User guide

SATELLINE-3AS NMS / NMS Epic / NMS 869 / VHF
User Guide, Version 1.0
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6.7
TX Delay
The radio modem can be configured to delay the beginning of a radio transmission by
1...65000 ms. This feature is for preventing packet contention in a system where all substations
would otherwise answer a poll query of a base station simultaneously. During this delay data
sent to the radio modem is buffered. If this function is not needed, the delay time should be kept
as 0 ms (default value).
6.8
Sync Interval
Normally the length of a single radio transmission is not limited by the radio modem. The
transmission ends when:
• The buffer of the transmitter is empty and a pause has been detected on the serial data
coming from the DTE (the default value of the Pause length parameter is 3 bytes (~1.5 ms at
19200 bps, ~3 ms at 9600 bps)
• Or the buffer overflows.
In some applications, especially in the remote control systems, the transmission can be of infinite
length. The radio modem transmits the synchronisation sequence and the radio message
identifiers (the type of the message, addresses etc.) only in the beginning of each radio
transmission. These signals might seem redundant to the user, but they are necessary for the
receiver to be able to receive the transmitted message. Thus, if the receiver momentarily looses
the reception of the transmitted message, it will not be able to synchronise back to the same
message again. The receiver will stop reception and starts looking for the beginning of the next
radio transmission, which in theory then might never come.
To overcome this problem the radio modem has the Sync Interval setting that limits the size of a
radio frames. When the limit is reached the modem completes the current radio frame properly
and starts the transmission of a new synchronisation sequence and the radio frame.
The drawback of this function is that the use of additional synchronisation sequences adds
delays among the data transmission i.e. approximately 20ms delay will be introduced in the
data flow (depending on the data speed at the serial port) each time an extra synchronisation
sequence is transmitted. The optimum value of Sync Interval setting must be derived by
experimental testing depending on the application.