SATEL Proof-TR4+/R4+/TR9
User Guide, Version 2.2
22
7
SERIAL PORT
The radio modem is referred to as DCE (Data Communication Equipment) whereas the device
connected to it, typically a PLC or a PC, is referred to as DTE (Data Terminal Equipment).
In order to transfer data, the physical interface between DCE and DTE must be compatible and
properly configured. This chapter describes shortly the basics of the physical interface options,
the related settings and the operation of the serial interface.
Before connecting DTE (Data Terminal Equipment) to the radio modem, make sure that the
configuration matches the physical interface (electrical characteristics, data speed, direction
and interpretation of signals).
7.1
Pause length
The modem recognises a pause on the serial line (a pause is defined as a time with no status
changes on the RS-232 interface TD-line). The pause detection is used as criteria for:
End of radio transmission - When the transmit buffer is empty and a pause is detected, the
modem stops the transmission and will then change the radio to the receive mode.
SL command recognition - For a SL command to be valid, a pause must be detected before the
actual command character string.
User address recognition - In order for the start character to be detected, a pause must precede
it in transmission.
Traditionally, in asynchronous data communication, pauses have been used to separate serial
messages from each other. However, the use of non-real-time operating systems (frequently
used on PC-type hardware) often adds random pauses, which may result in the user data
splitting into two or more separate radio transmissions. This may cause problems especially in
the systems including repeater stations.
In order to match the operation of the radio modem to the user data, the Pause length
parameter can be adjusted on the programming menu. It may have any value between 3 and
255 characters. The default value is 3 characters.
Notes:
The absolute time of Pause length is depending on the serial port settings. For example, 1
character is ~1.04 ms at 9600 bps / 8N1 (10 bits).
The maximum absolute time is always 170 ms independent from the value of the Pause length
given in the set-up.
An increase in the Pause length increases the round trip delay of the radio link correspondingly;
this is due to the fact that the radio channel is occupied for the time of the Pause length after
each transmission (the time it takes to detect a pause). If this is not acceptable, the TX delay
setting may also be useful in special cases.