Silicon Laboratories SI2493/57/34/15/04 - Manual (Page 130)

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130 Rev. 1.3
Given the example initialization settings shown in Table 87, after an ATDT command has been sent to establish a
connection, the modem responds with the following:
ATDT12345
CONNECT 1200
PROTOCOL: NONE
<0x19> <0xBE> <0x20> <0x20> <0x19> <0xB1>
The first indicator shows that the modem connected with a transmit rate of 1200 bps and a receive
rate of 1200 bps. The that occurs immediately after the indicates that a non-flag to flag
transition has occurred and that the receiver has now been synchronized. An indicator is applicable
only to the first occurrence of a non-flag to flag transition. Future occurrences of non-flag to flag transitions are
indicated with an instead. Also, this feature is unique to the U87 [8]=1 option. Also, with U87 [8]=1, the
Framed Submode is entered immediately upon connection. Otherwise, if U87 [8]=0, the Transparent Submode is
entered instead, and the host is expected to send an to switch to the Framed Submode.
After a connection has been established, the modem is ready to transmit and receive frames. For example, if it is
desired to send a frame whose contents are:
<0x10><0x11><0x12><0x13><0x14><0x15>
The host software sends the following:
<0x10><0x19><0xA0><0x12><0x19><0xA1>
<0x14><0x15><0x19><0xB1>
Note the bytes <0x11> and <0x13> are shielded because these bytes could have been used for XON /
XOFF handshaking. In this example, CTS/RTS hardware handshaking is used, so it is also possible for the host to
have sent this series of bytes instead:
<0x10><0x11><0x12><0x13><0x14><0x15>
<0x19><0xB1>
However, if the host does not shield the 0x11 and 0x13 characters, XON / XOFF software handshaking can
no longer be used.
In either of the above transmit frames, the is used to indicate that a logical frame has completed. The
modem does not begin transmitting the frame at the DCE until the is received or the number of bytes
sent to the modem exceeds the number of bytes programmed into U87 [7:0].
In the above example, the following transmission:
<0x10><0x19><0xA0><0x12><0x19><0xA1>
<0x14><0x15><0x19><0xB1>
meets both criteria of having 10 bytes received at the DTE and receipt of an command. In this
example, the transmission at the DCE begins approximately after the receipt of the <0xB1> byte.
Once an HDLC frame begins transmitting at the DCE, the host must ensure transmit overrun and underrun do not
occur. It is expected that the +ITF command be used to adjust the transmit flow control thresholds so that it is tuned
to the system's ability to process the interrupt.
If a transmit underrun occurs, the indicator always appears in the receive path, regardless of how
+ESA[C] is programmed.
If +ESA[C] = 0, the modem transmits an abort character at the DCE at the point of the transmit underrun. Additional
transmit frames can then be transmitted normally.
If +ESA[C] = 1, the modem transmits an HDLC flag at the point of the transmit underrun, and the DCE continues to
send only HDLC flags until the host sends an command. The is then followed by
the command so that the host software can correct this problem.
A transmit overrun can occur if the host does not properly implement transmit flow control. When a transmit
overflow occurs, the indicator always appears in the receive path. A transmit overflow is considered
to be a catastrophic failure and results in non-deterministic behavior at the DCE. It is recommended that the
session be terminated immediately.