Specifications

Chapter 7 EIA-485 asynchronous interface 33
General EIA-485 cable information
This section applies to Digi products equipped with EIA-485 asynchronous
interfaces.
Physical cable characteristics
EIA-485 interface cables should be shielded twisted-pair cables with a
maximum of 4000 feet in length. Each signal requires two leads (one
twisted pair of wires) to complete a balanced voltage digital circuit. The
shield should be connected to the Chassis Ground of the devices at both
ends of the interface cable.
Interface signaling
The EIA-485 interface provides four signals: Transmitted Data (TxD),
Received Data (RxD), Request To Send (RTS) and Clear To Send (CTS).
The functions of these signals are identical to their EIA-232 counterparts.
Grounding requirements
EIA-485 interface cables must provide a ground path between the devices
to be connected. This ensures the integrity of data transfers and control
signals. This should be connected to the Chassis Ground of each device.
Digi recommends using the cable shield for this purpose.
Termination information
A DIP switch controls the termination of many Digi devices. Termination is
recommended in a multi-drop line for the last networked device.
Termination also biases the line.
Most Digi devices use the #4 switch to control termination. When the switch
is down, termination is off and there is no termination. To set termination on
the device, move the #4 switch to the up position.
Check your Digi device for its ohm rating; 100 ohms is the nominal setting
in Digi products.
Chapter 6
EIA-485 asynchronous interface