Black Box MDU21-ROOF, MDU21-STD, MDU20G-918, MDU21-MAG Industrial Control Modem GSM, AT&Dn

Industrial Control Modem GSM

AT&Dn	Data Terminal Ready settings
&D0	Modem ignores DTR
&D1	Modem switches from data to command mode
	when DTR switches from ON to OFF
&D2	Upon DTR switch from ON to OFF, the call is
	cleared down
AT+IFC	Defines if Flow Control is used between DTE and DCE.
	+IFC=0,0	No flow control
	+IFC=2,2	RTS flow control (preferred)

Flow control is the use of RS232 signals to start and stop the flow of data to avoid data loss during buffering. Check if your DTE supports RTS/CTS flow control (Hardware flow control). With Flow Control the DTE will send data to the DCE. The DCE will store this data in a buffer. When this buffer is full, the DCE will drop the CTS-line, telling the DTE that it has to stop transmitting data until the signal raises again. If the RTS-signal is OFF, transmitting data to the DTE is stopped until the signal switches to ON.

AT+IPR	Defines the Baud rate used for communications between the DTE and DCE.
AT+IPR=0	The Baud rate is selected automatically
AT+IPR=9600	The Baud rate is fixed at 9600 baud or another speed.
	This is best left at its default setting of 0 (auto), unless your DTE demands
	otherwise.
AT+CBST	Defines the type of Bearer to be used. That is the communication between
	the DCE and the GSM Network.
AT+CBST=0,0,0	Auto baud rate selection, transparent mode
AT+CBST=0,0,1	Auto baud rate selection, non-transparent mode
AT+CBST=7,0,0	9600 (V32) baud rate selected, transparent mode
AT+CBST=4,0,1	2400 (V24bis) baud rate selected, non-transparent mode

Usually the baud rate is best left in the default Auto selection. Non-transparent copes with bad connections better than transparent mode, but transparent mode will pass data more quickly. Try either to see which is best for your application.

Non-Transparent communication utilises a special GSM network-based error correction facility called "Radio Link Protocol" (RLP) that ensures a more robust transmission. What this essentially means is that the GSM network will add special error correction codes to control the flow of data.

This ensures that the data transmitted first obtains a special acknowledgement signal from the receiver that informs the transmitter that the data has been received as transmitted and that the receiver is ready to receive the next set of data from the transmitter. If there is no such acknowledgement signal from the receiver, the networks will utilise their "data buffering" feature by using a special "forward correction" technique to ensure uninterrupted data transmission throughout.

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