CHAPTER 3. QUICK STARTêASYNCHRONOUS OPERATIONS ASYNCHRONOUS/SYNCHRONOUS COMMUNICATIONS Courier high speed desktop and rack modems are capable of asynchronous and synchronous communications. Terminals are usually capable of one type of communications or the other. The typical personal computer, however, is equipped to communicate only asynchron- ously. Most of the operational information in this manual is organized around asynchronous communications. During asynchronous communications, the computer adds Start and Stop bits to each data character before sending it to the modem over the serial port, and expects received data to be in the same format. Synchronous data transfer is based on synchronized timing between two communicating devices, not on data format, and is usually required by mainframes and other large computers. To exchange data synchronously, a personal computer requires specialized equipment, including the installation of a synchronous communications card with a synchronous port. If you will be making synchronous connections, you'll find the modem easy to use. Just follow the instructions in Chapter 7. INITIAL MODEM SETTINGS This manual covers the operation of three Courier models, the Courier HST Dual Standard, Courier V.32 bis, and Courier HST. The modems are factory-set for compatibility with their own type of modem. ù Dual Standard modemsêB0êso that they connect at high speeds with both V.32 bis and HST modems ù V.32 bis modemsêB0êto connect at high speeds with V.32 bis and V.32 modems ù HST modemsêB1êto connect at high speeds with HST modems The above settings do not affect how the modems connect at speeds of 2400 bps and below. Leave the modems set to their factory defaults, B0 or B1. The exception is when an HST modem is to answer overseas calls, in which case you should set it to B0. The modem is shipped ready for use except for Transmit Data flow control. This type of flow control is required for calls under error control, and when the computer sends data to the modem faster than the modem can send it over the phone link. How you set your modem depends on whether or not your machine or software support flow control, and what type of flow control they allow. Use the guidelines that follow. Recommended Settings The following recommended settings will yield the best data reliability and throughput. Check the quick- configuration guide that came with the modem to see if your communications program is listed and whether it supports both a fixed data rate between the computer and modem (setting the modem to &B1) and hardware flow control (setting the modem to &H1). If a program only supports software flow control (XON/XOFF characters), use the Alternative Settings after this section. If necessary, review your computer/terminal and software documentation as well to see if your system supports the recommended settings. If either your software or machine do not, use the Alternative Settings after this section. 1. Set your software to the highest rate your equipment and software support: 19.2K, 38.4K, or 57.6K bits per second. This sets a high serial port rate, that is, the rate of data transmission between the computer and the modem. 2. Set the software to a fixed serial port rate so that the computer and modem maintain the high rate, regardless of the rate on the phone line. Some programs call this locking the serial port. 3. Set the software for hardware flow control. This may be called RTS/CTS. The modem uses Clear to Send (CTS) to control the data flow from the computer. This is the most efficient and reliable form of flow control. 4. Set the modem to a fixed serial port rate (&B1) and hardware flow control (&H1). Have your software put the computer in Terminal mode, as shown in Step 3 of Chapter 2, Testing the Installation, and type the following command. ( indicates pressing the Enter key.) AT &B1 &H1 To have the modem always use these settings as defaults, type &W at the end of the command string, before you press the Enter key. The &W command causes the modem to write the settings to nonvolatile memory. Alternative Settings Use the above command format to set the modem. 1. Your machine or software doesn't support hardware flow control, but your software supports XON/XOFF signaling: &H2 The modem sends the computer Ctrl-S and Ctrl-Q (XOFF/XON) characters to control the flow of data. But see Guideline #2 below. &B1 As in Step 2 under Recommended Settings, if your software supports a fixed serial port rate. 2. Your machine or your software does not support hardware flow control, and you're using an Xmodem-type file transfer protocol: &H0 Flow control disabled (factory setting). Do not use software flow control (&H2) because the protocol control characters will be misinterpreted as flow control characters, and you risk losing data. &B0 The serial port rate switches to match the connection rate of each call. Required because there is no flow control. 3. Your system doesn't support either Clear to Send or XON/XOFF signaling. In this case, we recommend that the following configuration not be used for calls above 2400 bps. &H0 Flow control disabled (factory setting). &B0 The serial port rate switches to match the connection rate of each call. Required because there is no flow control. &M0 Error control disabled; recommended because there is no flow control. 4. Your software does not support a fixed rate: &B0 Factory setting. The serial port rate switches to match the connection rate of each call. This does not affect error control or flow control. You will not, however, gain the throughput efficiency possible with a serial port rate fixed higher than the link rate. In addition, at the factory setting for data compression, &K1, the modem disables com- pression when the link rate is set to &B0. 5. Your maximum computer rate is 9600 bps: S34=3 This disables higher-speed V.32 bis modulation. The modem operates as a V.32 modem, with a maximum serial port and link rate of 9600 bps. (Does not apply to HST modems.) &H1 Hardware flow control (CTS), or &H2 for software flow control. &B1 Fixed serial port rate of 9600 bps, recommended for HST modems. &M4 Error control factory setting required, or &M5. 6. You are calling a non-MNP modem: S27=16 Disable MNP handshaking. A non-MNP modem may misinterpret the MNP link request and prevent a successful connection. This does not apply for remote V.42 modems with error control enabled. 7. You are transferring compressed files: &K3 This disables MNP compression, which does not work successfully with files that are already compressed. If V.42 bis compression is negotiated, you will gain optimal throughput. DATA FORMAT The Courier and the remote modem must use the same ten-bit data format. The following table lists the allowable word lengths, parity and Stop bits. If a remote modem requires 2 Stop bits and the format totals 11 bits, the second Stop bit is not counted. StartWord Parity Stop BitsLength (1 Bit) Bits 1 7 Even, Odd 1 Mark, Space 1 7 None, Even, Odd, 2 Mark, Space 1 8 None 1, 2 STORING DEFAULTS IN NONVOLATILE MEMORY Write your default configuration to nonvolatile memory (NVRAM) with the &W command. The following example sets the modem for the current session and also stores the settings as defaults. The defaults are loaded from NVRAM when the modem is powered on if DIP switch 10 is UP. Be sure to insert &W last in the string, before pressing the Enter key. AT &B1 &H1 &W DIALING Use the following command format. The modem defaults to pulse dialing: the T in the command enforces tone dialing. The maximum number of characters in a command string is 40. ATDT phonenumber To redial the number, which the modem stores in a last-dialed number buffer, use the following command: ATDL LINK RATE NEGOTIATION During link negotiation (handshaking), the modems negotiate the highest possible rate, depending on each modem's capabilities. The Courier automatically switches rates to match the rate of the remote modem, within the following rate ranges: 16.8K (HST-to-HST only), 14.4K, 12K, 9600, 7200, 4800, 2400, 1200 bps and, without error control, 300 bps. For more detailed information on handshaking, see Appendix A. ONLINE FALLBACK When online at high speeds, the modems perform Adaptive Speed Leveling. They monitor the condition of the phone line. If they sense disturbances that threaten data integrity, they fall back and retrain (resynchronize) at the next lower speed in their fallback range: 14.4K, 12K, 9600, 7200, 4800 bps. If conditions remain poor, Courier modems continue to fall back to the next lower speed. As line improvements occur, they fall forward to the next higher speed, up to the link rate of the call.