Specifications
AN93
24 Rev. 0.9
the Framed Submode can be used in the same
applications currently using the Legacy Synchronous
DCE Mode.
Prior to sending the ATDT to establish a synchronous
access mode connection, the following commands and
registers require initialization: +MS, +ES, +ESA, +ITF,
+IFC, U87, and U7A.
As an example, the closest equivalent to the Legacy
Synchronous DCE Mode is the following initialization
setting.
With either Synchronous Access Submode, once a
connection has been established, payload data is
multiplexed with command/indicator information by use
of <EM> shielding. With <EM> shielding, either of the
two bytes <0x19> or <0x99> (used to represent <EM>)
precedes a special command or special indicator.
Note that the synchronous access mode <EM>
shielding is designed to support XON/XOFF
handshaking. As such, the bytes 0x13 and 0x11 (XON/
XOFF) are considered to be special characters in the
same way the 0x19 and 0x99 bytes, used for <EM>, are
special.
Since the payload data is multiplexed with <EM>
shielded command/indicator and possibly XON/XOFF
characters, Transparency <EM> codes are defined for
the purpose of allowing the host software to send 0x13,
0x11, 0x19 and 0x99 bytes to/from the DCE. For
example, if the desire is to send one <0x99> character
as a payload character, the host software sends
<EM><0x76> instead. For a complete list <EM>
commands and statuses, see Table 13.
In addition, a common Point-of-Sale V.22 Fast Connect
Handshake Protocol (with transparent HDLC) requires
these additional settings:
Table 11. Synchronous Access Mode Settings
AT\N0
Required to disable MNP,V42
and other protocols
AT+ES = 6,,8
Enable synchronous access
mode on originate or answer
AT+ESA = 0,0,0,,1,0
Send Abort on underrun/over-
run in Framed Submode.
Enable CRC generation and
checking.
AT+IFC = 2,2 CTS/RTS Flow Control
AT+ITF = 0383,0128
Controls CTS Flow Control
Threshold. CTS off at 383
bytes, CTS On at 128 bytes.
AT:U87,050A
Direct to Framed Sub-mode
upon connection. DCE starts
to transmit upon receipt of 10
bytes from the DTE.
Table 12. Fast Connect Settings
AT+MS = V22 V22 Protocol
AT:U7A,3
Set Fast Connect, Transmit
HDLC Flags instead of Marks
during handshake negotiation.