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# B.2. The Extended Hayes Command Set

Ampersand Commands

Table B-2. The Extended Hayes Command Set

Command | Description | Comments
--- | --- | ---
&B0 or &B | Retrain Parameters | Disable auto retrain function
&B1 | Retrain Parameters | Enable auto retrain function
&B2 | Retrain Parameters | Enable auto retrain, but disconnect if no line improvement over the period dictated by `S7`
&C0 or &C1 | Carrier detect | Signal always on
&C1 | Carrier detect | Indicates remote carrier (usual preferred default)
&D0 or &D | Data Terminal Ready (DTR | Signal ignored (This is modem specific, you must see your manual for information on this one!)
&D1 | Data Terminal Ready (DTR | If DTR goes from On to Off the modem goes into command mode (Some modems only)
&D2 | Data Terminal Ready (DTR | Some modems hang up on DTR On to Off transition (This is the usual preferred default)
&D3 | Data Terminal Ready (DTR | Hang up, reset the modem, and return to command mode upon DTR

&F0 or &F

Factory defaults

Generic Hayes-compatible defaults.

This is usually a good thing to use in your init string, since the &F1-&F3 settings can vary among modems, and they may actually be the cause of connection problems. (Since you never know exactly what Brand X's &F2 really changes.

On the other hand, it pays to try out the other options below; many people's problems can be solved by replacing a complicated init string with a simple &F2 or the like. However, if you're building an init string, it's best to start with a simple &F, and not use the "customized" form of defaults.

&F1 | Factory Defaults | Factory Defaults tailored to an IBM PC compatible user
&F2 | Factory Defaults | Factory defaults for a Mac w/software handshaking
&F3 | Factory Defaults | Factory defaults for a Mac w/hardware handshaking
&G0 or &G | Guard tones | Disable guard tones
&K0 or &K | Local flow control | Disable local flow control
&K1 | Local flow control | Enable RTS/CTS hardware local flow control
&K2 | Local flow control | Enable XON/XOFF software local flow control
&K3 | Local flow control | Enable RTS/CTS hardware local flow control
&K4 | Local flow control | Enable XON/XOFF software local flow control
&L0 or &L | Dial mode | Select dial-up mode
&M0 or &M | Error control mode | Select asynchronous non-EC mode (the same as &Q0)
&P0 or &P | Pulse dialing ratio | U.S./Canada pulse dialing 39% make / 61% break ratio
&P1 | Pulse dialing ratio | U.K./Hong Kong pulse dialing 33% make / 67% break ratio
&Q0 or &Q | Error control mode | Asynchronous non-EC more. No data buffering. ASB disabled.
&Q5 | Error control mode | Select V.42 EC operation (requires flow control)
&Q6 | Error control mode | Asynchronous mode with ASB (requires flow control)
&Q8 | Error control mode | Select alternate EC protocol (MNP)
&Q9 | Error control mode | Conditional data compression: V.42bis = yes, MNP5 = no.
&S0 or &S | DSR action select | Always on (default)
&S1 | DSR action select | Follows EIA specification (Active following carrier tone, and until carrier is lost.)
&T0 or &T | Self test | Model specific self test on some modems
&U0 or &U | Trellis code modulation | Enable V.32 TCM
&U1 | Trellis code modulation | Disable V.32 TCM
&V0 or &V1 | View active | (and often stored) configuration profile settings (or ATI4
&W0 or &W | Store profile | In NVRAM (&W0, &W1 etc. for multiple profiles) Some settings cannot be stored. These often don't show on &V or ATI4
&Y0 or &Y | Select configuration loaded at power-up | Load profile 0 (default)
&Y1 | Select configuration loaded at power-up | Load profile 1
&Z`n`=`x` | Soft reset and load stored profile number `n` | Note that all items after the &Z on the command line are ignored

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