Silicon Laboratories SI2493/57/34/15/04 - Manual (Page 131)

AN93
Rev. 1.3 131
It is expected that the <EM><tover> and <EM><tunder> indicators be encountered during system debug, and
designing the system software properly to avoid having these indicators occur should be the design goal.
In the receive direction, assuming that the remote modem is another Si2493/57/34/15, this is the expected
sequence at the remote receiver DTE, representing the frame sequence of:
<0x10><0x11><0x12><0x13><0x14><0x15>
<0x10><0x19><0xA0><0x12><0x19><0xA1>
<0x14><0x15><0x19><0xB1>
In the receive direction, the <EM><flag> indicates that the CRC check is successful, and the preceding frame was
received correctly. If there had been an error in the preceding frame, the <EM><err> would have been sent instead
of the <EM><flag>. The host is expected to discard the entire frame based on whether or not the frame is
terminated with an <EM><flag> or <EM><err>. The host should also expect to occasionally see the <EM><mark>
indicator if the sending modem experienced a transmitter underrun or overrun problem.
In general, the RTS flow control is not used. However, if it is used, and if RTS is negated for too long, the receive
buffers will eventually overflow. This is called a receiver overrun, and the modem sends an <EM><rover> indicator.
A receiver overrun is considered to be a catastrophic failure, and the host is expected to terminate the session.
Host software must be designed so that an <EM><rover> indicator never occurs.
It is expected that the <EM><rover> indicator be encountered during system debug, and designing the system
software properly to avoid having these indicators occur should be the design goal.
Please note that there is an option available in the U87 [10]. The reason for this option is to determine what the
modem sends to the DTE when the modem receives back-to-back occurrences of the special characters, 0x19,
0x99, 0x11, and 0x13, at the DCE.
As an example, if the following string is received at the DCE:
<0x19><0x19><0x11><0x11>
If U87 [10] = 0, this is what the host software will receive at the DTE:
<0x19><0x5D><0x19><0xA2>
If U87 [10] = 1, this is what the host software will receive at the DTE:
<0x19><0x5C><0x19><0x5C><0x19><0xA0><0x19><0xA0>
The choice of how to program U87 [10] is based on whether or not it is desired to simplify the host receive parsing
algorithm or to guarantee that the receive throughput is not overly affected by the <EM> <shielding>. In the worst
case, if there is a large frame consisting only of special characters, the required throughput at the DTE will have to
be at least 2x that of the DCE rate to account for the <EM> shielding overhead.
There are two methods of ending a call. One is to use the <EM><eot> command followed by an ATH. Sending the
<EM><eot> command will cause the modem to go to command mode and stop the transmitter; however, the
modem does not go back on hook until ATH. The other method is to use the <EM><esc> command to escape to
command mode, and then issue the ATH command. The main difference is that the <EM><esc> does not shut off
the transmitter. The <EM><esc> can also be followed by an ATO to resume the connection.