Comtech EF Data CRS-311 - Manual (Page 132)

CRS-311 1:1 Redundancy Switch Revision 7
Serial-based Remote Product Management MN/CRS311.IOM
6–2
characters. Control and status information is transmitted in packets of variable length, in accordance with the structure and protocol
defined in later sections.
6.2 Interface Overview
6.2.1 EIA-485
For applications where multiple devices are to be monitored and controlled, a full‐duplex (4‐wire plus ground) EIA‐485 is preferred.
Half‐duplex (2‐wire plus ground) EIA‐485 is possible, but is not preferred. In full‐duplex EIA‐485 communication, there are two
separate, isolated, independent, differential‐mode twisted pairs, each handling serial data in different directions.
It is assumed that a 'Controller' device (a PC or dumb terminal) transmits data in a broadcast mode via one of the pairs. Many 'Target'
devices are connected to this pair, and all simultaneously receive data from the Controller. The Controller is the only device with a line‐
driver connected to this pair; the Target devices have only line‐receivers connected.
In the other direction: On the other pair, each Target has a Tri‐State line driver connected; the Controller has a line‐receiver
connected. All the line drivers are held in high‐impedance mode until one – and only one – Target transmits back to the Controller.
Each Target has a unique address; each time the Controller transmits, the address of the intended recipient Target is included in a
framed 'packet' of data. All of the Targets receive the packet, but only one (the intended) will reply. The Target enables its output line
driver and transmits its return data packet back to the Controller in the other direction on the physically separate pair.
EIA‐485 (full duplex) summary:
• Two differential pairs – one pair for Controller‐to‐Target, one pair for Target‐to‐Controller.
• Controller‐to‐Target pair has one line driver (Controller), and all Targets have line‐receivers.
• Target‐to‐Controller pair has one line receiver (Controller), and all Targets have Tri‐State drivers.