Configuration via PSI-CONF
104275_en_c00
PHOENIX CONTACT
3-7
Data transmission mode
–
Character-based
data transmission is suitable for protocols where the interval be-
tween individual characters on the physical layer plays a lesser role. This is the case,
for example, when transmitting a file via an RS-232 interface or with the Modbus
ASCII protocol.
For character-based protocols, characters may also be transmitted individually or in
fragments via the SHDSL modems. Since fragments can be transmitted, the length
of a data packet that is to be transmitted in this mode may be of any size. In the case
of these protocols, it is first determined on a higher layer whether and when a data
packet has reached the end or whether an error has occurred.
–
Frame-based
data transmission must be selected for many fieldbus systems, for ex-
ample, in the case the Modbus RTU protocol.
For these protocols it is important that the interval between the characters in a frame
is limited, so that a frame can still be recognized as such. Here the SHDSL modems
transmit the entire frame in one go. The modems support frames up to a maximum
length of 504 characters.
There are further setting options here:
T
IdleMin
and
T
FrameEnd
–
The
T
IdleMin
parameter relates to the minimum time that must elapse between
two successive frames. A certain amount of time must elapse between two
frames to prevent the concatenation of multiple frames.
–
Once the data has been received by the SHDSL modem, after a specified time
has elapsed the modem assumes that the received frame has arrived in its en-
tirety and then transmits it. This time is referred to as
T
FrameEnd
.
The
T
FrameEnd
time must be shorter than
T
IdleMin
.
Handshake
Handshake (flow control) can be set for RS-232 and RS-422 interfaces.
Flow control manages the pace of data transmission between two termination devices in
a data network. In order to prevent a slow receiver being overloaded with data by a fast
transmitter, data transmission must be interrupted occasionally. This method enables
data to be transmitted as continuously as possible and without losses.
To control data flow, there are two methods for hardware or software. The hardware
method is only suitable for RS-232 interfaces.
–
In the case of the
hardware method
, additional control lines are used to control data
flow: RTS = Request to send/CTS = Clear to send. With this method, the RTS/CTS
cables of the relevant RS-232 cable must be utilized.
–
In the case of the
software method
, additional control information is added to the
user data. The software method is known as "Xon/Xoff".
Configuration >> Point-to-point wizard or Line structure wizard