TC35i Module TC35i Terminal

TC35i AT Command Set
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1.6 Flow control
Flow control is essential to prevent loss of data or avoid errors when, in a data or fax call, the sending
device is transferring data faster than the receiving side is ready to accept. When the receiving buffer
reaches its capacity, the receiving device should be capable to cause the sending device to pause until
it catches up.
There are basically two approaches to regulate data flow: software flow control and hardware flow
control. The High Watermark of the input / output buffer should be set to approximately 60% of the total
buffer size. The Low Watermark is recommended to be about 30%. The data flow should be
stopped when the capacity rises close to the High Watermark and resumed when it drops below the
Low Watermark. The time required to cause stop and go results in a hysteresis between the High and
Low Watermarks.
In Multiplex mode, it is recommended to use hardware flow control. For details please refer to [4].
1.6.1 Software flow control (XON/OFF flow control)
Software flow control sends different characters to stop (XOFF, decimal 19) and resume (XON, decimal
17) data flow. The only advantage of software flow control is that three wires would be sufficient
on the serial interface.
1.6.2 Hardware flow control (RTS/CTS flow control)
Hardware flow control sets or resets the RTS/CTS wires. This approach is faster and more reliable,
and therefore, the better choice. When the High Watermark is reached, CTS is set inactive until the
transfer from the buffer has completed. When the Low Watermark is passed, CTS goes active once
again.
To achieve smooth data flow, ensure that the RTS/CTS lines are present on your application platform.
The application should include options to enable RTS/CTS handshake with the GSM engine. This
needs to be done with the AT commands AT\Q3 or AT+IFC - it is not sufficient to set RTS/CTS handshake
in the used Terminal program only. For details refer to Chapter 2.3 and Chapter 2.45.
Often, fax programs run an intialization procedure when started up. The intialization commonly includes
enabling RTS/CTS hardware handshake, eliminating the need to set AT\Q3 once again. However,
before setting up a CSD call, you are advised to check that RTS/CTS handshake is set.
Note:
After deactivating the RTS line, the ME may still send up to 264 bytes (worst case). This can
be easily managed if the buffer of the host application is sufficiently sized, and if a hysteresis
is implemented in its Rx buffer as mentioned in Chapter 1.6. For host applications that are required
to handle a large amount of data at high speed, a total buffer capacity of at least 512
bytes is recommended.
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