point-to-point where you can simply stream individual asynchronous characteristics.

• Since higher level protocol are used and the technology is primarily a LAN, no MODEM control

signals are implemented in RS485.

Duplex), on page 38

Failsafe Bias Resistor

Figure 14: 2 Wire RS485 (Half

Device Cable (Maximum

of 5 m)

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