AT Command Set
MC35_ATC_01_V05.00 Page 18 of 256 15.07.2002
1.6 Flow control Flow control is essential to prevent loss of data or av oid errors when, in a data or fax call, the sending
device is transferring data faster than the receiving side is read y to accept. When the receiving buffer
reaches its capacity, the receiving device should be capable to cause the sending devic e to pause un-
til it catches up.
There are basically two approaches to regulate data flow: software flo w control and hardware flow
control. The High Watermark of the input / output buffer should be set to approximately 60% of the to-
tal buffer size. The Low Watermark is recommended to be about 30%. The data f low should be
stopped when the capacity rises close to the High Watermark and resum ed 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 contro l. For details please refer to [5].
Software flow control sends different characters to stop (XOFF, decimal 19) and res ume (XON, deci-
mal 17) data flow. The only advantage of software flow control is that three wires would be sufficient
on the serial interface.
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 th e
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 en gine. This
needs to be done with the AT command AT\Q3 - it is not sufficient to set RTS/CTS handshak e 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 res tored each time af-
ter 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 ac-
tive profile with AT&V.
Often, fax programs run an intialization procedure when started up. The intialization com monly in-
cludes enabling RTS/CTS hardware handshake, eliminating the need to set AT\Q3 once again. How-
ever, before setting up a CSD call, you are advised to check that RTS/CT S handshake is set.
RTS/CTS hardware handshake must also be set if you want to take advant age of the CYCLIC SLEEP
modes. For further details refer to Chapter 4.11, AT+CFUN.
Note: After deactivating the RTS line, the ME may still send up to 32 b ytes. This can be easily han-
dled if the buffer of the host application is sufficiently sized, and if a hysteresis is im plemented
in its Rx buffer as mentioned in Chapter 1.6. A total buff er capacity of 256 bytes has been
proved to work well.