Comtech EF Data MBT-4000B - Manual (Page 54)

MBT-4000B Multi-band Transceiver System Revision 1
Serial-based Remote Product Management MN/MBT4000B.IOM
6–2
6.2.1 EIA-485
For applications where multiple devices are to be monitored and controlled, a full-duplex (or 4-wire) EIA-485 is preferred. Half-duplex (2-
wire) 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 there is a ‘Controller’ device (a PC or dumb terminal), which transmits data, in a
broadcast mode, via one of the pairs. Many ‘Target’ devices are connected to this pair, which all simultaneously receive data from the
Controller. 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-Stateable 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, in a framed ‘packet’ of data, the address of the intended
recipient Target is included. 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 Pair has one line driver (Controller), and all Targets have line-receivers.
Target-to-Controller pair Pair has one line receiver (Controller), and all Targets have Tri-State drivers.
6.2.2 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.