Silicon Laboratories SI2493/57/34/15/04, SI2494/39 manual ATDT12345 Connect 1200 Protocol None

AN93

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 <EM><rate> indicator shows that the modem connected with a transmit rate of 1200 bps and a receive rate of 1200 bps. The <EM><flag> that occurs immediately after the <EM><rate> indicates that a non-flag to flag transition has occurred and that the receiver has now been synchronized. An <EM><flag> 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 <EM><err> 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 <EM> <flag> 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 <EM> 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 <EM> shield the 0x11 and 0x13 characters, XON / XOFF software handshaking can no longer be used.

In either of the above transmit frames, the <EM><flag> is used to indicate that a logical frame has completed. The modem does not begin transmitting the frame at the DCE until the <EM><flag> 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 <EM><flag> 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 <EM><tunder> 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 <EM><resume> command. The <EM><resume> is then followed by the <EM><unum> 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 <EM><tover> 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.

130 Rev. 1.3