COURIER HST CHAPTER 3 OPERATIONS AT A GLANCE Here are some brief guidelines for immediate operation of the modem. DATA FORMAT Ten-bit data units: check the table on page 6-1 or page F-1. ERROR CONTROL Both modems must be set to error control mode for error detection and retransmission to occur. Always use error control for 9600- bps communications. We use the term ARQ (automatic repeat request) for error control. The three ARQ settings are as follows: &M0 Normal Mode. No error control. &M4 Normal/ARQ Mode -- Default. The Courier attempts an ARQ connection; if the signal isn't recognized, the modem continues in Normal Mode (&M0). &M5 ARQ Mode. The Courier attempts an ARQ connection; if the signal isn't recognized, the modem hangs up. DATA COMPRESSION The modem is set at the factory to negotiate for data compression. Leave the modem set to its default unless you are transferring files that are already compressed, in which case you should disable data compression by setting the modem to &K0. For more information, see the throughput guidelines at the beginning of Chapter 4 and the &K command description in that chapter. DIALING Use the following settings/commands to call the specified type of modem at the indicated speeds. HST-Compatible, 9600 bps Terminal/software: 19.2k (preferable) or 9600 bps Type: AT &H3 D phone number 3-1 COURIER HST The command includes the setting for hardware/software flow control (&H3). See the flow control guidelines that follow. If the modem isn't set for error control, include &M4 or &M5 in the command line. MNP-Compatible, 2400 or 1200 bps Terminal/software: 19.2k, 9600, 2400 or 1200 bps Type: AT &H3 D phone number The command includes the setting for hardware/software flow control (&H3). See the flow control guidelines that follow. Non-MNP Compatible, 2400, 1200 or 300 bps See the flow control guidelines that follow. Terminal/software: 2400, 1200 or 300 bps Type: AT &M0 D phone number NOTE: &M0 suppresses the MNP signals that may be misinterpreted by the remote system and prevent a successful connection. FLOW CONTROL Hardware and/or software flow control can be used for transmitted and received data. We recommend hardware Transmit data flow control (&H1) since this setting doesn't affect the data stream. This is especially important if you're transmitting binary data. Transmit Data flow control (&H) is required in the following situations. Received Data flow control is optional. o You're using error control (9600/2400/1200 bps), with or without data compression. Flow control prevents buffer overflow in the event that line disturbances cause frequent retransmissions. o The rate at the local terminal interface is higher than the link rate. o The volume of data transfer is high. Flow control commands are explained in Chapter 4. MAXIMUM THROUGHPUT Use these settings for both Originate and Answer Modes. The default data compression setting, &K1, is assumed. Terminal/software: 19.2k bps 3-2 COURIER HST Modem: Fixed terminal interface rate (&B1) Variable link rate (&N0) Transmit Data flow control (&H1 preferred, &H2 or &H3) HIGH-SPEED CHANNEL TURNAROUND As described in Chapter 1, HST modems use asymmetrical modulation. The modems allocate the high-speed channel on demand so that the modem with the greatest amount of data in its buffer transmits at 9600 bps. Turnaround of the channel is automatic and requires no user intervention. If the remote modem has a 300-bps back channel, the Courier automatically drops its back channel rate from 450 to 300 bps. LINK NEGOTIATION When a Courier HST calls an HST-compatible modem and both are operating at 9600 bps, the modems negotiate the link connection (shake hands) at 2400 bps and automatically resume operation at 9600 bps. The lower handshaking rate maintains compatibility with the existing telephone network as well as existing hardware and software. It's possible for two HST-compatible modems to connect directly at 9600, without 2400-bps handshaking, but both modems must be set as follows: Terminal/software: 9600 or 19.2k bps Modem: Fixed link rate of 9600 bps (&N6) Error control, either &M4 or &M5 Transmit Data flow control, preferably hardware, &H1 DATA RATE DEFAULTS Terminal interface: &B0, detect rate from the AT command, then follow the connection rate. Link interface: &N0, negotiate the highest possible rate with the remote modem in both Originate and Answer Modes. The terminal interface and link rates may be set to fixed rates using &B1 or &N1-6, respectively. NONVOLATILE MEMORY (NRAM) The modem is factory set (DIP switch 10 OFF) to load the settings stored in NRAM on power up. Write your own configuration defaults to NRAM with the &W command. To review the NRAM 3-3 COURIER HST settings, refer to Appendix B-4 or send the modem the I5 option of the Inquiry command: ATI5 PHONE NUMBER STORAGE Use &Zn=s to store four frequently called phone numbers in NRAM (nonvolatile memory); n = positions 0 through 3, s = the number- string. The string may contain a maximum of 36 characters and any Dial command options, but no other operational settings. Example: AT&Z=9,,1 312 5551234 (Store at position 0) Example: AT&Z1=5551234 (Store at position 1) DIALING STORED NUMBERS To dial, issue the DSn command, where n indicates the position of the number in NRAM. Example: ATDS1 (Dial number at position 1) The first command in the following sequence stores the phone number and access code for a long distance service. The second command dials the stored number/access code and continues dialing the long distance phone number. AT&Z2 = 5551234,,9876,, ATDS2 D 1 312 5556789 BUSY ANSWER Use A> instead of A/ and the modem redials up to 10 tries instead of once. Neither A> nor A/ takes the AT prefix or a Carriage Return. Insert the Repeat Mode command (>) before or after the Dial string and the modem automatically redials up to 10 tries: AT>D5551234 ATD5551234> You may include the Repeat command when you store a phone number- string in NRAM: AT&Z2=5551234> INACTIVITY TIMER Set the inactivity timer (S-Register 19) if you suspect that a connection may be inadvertently left open without data transfer. 3-4