Comtech EF Data CDM-760 - Manual (Page 236)

Serial-based Remote Product Management Revision 2
CDM-760 Advanced High-Speed Trunking Modem MN-CDM760
8–2
data in asynchronous serial form, using ASCII characters. This data consists of control and status information, transmitted in packets of variable
length in accordance with the structure and protocol explained later in this chapter.
8.2 EIA-485
For applications where multiple devices are to be monitored and controlled, a full-duplex (or 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. Multiple '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, and 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,
and 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.
o
Controller-to-Target pair has one line driver (Controller), and all Targets have line-receivers.
o
Target-to-Controller pair has one line receiver (Controller), and all Targets have Tri-State drivers.
8.3 EIA-232
This is a much simpler configuration in which the Controller device is connected directly to the Target via a two-wire-plus-ground connection.
Controller-to-Target data is carried, via EIA-232 electrical levels on one conductor, and Target-to-Controller data is carried in the other direction
on the other conductor.