Comtech EF Data CDM-750 - Manual (Page 183)

CDM-750 Advanced High-Speed Trunking Modem Revision 2
Serial-based Remote Product Management MN-CDM750
7–3
7.3 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.
7.4 Remote Commands and Queries Overview
7.4.1 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 Universal Asynchronous Receiver/Transmitter (UART). The asynchronous character in the form 8‐N‐1 (8 data bits, no parity, 1 stop
bit). The baud rate may vary from 1200 to 38400 baud.
All data is transmitted in framed packets. The Controller is in charge of the process of monitor and control, and 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 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.
7.4.2 Packet Structure
The exchange of information is transmitted, Controller‐to‐Target and Target‐to‐Controller, in ‘packets’. Each packet contains a finite
number of bytes consisting of printable ASCII characters, excluding ASCII code 127 (DELETE).
In this context, the Carriage Return and Line Feed characters are considered printable. With one exception, all messages from Controller‐
to‐Target require a response – 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.