Flow Control; Software Flow Control (Xon/Off Flow Control); Hardware Flow Control (Rts/Cts Flow Control) - Siemens TC37 Module At Command Reference

AT Command Set

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 [3].

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 command AT\Q3 - it is not sufficient to set RTS/CTS handshake in the
used Terminal program only.
The default setting of the GSM engine is AT\Q0 (no flow control) which must be altered to AT\Q3
(RTS/CTS hardware handshake on). The setting is stored volatile and must be restored each time
after the GSM engine was switched off. For further details refer to Chapter 2.3.
AT\Q has no read command. To verify the current setting of AT\Q, simply check the settings of the
active profile with AT&V.
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 32 bytes. This can be easily
handled 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. A total buffer capacity of 256 bytes
has been proved to work well.
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