Comtech EF Data Vipersat SLM-5650A, Installation And Operation Manual

Page: 196 / 516

SLM-5650A Satellite Modem
Revision 10
Serial Remote Control 10–2 MN-SLM-5650A
10.2.2 EIA-485
For applications where multiple devices are to be monitored and controlled, a 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) 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 Target s have line-receivers.
•
Target-to-Controller pair has one line receiver (Controller), and all Targets have Tri-State drivers.
10.2.3 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 38,400 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.