Basic Protocol - Comtech EF Data CDM-750 Installation And Operation Manual

CDM-750 Advanced High-Speed Trunking Modem
Appendix D
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 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.
D.4

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. The baud rate may vary between 1200 and 38,400 baud.
All data is transmitted in framed packets. The controller is assumed to be 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.
D–2
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MN-CDM750