Basic Serial Protocol - Comtech EF Data LPOD Series Installation And Operation Manual

5.3.2 EIA-485
For applications where multiple devices are to be monitored and controlled, 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) operation is summarized by its 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.

5.3.3 Basic Serial Protocol

Serial data can be transmitted and received by a Universal Asynchronous Receiver/Transmitter (UART).
Serial data is transmitted as asynchronous serial characters:
•
Asynchronous character format is 8-N-1 (8 data bits, no parity, 1 stop bit)
•
Baud rate can vary from 1200 through 57600 baud.
Serial data is transmitted in framed packets. All bytes within a packet are printable ASCII characters less than ASCII code 127 (DELETE).
The Carriage Return and Line Feed characters are considered printable.
The Controller device manages the monitor and control processes. It is the only device that can start data transmission at will. Messages
sent from the Controller to the Target require responses, except in these cases:
•
Acknowledging receipt of a "command" – an instruction message issued by the Controller – to change the configuration of the
Target.
•
Returning data that was requested by the Controller – response to a "query" message – that requests information from the Target.
Target devices can transmit data only when the Controller tells them to.
Serial Interface Operation
Low Power Outdoor (LPOD) Amplifier/Block Up Converter (BUC)
5–5
Revision 15
MN-LPOD