Comtech EF Data LPOD-R - Manual (Page 114)

LPOD-R Outdoor Amplifier / Block Up Converter (BUC) MN-LPODR
Optional Serial-based Remote Product Management Revision 1
5–4
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.
5.3.2 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.
5.3.3 Basic Protocol
Whether in EIA-232 or EIA-485 mode, all data is transmitted as asynchronous serial characters, suitable for transmission and reception
by a UART. The character format should be 8N1 (8 data bits, no parity, 1 stop bit). The baud rate may vary from 2400 to 38400 baud.
All data is transmitted in framed packets. The Controller is assumed a PC or ASCII dumb terminal, which is in charge of the process of
monitor and control. The Controller is the only device that is permitted to initiate, at will, the transmission of data. Targets are only
permitted to transmit when they have been specifically instructed to do so by the Controller.
All bytes within a packet are printable ASCII characters, less than ASCII code 127. In this context, the Carriage Return and Line Feed
characters are considered printable.
All messages from Controller-to-Target require a response – with one exception. This will be either to return data that has been requested by
the Controller, or to acknowledge reception of an instruction to change the configuration of the Target. The exception to this is when the
Controller broadcasts a message (such as Set time/date) using Address 0, when the Target is set to EIA-485 mode.