Eia-485 - Comtech EF Data CDM-710 Installation And Operation Manual

CDM-710/L Broadcast Satellite Modem
Serial Interface Operation

9.1.2 EIA-485

A full-duplex (or 4-wire plus ground) EIA-485 is best when multiple devices are monitored and controlled. In full-
duplex EIA-485 communication, there are two separate and isolated differential-mode twisted pairs. Each
independent pair carries serial data in different directions:
• A Controller device (a PC or dumb terminal) transmits data in a broadcast mode on one of the pairs.
• Multiple Target devices are connected to this pair. All Targets receive data from the Controller at the same
time.
The Controller is the only device with a line driver connected to this pair. The Target devices only have 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 just one Target transmits
back to the Controller. Each Target has a unique address. Each time the Controller transmits, the address of the
intended Target is included in a framed packet of data. All of the Targets receive the packet, but only the intended
one replies.
The Target starts its output line driver and transmits the return data packet back to the Controller in the other
direction, on the physically separated pair.
EIA-485 (full duplex) summary:
• Two differential pairs – one pair for Controller-to-Target, one pair for Target-to-Controller.
• The Controller-to-Target pair has one line driver (Controller), and all Targets have line receivers.
• The Target-to-Controller pair has one line receiver (Controller), and all Targets have Tri-State drivers.
The use of Half-duplex (2-wire plus ground) EIA-485 is possible, but is not preferred.
9–2
MN/CDM710.IOM
Revision 12