I-O Display Systems Basic I/O Product - Manual (Page 33)

Setup & Installation 2
2-16(Vol.1)
Network Type Switch:
Based upon the selected network configuration, each BASIC I/O must be setup before
communications can begin. This is done with the network switches shown below. The three
basic communication connections for individual BASIC I/Os are:
L1 Repeater
Used in RS-422 networks to extend
range to5000 feet between units
L2 Multidrop
Used in RS-422 or RS-485 networks.
Provides a total network range of 5000
feet.
L3 Last Unit
Must be used in RS-422 and RS-485
networks for the unit most distant from
the host
If there is only one BASIC I/O in a network it is designated L3.
For the network layout, the sequential display indicates the letter “L” followed by “1", “2", or
“3". As shipped, BASIC I/Os are setup as “l3", Last Unit. This parameter is not changed by
push button but is displayed as “l1", “L2", or “L3" after the dip switches have been set and the
unit has undergone a power cycle.
When the unit is configured for “L3" a network termination resistor is placed across the
differential receiver. A value of 150 Ohms was selected to suit most applications. A detailed
analysis of transmission line effects (reflections), which is beyond the scope of this document,
would be necessary to select the ideal termination resistor for any given application.
If the customer wishes to terminate the communication bus externally (at the terminal block
location) then it is imperative that no BASIC I/O network switches be set to “L3".
EIA standards dictate that the total bus impedance for RS-485 can be no less than 60 Ohms.