Comtech EF Data CPA - Manual (Page 102)

CPA C-Band Solid State Power Amplifier Revision 1
Serial-based Remote Product Management MN/CPA-IN.IOM
7–2
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.
• Controller-to-Target pair has one line driver (Controller), and all Targets have line-receivers.
• Target-to-Controller pair has one line receiver (Controller), and all Targets have Tri-State drivers.
7.3 Basic Protocol
All data is transmitted as asynchronous serial characters, suitable for transmission and reception by a UART. The asynchronous character
format is 8-N-1 (8 data bits, no parity, one stop bit). Five baud rates are supported: 2400 baud, 4800 baud, 9600 baud, 19200 baud, and
38400 baud.
All data is transmitted in framed packets. The Controller is assumed a PC or ASCII dumb terminal that 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.