Innotech Device Network Cabling - Manual (Page 12)

© Mass Electronics Pty Ltd 2007
Innotech Device Network Cabling Installation Instructions
Edition 3.0 dated 05.12.2013 Page 12
1-1 Definition of RS485 or EIA-485
The term RS485 is outdated and was replaced with the latter specification called EIA-485 [TIA-485]
balanced (differential) communication interface however the industry still uses RS485 and it is still
used in this manual.
‘Balanced’ means that two signal wires are required to produce an alternating signal pattern. i.e. if
the [+] terminal is measuring +5V then the [-] terminal is reading 0V, conversely if the [-] terminal is
measuring +5V then the [+] terminal is reading 0V.
For reliability, when interconnecting different equipment, 3 wires are required. Typically those are
named [+, -, 0V or S]. [+/-] and are used for data. [S] is the signal reference. Instead of having an
individual third wire for [S], the shield from the Shielded Twisted Pair (STP) cable is used.
The RS485 interface is operating in ‘half duplex’ mode. Meaning only one transmitter can be active
at the time but many receivers can listen simultaneously. A period of ‘silence’ is used to free the
interface for another transmitter to become active.
Two more terms are frequently mentioned in conjunction with RS485.
1-1.1 Termination
EOL: Will be referred to as ‘End of Line’ termination, in this manual.
A resistor (load) connected between terminals [+/-] of typically 120 Ohms.
There is one resistor connected at each end of a cable.
It is sometimes user selectable or fixed internally to the device.
1-1.2 Biasing
These are referred to as ‘Pull-up/Pull-down’ or ‘Idle-line failsafe’ resistors.
A weak current limited power supply individually connected to terminals [+ and -] to define the 485
interface to be Idle or silent when no transmitter is active, which is important to operate reliably in
‘half duplex’ mode.
The idle bias level can be checked with a voltmeter measuring between terminals [+ and -].
The reading is to be greater than +/-200mV.
Ensure that there is no comms activity while measuring.
NOTE
1-1.3 Communications speed
Faster is not always better. Speed is a trade off between cable length and cable quality, link
reliability, controller processing capability, cost and data quantities.
There is no doubt that increasing network speed makes networks more difficult to set up or fault find.
Therefore it is important to take care during installations on where, how and what cables are routed
between controllers.
The selected speeds of Innotech networks of controllers and wiring instructions in this manual are
set out to achieve the best reliability.