CHAPTER 11 CARRIER CONTROL Most modems have an output control signal which goes "high" or "on" whenever a remote carrier is detected. This indicates a connection has been made with another system. This carrier-detect signal (DCD) can be used to control some of the functions of MODEM MGR. The advantages of carrier control are: 1) You will realize more readily when an unanticipated disconnect occurs. 2) With external non-smart modems, this is the only way for this software to determine a connection has been made. The connect status is required to support some macro commands and the unattended mode with external non-smart modems. 3) In the unattended mode, your system will reset for the next call immediately after a disconnect instead of relying on timeout. 4) If you are using a macro or performing a file-transfer operation, it will terminate immediately if a disconnect occurs instead of continuing to the end of the operation. 5) If you are receiving files using a protocol transfer, the files will be closed immediately if carrier is lost. Otherwise they will be closed when you cancel the transfer or timeout occurs. The disadvantages of carrier control are: 1) Some "smart" modem result codes and status messages may not be received and displayed when there is no carrier. These include DIALING, NO DIALTONE, RING, BUSY, NO ANSWER, NO CARRIER, CONNECT and other messages. 2) If the carrier-detect signal is not properly connected to your computer, your system may fail to operate. The proper connection is already accomplished if you have an internal plug-in modem, but you will have some additional interfacing effort with an external modem. The second disadvantage above is the reason we have promoted the initial operation of this program with carrier control disabled. After you become familiar with this program you may wish to try it with carrier control enabled to see if you prefer this mode of operation. If all you want is attended communications, you can get by with carrier control disabled. If you have a good feel for what your hardware is doing, and have a method of monitoring your modem status, you don't need the extra benefits of carrier control. If you are aware of the advantages (and disadvantages) of carrier control and are reasonably sure you can interface your hardware correctly, then proceed with the remainder of this chapter. In order to utilize carrier control, you must do three things: 1) You must set carrier control ON (enabled). This is one of the defaults you can set when you use the INSTALL program. 2) If you have an external modem, you must wire the carrier-detect signal (DCD) from your modem to your computer so MODEM MGR can determine the carrier status. 3) On some modems, you must set a carrier detect switch to enable carrier status output. ENABLING CARRIER CONTROL You can set carrier control ON (enabled) by executing the INSTALL program. When prompted by the program, insert your work disk to use your currently installed drivers. Insert the install disk when prompted and select option 7 to customize the default settings. When the default settings menu appears, select 12 to toggle carrier control from OFF to ON. Finally, insert the work disk and save the newly installed program. Whenever carrier control is set ON, the status command (L) in the terminal comm and mode will display CARRIER ON or CARRIER OFF as the last status message. If carrier control is disabled, there will be no carrier status shown. This is one way you can determine if carrier control has been enabled. INTERNAL MODEM CARRIER CONTROL INTERFACING Carrier control interfacing is easily implemented if you have an internal plug- in modem. Carrier-detect control interfacing is automatically accomplished when the card is plugged into your computer. On the Prometheus Pro Modem 1200A, set configuration switch 3 OFF. On the Cermetek AppleMate 1200, set SW10 open. On the Hayes Smartmodem 1200A, set switch 2 UP. EXTERNAL MODEM CARRIER CONTROL INTERFACING If you have an external modem, you will have to connect the carrier-detect signal from the modem to the appropriate input on your computer or serial interface card. In some cases, this is accomplished simply when you connect your cable between the modem and the card. In other cases, you will have to add or modify wires in your interface cable. Most serial cards have several input control lines. These include a receiver control input and a transmitter control input. A communications program can use any of these control inputs for carrier control. MODEM MGR uses the receiver control input for carrier control. The reasons for this choice are: 1) Some serial cards have only one control input--the receiver control input. 2) If you are using a "smart" modem, you should be able to transmit commands to the modem even when no carrier is present. Therefore, the transmitter control input must always be "high". On most external modems, the carrier-detect signal is on pin 8 of the modem DB- 25 connector. However, the receiver control input is not always on pin 8 of the connector at the serial card. Some common serial cards are listed at the end of this chapter in Table format along with the proper pin to connect the modem carrier-detect signal to. On some cards you must also set a switch or jumper to enable the input. This is noted where applicable in the Table. In many cases, you cannot use a "standard" interconnecting cable with straight through pin-to-pin wiring to connect the carrier control signal. For example, if you want to use carrier control with the PSIO card, the Table shows you must wire pin 8 of the modem to pin 5 of the PSIO cable connector. You must also install card jumper J2 between the center pin and the right-most pin. In the Table, note the Apple //c and the Alphabits card have a five-pin round DIN connector instead of a DB-25 connector. The numbering convention used for the DIN connector corresponds to Apple's convention. Note also the Apple Communications card does not support carrier control. If your serial card is not listed in the Table, contact the card manufacturer to determine which pin is used for receiver control or inspect the schematic diagram (if there is one) in the card instruction manual. Find the ACIA chip on the schematic (usually a 6551 or 6850) and trace the DCD signal on the chip through the various circuit stages to the connector pin. Sometimes this pin is on the card connector and not the cable DB-25 connector so you will have to trace the signal path through the cable. The Apple Modems 300 and 1200 do not use DB-25 connectors. The carrier detect signal on these modems is on pin 7 of the nine-pin DB-9 modem connector. If you are using the interface cable (Apple Part No. 590-0121) from the //e modem accessory kit, this signal will appear on pin 5 and pin 8 of the DB-25 connector on that cable. If you are using the interface cable (Apple Part No. 590-0192) from the //c modem accessory kit, the carrier detect signal is not cabled to the five-pin DIN connector. The modem DSR signal (which is always high) is cabled to the DIN connector so the carrier status will always be on. Therefore, you cannot use carrier control with this cable unless you modify the cable or use an adapter cable between the modem and the cable. To achieve carrier control with this cable, an adapter cable with a male DB-9 connector on one end and a female DB-9 connector on the other end should be wired as follows: DB-9 male DB-9 female Function 3 3 Signal ground 5 5 Data from modem to //c 6 6 DTR modem input 7 2 DCD carrier detect 8 8 Chassis ground 9 9 Data from // c to modem Note five wires are wired between the same pin numbers, but one wire (the carrier detect signal) is not. CABLE PROBLEM ][+ OR //E) If you are using the Apple Part No. 590-0121 cable from the Apple Modem 300 or 1200 accessory kit, you may have some problems if you try to use carrier control. This cable connects the modem DCD signal to both pins 5 and 8 at the DB-25 connector. Some serial cards use one of these pins for the transmitter control input. If your modem is set for carrier control, the transmitter control will be set "low" when you are not on-line and you will not be able to transmit commands to the modem. If you set your modem for a continuous "high" carrier output, you can send commands to the modem but you cannot use carrier control. If you wish to use carrier control, you must use a special cable or modify the card or its output cable. For example, on an Apple Super Serial Card, the transmitter control is on pin 5. You can disconnect the wire on pin 5 of the short cable that goes from the card to the rear of the computer. MODEM DCD ENABLE If you are using an external modem, you will probably have to enable the carrier-detect signal output on the modem. Most modems have a switch which will either set this DCD signal permanently high or allow it to follow the carrier status. In Chapter 3, you were directed to set this switch to output a continuous high signal. To use carrier control, change the setting of this switch so the carrier-detect signal will provide the proper carrier status. Some "smart" modems do not have this switch so the carrier-detect signal always follows the carrier status. If you have a Hayes Smartmodem 2400, the modem DCD function will be set automatically by this program depending on whether you have carrier control enabled or disabled. Always power the Smart-modem 2400 on before starting the program. REMOVING CARRIER CONTROL If you have interfaced your system so it will respond to carrier control and you wish to remove carrier control, set the modem switch so the carrier-detect signal is high all the time and run the INSTALL program to set carrier control OFF (disabled). Serial Card Carrier Control Input Apple IIGS Modem Port -pin 7 Apple Communications -none Apple Super Serial (Switch SWl-7 ON) -pin 8 Apple // c -pin 5 * Apricorn Serial -pin 6 Apricorn Super Serial Imager (Switch SWl-7 ON) -pin 8 AST Multi I/O -pin 6 CCS 7710 -pin 20 CCS 7711 -pin 6 or 8 (DTR jumper ON) Franklin Dual I/O -pin 8 Microtek SV-622 pin 8 Mountain CPS -pin 8 Pract. Periph. Serial1 (HHS jumper on 5) pin 8 Prometheus Versacard -pin 6 Quadram Multicore -pin 6 Street Alphabits -pin 5 * Street Businesscard -pin 5 * Transend AIO -pin 8 Transend ASIO -pin 8 Videx PSI0 -pin 5 (Set J2 to right-most pin) * Five-pin round DIN connector