Cirrus MD56xx Modem Commands

Cirrus, then Ambient and now Intel modems based upon the 56xx chipset are V.90, x2, and V.34-compatible. The command documentation for 2 of the important extended commands (+MS and +ES) are very hard to find. (Most documentation available on the web is either outdated, or refers to a hard-to-find Programmer's Reference Manual.) Here is a summary of the +MS and +ES commands, along with related V.42/MNP commands.

+MS=p,a,mintx,maxtx,minrx,maxrx
Where
p = protocol: valid values include:
V34, X2, and V90. Also valid: V34S, V32B, V32, V23C, V22B, V22, V21, Bell212A, and Bell103
a = automode: 1 = only connect at rates/protocol specified; 0 = allow fallback if connection cannot be achieved at specified rates
mintx: Minimum upstream rate defines the lowest data rate at which a modem connection can occur. Setting <min rate> to ‘0’ has one of two meanings depending on the <automode> setting. When both <automode> and <min rate> are set to ‘0’, then the lowest data rate at which the connection can occur is the lowest data rate specified by the <carrier> parameter. If <automode> is set to ‘1’ and <min rate> is set to ‘0’, then the lowest data rate is 300 bps.
maxtx: Maximum upstream rate defines the highest data rate at which a modem connection can occur. If the <max rate> is set to ‘0’, the modem uses the DTE data rate or a slower <carrier> data rate as the highest permitted connection data rate. This highest permitted data rate means the modem attempts to connect at this data rate but can connect at a slower rate because of line impairment. If <max rate> and <auto-mode> are set to ‘0’ and the DTE data rate is below the lowest data rate supported by the modulation rate, then the modem’s connection attempts always fail and the modem reports a "NO CARRIER" message.
minrx and maxrx: Same as above, except for downstream rate (Valid only for V.34, x2, and V.90 connects).
Valid rates include: 0, 300, 1200, 2400, 4800, 7200, 9600, 12,200, 14,400, 16,800,19,200, 21,600, 24,000, 26,400, 28,800, 31,200, 33,333, 33,600, 37,333, 41,333, 42,666,44,000, 45,333, 46,666, 48,800, 49,333, 50,666, 52,000, 53,333, 54,666, 56,000, and 57,333 bps.

ATW3 will report the actual receive and transmit rates in the CONNECT string. AT+MS? will report the current settings for Modulation Select.

If the +MS=m parameters contain conflicting information like "+MS=V34,1,14400,0" with a DTE data rate of 2400 bps, then the modem’s connection attempts always fail and the modem reports a "NO CARRIER" message. When the modem receives the +MS=m command, the modem does not check for conflicts of valid parameter information.

+ES= Controls the Error Correction protocol:

Buffer (Normal) +ES=1, 0, 1	No error correction/data compression, but speed buffering is supported.
MNP Reliable +ES=4, 4, 6	MNP2–4, 5 connection only. If an MNP connection cannot be established, the modem hangs up.
V.42 Auto-reliable +ES=3, 0, 2	V.42/V.42 bis with fallback to MNP2–4, 5 or normal mode.
V.42 Reliable +ES=3, 2, 4	V.42, V.42 bis or MNP2–4, 5 only connection. If a V.42/V.42 bis/MNP2–4, 5 connection cannot be established, the modem hangs up.

1) MNP5 requires the modem to be configured for %C1.
2) V.42 bis requires the modem to be configured for "H3.
3) Refer to the \Cn and %An commands for more information about Auto-reliable mode.

The list of commands needed to enter a specific error correction or data compression mode are as follows:

Protocol:	Command
V.42 bis with fallback to MNP5,MNP2–4 or V.42:	+ES=3,0,2 "H3 %C1
V.42 bis with fallback to V.42/MNP2–4:	+ES=3,2,4 %C0 "H3 -J1
V.42 bis only:	+ES=3,2,4 -J0 "H3
V.42 only:	+ES=3,2,4 -J0 "H0
MNP5 with fallback to MNP2–4:	+ES=4,4,6 %C1
MNP2–4 only:	+ES=4,4,6 %C0

V.42 and MNP Data Mode Command Descriptions

%An (Default = 13)	Set Auto-Reliable Fallback Character: In Auto-reliable mode (+ES=3,0,2) with auto-reliable fallback character enabled (\C2), receipt of the fallback character from the line during the V.42 detection phase causes the modem to switch to buffer (normal) mode. This allows a remote user with a non-V.42 modem to immediately connect with a V.42 modem. A space or carriage return is usually chosen for the fallback character. n = 0–127 (ASCII character.)
%Cn (Default = 1)	MNP 5 Data Compression Control: This command controls whether the data sent during the MNP frames is compressed using the MNP Class 5 compression standard. MNP 5 data compression can improve throughput by as much as 150%. n = 0, 1 n = 0 No compression. n = 1 MNP Class 5 compression.
\An (Default = 3)	MNP Block Size: This command specifies the maximum number of data bytes in an MNP data frame. A smaller frame size can improve throughput on high-impairment (noisy) telephone lines. n = 0–3 n = 0 Maximum 64 characters. n = 1 Maximum 128 characters. n = 2 Maximum 192 characters. n = 3 Maximum 256 characters.
\Cn (Default = 0)	Set Auto-Reliable Buffer: (requires a license from Microcom) In Auto-reliable mode (+ES=3,0,2), this command determines the fallback method and enables data buffering. The settings for this command are used by the modem during the V.42 detection phase. n = 0–2 n = 0* Does not buffer data. n = 1 Reserved. n = 2 Does not buffer data. Switches to Buffer (normal) mode upon receipt of auto-reliable fallback character and passes it to the serial port. This feature allows non-V.42 modems to immediately connect to a V.42 modem without data loss.
\Gn (Default = 0)	Set Modem Port Flow Control: In Buffer (normal) mode (either +ES=1,0,1 or after fallback), this command enables modem-to-modem flow control using XOFF (13h) to stop and XON (11h) to start transmission between modems. n = 0, 1 n = 0 Disables port flow control. n = 1 Sets port flow control to XON/XOFF.