Specifications

AN93
22 Rev. 0.8
The “n” in the /Zn and /Tn is a single-byte, host-defined
tag that can be used to track multiple HDLC frames.
To facilitate transmit flow control, the modem sends the
/S and /Q metacharacters to the host. If the transmit
buffer (512 bytes) is three quarters full, the
/S metacharacter is sent to the host. The host must then
stop transmitting. When the transmit buffer empties
down to half full, the /Q metacharacter is sent to the
host to indicate that it is okay to begin transmitting
again. If a transmit underrun occurs, the current frame is
aborted, and a /Un is sent to the host. All data from the
underrun to the receipt of the /Zn metacharacter is
discarded by the modem. A design goal of the host
software should be to eliminate any occurrence of the /
Un metacharacter.
Because the “/” is an escape character, the host must
send a “//” when a “/” appears in the transmit data
stream. The Si2493/57/34/15/04 removes one “/” for
each instance of “//” that appears on TXD.
On the receive side, as long as HDLC flags are received
by the Si2493/57/34/15/04 at the DCE, it does not pass
the data out RXD. Once the first non-flag word is
detected, the Si2493/57/34/15/04 performs zero
deletion, calculates the CRC value, and passes the data
out RXD. The Si2493/57/34/15/04 continues in this
manner until detecting the HDLC flags, which indicate
the end of the frame. At this point, the HDLC frame is
complete, and the Si2493/57/34/15/04 calculates the
final CRC and compares it to the CRC value received in
the frame. If the CRC matches, the Si2493/57/34/15/04
passes /G to the host. If the CRC does not match, the
Si2493/57/34/15/04 passes /B to the host to initiate a
retransmit request.
Because the / is an escape character, the Si2493/57/34/
15/04 sends a // when a / appears in the receive data
stream. The host must remove one / for each instance
of // that appears on RXD. Table 12 lists additional
escape characters that are used to control the flow of
data between the Si2493/57/34/15/04 and the host in
the "transparent HDLC" mode.
Table 12. Legacy Synchronous DCE Mode Metacharacters
Character* Direction Description
/Zn TX Follows the last character of a transmit frame. Once the frame has been sent, a /T
metacharacter is sent to the host. n denotes a frame tag. n is echoed back later
with the /U or /T metacharacters to make frame tracking easier.
// TX A forward slash character is to be transmitted.
/E TX Escape back to command mode. Si2493/57/34/15/04 returns to command mode.
/Un RX A transmit underrun has occurred, but a /Z metacharacter was not received. When
an underrun occurs, the current frame is aborted; a /Un is sent to host, where n is
the frame tag. All data following the underrun, up to the /Z metacharacter, is dis-
carded by the modem.
/Tn RX The transmit frame, n, has been sent. The n from the /Z is echoed with the /Tn to
allow tracking frames.
/G RX The previous receive frame CRC check was successful.
/B RX The previous receive frame CRC check was unsuccessful.
/S RX Transmit buffer is almost full; the host must pause transmission to prevent an over-
flow. If hardware flow control is used, the host may ignore this metacharacter.
/Q RX The host may begin transmitting again after a /S (pause) has been sent. If hard-
ware flow control is used, the host may ignore this metacharacter.
// RX A forward slash character was received.
/A RX Receive frame aborted.
*Note: Characters after “/” must be uppercase.