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CHAPTER 4. INTERFACE CONTROLSê ASYNCHRONOUS OPERATIONS Use the commands explained in this chapter to select modem settings at the terminal or computer and phone link interfaces. These settings apply only to asynchronous operations. For syn- chronous calls, see Chapter 7. TERMINOLOGY ARQ (automatic repeat request, or retransmission) is the term used by USRobotics in error control commands and response codes. An ARQ connection indicates a call under error control. The terminal or computer is referred to as the DTE (Data Terminal Equipment). The DTE rate is the rate at which your DTE and Courier modem communicate, for example, 19.2K bps. The Courier modem is referred to as the DCE (Data Communications Equipment). Figure 4.1 indicates which commands control modem operations at the DTE/DCE (computer-to-modem) and phone link (modem-to-modem) interfaces. Figure 4.1êInterface Control Commands Detailed command descriptions are in this chapter. If you're familiar with modem operations, you may want to review the Quick-Start instructions in Chapter 3 instead. CHANGING SETTINGS When you change a setting, the modem retains it until you do one of the following: ù Change the setting. ù Issue the ATZ command to reset the modem. If DIP switch 10 is UP, the modem resets to the defaults stored in nonvolatile memory (NVRAM). If DIP switch 10 is DOWN, the modem resets to the factory settings stored in read-only memory (ROM). ù Issue the AT&F command to reset the modem to its factory settings. ù Turn the modem off, and power it on again. The modem then loads either the nonvolatile memory defaults (DIP switch 10 UP) or the factory settings (DIP switch 10 DOWN). HIGH SPEED MODULATION: V.32 BIS/HST At high speeds, USRobotics modems use either V.32 bis modulation or HST modulation, unless they are Dual Standard modems that have both capabilities. By high speeds we mean calls at 16.8K (HST-to-HST only), 14.4K, 12K, 9600, 7200 or 4800 bps. (The modems use identical, standard modulation at 2400 bps and below.) Modulation SettingsêBn The modems are set at the factory as follows: V.32 bis modulationêB0. V.32 bis modems are always set to B0. Dual Standard modems are factory set to B0 so that they connect with both V.32 bis modems and HST modems. If a Dual Standard modem is set to B1, it can call but cannot answer V.32/V.32 bis modems. HST modulationêB1. HST modems are usually set to B1. However, they can be set to B0 to answer international calls. A Dual Standard modem set to B1 can successfully call V.32/V.32 bis or HST modems but, as mentioned above, cannot answer V.32/ V.32 bis calls. Under V.32 bis modulation, data flows at the same rate in both directions. Under HST modulation, data flow is asymmetrical: the high speed in one direction and 450 bps in the other. The modems switch the fast and slow channels when the volume of data flow changes. Interactive applications appear faster under V.32 modulation, while connection times are faster under HST modulation. WARNING: To connect above 9600 bps, the DTE rate must be 19.2K, 38.4K or 57.6K bps. If the local computer is limited to 9600 bps, V.32 bis modems are limited to 9600 bps maximum, that is V.32. Disable V.32 bis modulation so that the modem does not switch its DTE rate up higher than 9600 bps. Do this by setting Register S34 to 3 (S34=3) and including that setting in the defaults you write to nonvolatile memory, as shown in the next chapter. MODULATION AT 2400 BPS AND BELOW The Bn setting is not significant at speeds of 2400 bps and lower except for answering calls from overseas. (See International Calls at the end of Chapter 6.) The only difference you might notice is that the B1 setting (Bell answer tone) allows the modems to connect slightly faster. Error control, DTE rates, flow control and other settings are the same for both HST and V.32 bis modems. THROUGHPUT GUIDELINES The following guidelines should help you to make the most of your modem's advanced performance features. In many instances, experimentation and experience will indicate what works best for your applications. You'll obtain optimal throughput under the following conditions. 1. Your communications software supports a fixed DTE rate higher than the link rate, for example, setting your software to 57.6K, 38.4K or 19.2K bps and setting the modem to &B1. Check the quick-configuration guide that came with the modem or your software manual, and see Data Rate Commands in this chapter. 2. The call is under data compression. See Data Compression (&Kn) in this chapter. 3. You're transmitting text files. Throughput is higher for text files than for other types of files, such as .EXE or .COM binary files. For a comparative table, see the end of Appendix A. 4. File transfer is not slowed down by a file-transfer protocol. Many non- text files require a file transfer protocol, but throughput result vary. Certain public domain file transfer protocols, for example, have the following effects: Kermit Throughput is severely reduced due to Kermit's short block lengths (under 128 bytes) and acknowledgment turn-around time. Xmodem Throughput may be reduced if your version uses short block lengths, for example 128 bytes. Some versions use blocks of 1K bytes, which is much better, although overhead (error control protocol information) still affects overall throughput. Ymodem This protocol is better than some because of larger block lengths (1K bytes), although overhead (error control protocol information) still affects overall throughput. The above protocols further reduce throughput during error control (ARQ) connections. The accuracy of the data is checked twice, by the file transfer protocol and the modem. To avoid redundancy, use the above protocols only for non-ARQ connections, and only at speeds of 2400 bps and lower. For the best throughput, but on ARQ connections only and with hardware flow control, we recommend the most current version of Zmodem. Overhead is minimal with this protocol, with throughput almost equal to that obtained with no file-transfer protocol. Leave the modem at its &M4 and &K1 settings for both error control and data compression. Ymodem-G is another good choice, but only on error-controlled calls, because Ymodem-G aborts the connection if it detects an error. WARNING: If you are using an X, Y or Zmodem-type protocol, do not use the modem's software flow control. See the more detailed warnings in this chapter under Transmit Data Flow Control (&Hn) and Received Data Software Flow Control (&In). ERROR CONTROL (&Mn) The Courier first attempts a connection using V.42 (LAPM) error control and, if that doesn't succeed, it attempts an MNP connection. If that doesn't succeed, the Courier tries to connect without error control. (The process takes much less time than it takes to key in this paragraph.) The exception to the above is that HST modems, at high speeds, do not use V.42 error control. They use USRobotics' proprietary HST technique, an MNP-type of protocol adapted for its high speed/low speed asymmetrical modulation. Error control is possible at speeds of 1200 bps and higher. For more detailed information, see Appendix A. The following options are available. &M0 Normal mode, no error control. Because of the nature of phone line channels, this is never recommended for calls above 2400 bps. &M1 This setting is exclusive of the modems' error control and is used only for Synchronous mode. See Chapter 7, Synchronous Operations, for instructions. &M2 Reserved. &M3 Reserved. &M4 Normal/ARQ mode. This setting is the default. If the remote modem doesn't recognize the Courier's error control signalsêV.42, HST or MNPêthe modem operates in Normal mode, as though it were set to &M0. NOTE: V.32 bis modems reverting to Normal mode transfer data at high speeds without the reliability of error control. To avoid this, V.32 bis and V.32 modems, local and remote, should always be set for error control. HST modems, if unable to establish an error control connection, drop to 2400 bps. &M5 ARQ mode. If the remote modem doesn't recognize the error-control requestêV.42, HST or MNPêthe Courier hangs up. With Auto Answer When set to &M4 or &M5 and a call comes in, the modem goes off hook and responds to received error control signals. If the modem doesn't receive those signals and is set to Normal/ ARQ mode (&M4), it answers the call in Normal mode (&M0). If it doesn't receive the signals and is set to ARQ mode (&M5), it hangs up. FLOW CONTROL OVERVIEW This feature controls the flow of data into and out of the modem's Transmit and Receive data buffers. Due to variations in computer and terminal equipment and in software requirements, the Courier is shipped with all flow control options disabled. Users at Hewlett Packard installations that use the ENQ/ACK protocol should note the HP settings, &I3 and &I4, described under Received Data Software Control (&In). NOTE: Flow control of data from the DTE to the modem is required under the circumstances described under Transmit Data Flow Control (&Hn). Received Data flow control is not as critical unless, for example, you are writing incoming files to disk on a very slow computer. Transmit Data Buffer Sizes The size of the Transmit data buffer depends on whether the connection is under error control or not, as follows. ù ARQ connections: 3.25K bytes. ù Non-ARQ connections: 1.5K bytes, allowing use of error control file transfer protocols such as Xmodem and Ymodem without flow control. If bit 3 of Register S15 is turned on, the non-ARQ buffer size is reduced to 128 bytes, for the convenience of some remote users of slower modems. See S- Register Summary, S15, in Appendix B. Received Data Buffer Size The size of this buffer remains constant at 2K bytes. TRANSMIT DATA FLOW CONTROL (&Hn) This type of flow control is for data transmitted to the modem by its attached DTE, that is, by your computer or terminal. The modem monitors its buffer as data comes from the DTE. If the buffer approaches 90% capacity, the modem signals the DTE to stop transmitting. When the modem has sent enough data over the link to half empty the buffer, it signals the DTE to resume transmitting. When it is Required Transmit Data flow control should be enabled in the following situations. ù You're using error control (any allowable speed above 300 bps), with or without data compression. ù The DTE rate is higher than the link rate, for example, the DTE is sending data to the modem at 38.4K or 19.2K bps and the link rate is 14.4K bps. Another example is a DTE rate of 19.2K bps and a link rate of 2400 bps. NOTE: The modem uses either hardware or software flow control. Your software and machine must support whichever type you select, although we recommend hardware flow control, if possible. Review the quick-configuration guide that came with the modem and, if necessary, your software and computer manuals. Hardware Control The modem drops the Clear to Send (CTS) signal it's been sending to the DTE when the modem's buffer nears 90% capacity. It starts sending CTS again when the buffer is about half full. Software Control The modem sends the DTE the standard ASCII Transmit OFF (XOFF) character, <CTRL-S>, when its buffer nears 90% capacity. The modem sends the ASCII Transmit ON character, <Ctrl>-Q, when the buffer is about half full. ASCII definitions are as follows: XON <Ctrl>-Q (ASCII 17 Decimal, 11 Hex) XOFF <Ctrl>-S (ASCII 19 Decimal, 13 Hex) NOTE: You may have to set your software as well to either hardware or software flow control. Some programs also require that you turn off the type you are not using. &H0 Transmit Data flow control disabled. Default. &H1 Hardware flow control. Recommended setting. Requires that your DTE and software support Clear to Send (CTS) at the RS-232 interface. &H2 Software flow control. Requires that your software support XON/XOFF signaling. See the guidelines that follow. The ASCII characters may be user-defined. See Registers S22 and S23 in Appendix B. That appendix also includes an ASCCI chart. &H3 Use both hardware and software flow control. If you are unsure about what your equipment supports, select this option. But keep the warning, above, in mind about software flow control. WARNING: If possible, always use hardware flow control. You may lose data if XON/XOFF characters occur in the data stream from other sources. They may, for example, come from the remote system: an XON from the remote system, after your modem has sent an XOFF, can result in buffer overflow. Ctrl-S (XOFF) and Ctrl-Q (XON) characters also occur in binary files, and are used by Xmodem-type protocols. You risk having these characters misinterpreted as modem flow control characters and dropped from the data stream. Guidelines If your terminal or software does not support Clear to Send, use of software flow control may prove satisfactory if you're only transferring text files. However, if you're transferring non-text (binary) files, or using an Xmodem-type protocol, disable flow control entirely (&H0). In addition, be sure the modem is set to &B0 and &N0, so that the DTE and link rates are equal. RECEIVED DATA FLOW CONTROL Separate commands, &Rn (hardware) and &In (software), control the flow of Received Data passed by the Courier to your DTE. Because most computers handle incoming data at a much faster rate than the modem receives it over the phone line, you may want to leave this type of flow control disabled. Your software and machine must support whichever type you select, although we recommend hardware flow control, if possible. Review the quick- configuration guide that came with the modem and, if necessary, your software and computer manuals. Hardware Control (&Rn) When the DTE drops its Request to Send (RTS) signal, the modem stops passing along received data. The DTE sends RTS again when it is ready to receive. Your DTE and software must support RTS. You cannot use this type of flow con- trol, however, if your software requires a constant RTS signal. WARNING: Use this type of flow control only for ARQ (error-control) calls, because under error control the modems use flow control across the phone link. During non-ARQ connections, there is no way to signal the remote modem to stop sending data. If your modem stops passing data to the DTE and the remote modem keeps sending across the link, the Received Data buffer will overflow. &R0 Delay Clear to Send Response after Request to Send signal (RTS/CTS delay). The delay is required by some synchronous mainframes and does not apply to asynchronous calls. See Chapter 7. &R1 The modem ignores RTS. Default. This setting is required if your DTE or software does not support RTS. &R2 Hardware flow control of received data enabled. The modem sends data to the DTE only on receipt of the RTS signal. Software Control (&In) When you send the modem a <Ctrl>-S (XOFF) command from the keyboard, the modem stops passing received data to the DTE. When you send a <Ctrl>-Q (XON) command, the modem resumes. (Hold down the Ctrl key and press the letter. Don't type the angle brackets. The brackets indicate a special, named key on the keyboard.) WARNING: In ordinary operation, the only characters the modem recognizes during a call are the three pluses (+++) of the escape code. But when software flow control is enabled, the modem also looks for <Ctrl>-S or <Ctrl>-Q characters. If these characters occur in a file or as part of a protocol, the modem reads them as XON/XOFF characters and acts on them. In some cases, the modem drops them from the data stream. &I0 Disables XON/XOFF flow control of received data. Default. Recommended for non-ARQ (Normal mode) calls, but see &I5. The I0 option provides transparency for all characters except the escape code sequence (+++), because at this setting the modem does not look for control characters. NOTE: Because of the risk described in the above warning, the settings that follow are only recommended for users whose data does not include XON/XOFF control characters. &I1 The Courier acts on your typed XON/XOFF commands, and passes them to the remote computer. Use in ARQ mode only, but keep in mind that the XON/XOFF characters sent to the remote computer may interfere with XON/XOFF signaling between the remote computer and remote modem. See &I2. &I2 The Courier acts on your XON/XOFF commands, but removes them from the data stream instead of passing them to the remote computer. This ensures that the remote computer does not confuse your XON/XOFF characters with those from its attached modem. This is the recommended setting for ARQ mode. However, if the call is not in ARQ mode, there is no flow control on the phone link. If you send an XOFF to your modem and it stops passing data, it has no way to tell the remote computer and modem to stop sending for a while, and your modem's buffer may overflow. For more reliable control in non-ARQ mode, see &I5. &I3 Hewlett PackardêHost Mode. Applies only to modems attached to an HP mainframe that uses the ENQ/ACK protocol. See Appendix E. &I4 Hewlett PackardêTerminal Mode. Applies only to modems attached to terminals in an HP system that uses the ENQ/ACK protocol. See Appendix E. &I5 This setting is designed to enable flow control on the phone link when the connection is not under error control. For this to work for you, the remote modem must have &I5 capability. In ARQ mode, a Courier set to &I5 operates the same as it does when set to &I2. It acts on your XON/XOFF commands, but does not pass them to the remote system. The error control protocol enables the modems to control the flow of data on the phone link. In non-ARQ mode, a Courier set to &I5 operates as though flow control were disabled (&I0): it does not look for your typed XON/XOFF commands. However, it looks for XON/XOFF characters coming in over the phone link. When the remote operator sends XON/XOFF commands, the Courier either resumes or stops transmitting data over the link and drops the characters from the data stream. If both modems are set to &I5, operators at each end can signal the remote modem to stop sending, thereby controlling the data flow on the phone link and preventing their own modem's buffer from overflowing. At the DTE/DCE interfaces, the modems independently control the flow of data through their Transmit Data (&H) settings. Guidelines Use of software flow control may prove satisfactory if you're only transferring text files. However, if you're transferring non-text (binary) files, or using an Xmodem-type protocol, disable flow control entirely (&R1, &I0). In addition, set the modem to &B0 and &N0, so that the DTE and link rates are equal. DATA COMPRESSION (&Kn) When data compression is enabled, the transmitting modem detects redundant bits of data and recodes them into shorter units of fewer bits. The receiving modem decompresses the redundant data units before passing them to the receiving DTE. Compression does not occur unless the modems are able to establish an error control connection. In addition, the modem should be set for Transmit Data flow control (&H1, 2 or 3), as described earlier in this chapter. &K0 Data compression disabled. &K1 Auto enable/disable. This is the default. The modem enables compression if the DTE rate is fixed, &B1, and disables compression if the DTE rate follows the link rate, &B0. The reason is that compression offers no throughput advantage when the DTE and link rates are equal: compression may even degrade throughput. &K2 Data compression enabled. Use this setting to keep the modem from disabling compression. &K3 Selective data compression. The modem negotiates only for V.42 bis compression, and disables MNP Level 5 (MNP5) compression. Use this setting to transfer 8-bit binary files, .ARC files, and other files that are already compressed. See the note below. If the Courier is connecting with a V.42 or HST modem, it negotiates V.42 bis compression. If V.42 bis is not feasi- ble, the connection remains under error control, but without data compression. If the Courier is connecting with an MNP modem, it negotiates for MNP Level 5 (MNP5) data compression. If the remote modem does not have MNP5, the connection remains under MNP error control, but without compression. NOTE: MNP5 compression is not useful when transferring files that are already compressed, such as the .ARC files downloaded from many Bulletin Boards and 8-bit binary files, which appear to the modem to be compressed. MNP5 tends to add data to the transmission so that throughput over the link degrades. V.42 bis compression dynamically detects when data is already com- pressed and turns off until it detects that compression will work to advantage. The special &K3 setting enables the best throughput for already-compressed files. See Throughput Guidelines and Data Compression in Appendix A for more information, including throughput to expect for different kinds of files. DATA RATE COMMANDS The &Bn and &Nn commands allow you to select variable or fixed data rates at the DTE and link (modem-to-modem) inter- faces. Throughout this manual we recommend that, if your software allows, you select a fixed rate at the DTE interface (&B1) so that it can remain higher than the rate on the phone link. You'll get greater throughput regardless of the call's rate on the phone link. Just remember to also enable Transmit Data flow control, preferably hardware (&H1). For the phone link we recommend variable rates (&N0). This enables the modem to switch its link rate to match the rate of a remote modem, no matter what the remote modem's rate is. If the link rate is fixed, for example at 9600 bps (&N6), the modem only connects with modems operating at that rate. Of course, if your application requires connections with modems at only one rate, you may wish to set the modem to a fixed link rate. Software Requirements Both variable and fixed rates require communications software support. Most communications programs support variable rates. Not all software supports fixed rates, although this support is increasing in program updates. Check the quick-configuration guide that came with your modem and/or your software documentation if your software is a later version than the version in the quick-configuration guide. DTE RATE SELECT (&Bn) Use this command to select variable or fixed rates at the DTE interface. Initially, the modem's DTE interface rate depends on your terminal or soft- ware setting. The modem detects this rate from the rate at which it receives the AT command. After that, the DTE rate depends on the modem's &B setting, as follows. &B0 Variable rates. Default. When the modem switches its link rate to connect with a modem operating at a different rate, it also switches its DTE rate. The software or terminal also switches DTE rates to match the link rate. This setting requires variable rates on the phone link, &N0, so that the Courier can switch link rates to match the rate of the remote modem. &B1 Fixed rate. The modem always communicates with the DTE at the rate at which you have set the terminal or software, regardless of the link rate. For the greatest throughput, set the DTE to 57.6K, 38.4K or 19.2K bps for high speed calls and to at least 9600 bps for 2400-bps calls. This setting is not affected by the &N setting. However, the DTE rate must be equal to or higher than the link rate. In addition, this setting requires Transmit Data flow control enabled, preferably hardware control (&H1), so that the modem's buffer doesn't overflow. &B2 Fixed for ARQ calls/Variable for non-ARQ calls. Answer mode only. When the modem goes off hook and connects in ARQ mode, it shifts its DTE rate up to a user-specified rate, for example, 19.2K bps. If the connection is not under error control, the modem behaves as if it were set to &B0 and switches its DTE rate to match the connection rate of each call. This option is designed for installations such as Bulletin Boards that receive calls from a wide variety of modems, ranging from the very slow to those with the Courier's advanced design. To implement this feature, set the modem to return ARQ CONNECT result codes, &A1 or &A2, and to Transmit Data flow control, preferably &H1. Set your software to the desired rate, and send the modem an AT &B2 [other settings] &W command string. The modem stores the rate of the command in NVRAM along with the settings. Each time it makes an ARQ connection, the modem checks NVRAM for the specified DTE rate. When sending subsequent configurations to NVRAM, be sure your software is set to your selected DTE rate, so that the correct rate is maintained. Link Rate Select (&Nn) Use this command to select variable or fixed rates at the link interface. Variable rates allow the modem to connect with a variety of remote modems. &N0 Variable rates. Default. The Courier negotiates with the remote modem for the highest possible link rate, depending on the capabilities of the remote modem. This is the recommended setting. &N1-8 Fixed rate. The modem only connects if the remote modem is operating at the same rate. If not, the modem hangs up. The link rate must always be lower than, or equal to, the DTE rate, never higher. If the link rate is lower than the DTE rate, enable Transmit Data flow control, preferably hardware control (&H1), so that the modem's buffer doesn't overflow. If you wish, you can filter out calls at other than a specific speed, for security or other reasons, by fixing the link rate. The options are as follows. Rates above 9600 bps are valid only for V.32 bis and HST operations, not V.32 operations. &N1 300 bps & N2 1200 bps &N3 2400 bps & N4 4800 bps &N5 7200 bps & N6 9600 bps &N7 12.K bps & N8 14.4K bps &N9 16.8K bps (HST-to-HST only) Guidelines For Low-speed Calls Calls at 300 bps: These guidelines apply to all Courier high speed modems for standard 300-bps calls. The modem must be set to &B1. If your software rate is higher than 300 bps, set the modem for Transmit Data flow control, &H1 or higher. Set a variable link rate (&N0) or, if you know the remote modem is operating at 300 bps, set a fixed link rate of 300 bps (&N1). NOTE: Substitute the &B0 setting only if the remote modem is operating at 300 bps and the system it is attached to (usually older equipment) operates at 110-300 bps. In addition, set your software to 300 bps if you plan to use the escape code sequence (+++) to hang the modem up or bring it back to Command mode. When set to &B0 and operating at 300 bps, the modem ignores all flow control settings. Calls at 110 bps: Set the modem to &B0 and &N0, and your software to 110 bps. The Courier then will only connect with a modem that is operating at 110 bps. The modem ignores all flow control settings. If the Courier is set to &B1 for a 110 bps call, the call will be unsuccessful. When the modem is set to &B1, even if the DTE transmits to the modem at 110 bps, the modem transmits the data over the phone link at 300 bps using Bell 103 modulation. Data Set Ready (&Sn) The modem sends the DTE a Data Set Ready (DSR) signal via the RS-232 interface. (Data Set is industry jargon for modem.) Few, if any, commercial communications programs require the modem to control DSR, &S1. Leave the modem set for DSR overridden, &S0, unless you know that your installation requires a different setting. &S0 DSR is always ON (override). Default. &S1 In Originate mode, the modem sends DSR after it dials, when it detects the remote modem's answer tone. In Answer mode, the modem sends DSR after it sends an answer tone. &S2 This option is for specialized equipment such as automatic callback units. On loss of carrier, the modem sends a pulsed DSR signal with Clear to Send (CTS) following Carrier Detect (CD). &S3 This is the same as &S2, but without CTS following CD. DATA TERMINAL READY (&Dn) This command, like DIP switch 1, controls Data Terminal Ready (DTR) signaling from the DTE to the modem. The modem is factory set with DIP switch 1 UP, for normal operations and the override disabled. If you want to change the modem's DTR operations during a session, you can either change the DIP switch position or use the &D command. Whichever method you choose, check either the quick- configuration guide that came with the modem or your communications software manual to see which setting is required. NOTE: At power-on and reset, the modem operates according to the DIP switch setting. This command is not stored in nonvolatile memory as a power-on/reset default. &D0 DTR override. The modem operates as though DTR is always ON. &D1 Reserved. &D2 Normal DTR operations. The terminal or computer must send a DTR signal for the modem to accept commands. Dropping DTR terminates a call. CARRIER DETECT (&Cn) This command, like DIP switch 6, controls Carrier Detect (CD) signaling from the modem to the DTE. The modem is factory set with DIP switch 6 UP, normal CD operations and the override disabled. If you want to change the modem's Carrier Detect operations during a session, you can either change the DIP switch position or use the &C command. Whichever method you choose, check either the quick-configuration guide that came with the modem or your communications software manual to see which setting is required. NOTE: At power-on and reset, the modem operates according to the DIP switch setting. This command is not stored in nonvolatile memory as a power-on/reset default. &C0 CD override, CD always ON. &C1 Normal CD operations. The Courier sends a CD signal when it connects with another modem and drops CD on disconnect. RACK CONTROLLER UNIT ACCESS (%Rn) If the Courier modem is part of a USRobotics Total Control Modem Management System, setting the modem to %R1 allows an operator to use the modem to call a Rack Controller Unit (RCU) at a remote site. %R0 Normal operations, RCU access disabled. This is the default. %R1 RCU access enabled. TOUCH-TONE RECOGNITION MODE (%T) This command enables the modem, when off hook, to detect the Touch-Tone frequencies of dialing modems. %T is meant primarily for use with network applications, but may also be integrated into certain software programs. For example, %T could be used in a security program to identify incoming Touch-Tone security codes. To enable this feature, type ATH1 <Enter> to force the modem off hook. Then type AT%T <Enter>. To return the modem to Command mode, press any key or drop the DTE's DTR signal. The modem responds OK.