What Is A MODEM, And Why Do I Need One?
Michael E. Day
West Linn, OR
How To Use A Modem
The standard modem that the hobbyist normally encounters is the BELL 103 type modem. Additionally, the BELL 113A (originate) and the 113B (answer) are normally included in this group, with the specification of a 103 compatible modem often being used to indicate the 113A or 113B type modem. The 103 type modem is rated for speeds of 0 to 300 BPS, with most modems being able to operate up to 400 BPS, and the more expensive models being able to operate at up to 600 BPS. The maximum theoretical limit of the 103 type modem is 1000 BPS. However, due to the great amount of filtering and special line conditioning required to operate as the speed approaches this level, it becomes impractical to operate at these speeds. Due to this, 600 BPS is the maximum reliable speed that can be expected from the 103 type modem. It should be remembered, however, that 300 BPS is the maximum guaranteed speed of the 103 type modem. Speeds faster than this will not always work (depending on the phone line condition), and those modems capable of operating at greater speed generally cost twice as much as the lower speed types. The modem may be used at any speed less than 300 BPS. Dropping the speed to 150 or 110 BPS can often improve the reliability if the connection is very poor, and 300 BPS does not work.
The 103 has two modes of operation, the "answer" mode and the "originate" mode. The 113 modem will only work in one of the modes (113A for originate and 113B for answer) and not the other. Most of the acoustic coupled modems found on the surplus market are the originate type. This type of modem is what you need to talk to most of the computers that the hobbyist has access to (such as CBBS/NW). The originate modem is so named because it is used by the person or device that places the call (or originates the call) to the remote computer or person. The answer modem is used by the person or device that receives the call (or answers the call). The two modes could be reversed, as the phone line doesn't care. They were set that way to provide a standard as to which modem should use which mode. Since the 103 is a full duplex modem (two-day communications), two separate communication links must be established, thus the two different modes. The answer modem transmits on the high frequency link and receives on the low frequency link.
A true 103 type modem will be capable of operation in either mode depending on a control function. (This could be as complex as a control sequence or as simple as a switch).
Of all the generally available modems, the 103 is the most forgiving. It will operate at any speed less than its designed maximum. It is totally transparent to any protocols that might be used as long as it is asynchronous type transmission and it requires no special handshaking (control) signals in its basic configuration. This is ideal in a portable application where it might be used in a wide variety of configurations.
|PIN 1||FRAME GROUND||Tied to the modem case (if metal). Can be ignored normally.|
|PIN 2||TXD||Transmit Data.
The data to be transmitted is presented to this pin.
|PIN 3||RXD||Receive Data. The received data is present on this pin.|
|PIN 4||RTS||Request to send.
Generally ignored by modem, it can Sometimes be used to turn the transmitter on and off (1 = on; 0 = off).
|PIN 5||CTS||Clear to send.
This pin is normally held high (on). Alternately it can follow RTS, or DCD, or both (depending on modem).
|PIN 6||DSR||Data set ready.
This signal will always be on when the modem is operational (power on).
|PIN 7||LOGIC GROUND||This is the common reference ground for all the signals listed.|
|PIN 8||DCD||Data carrier detect.
This signal will be on when the communications link has been established (the carrier signal from the remote modem is being received.) On some modems this is always on.
|PIN 20||DTR||Data Terminal Ready.
Depending on the modem, this can be ignored, the modem on or off (I =on; 0 = off). Also, in some turn modems, it is used in conjunction with RI to set the operating mode.
|PIN 21||SQD||Signal Quality Detector.
Generally this signal is not provided. On those modems that do provide it, it indicates that a poor communications link has been established, and that there is a high probability of errors occuring.
|PIN 22||RI||Ring Indicator.
Those modems that do not have mode control generally ignore this signal. Those modems that do use this signal generally use it to determine which mode to place the modem in. If the RI signal was present prior to the DTR signal, then the modem is placed in the answer mode. If the RI signal was not on prior to the DTR signal, then the modem is placed in the originate mode.
All other pins are undefined in their actions and no connections should be made to them to prevent possible malfunction of the modem.
Sometimes large amounts of data transfer is desired. With a 103 type modem limited to 300 BPS, this can get to be a bit tedious after a while. The 202 type modem serves as a compromise for this type of operation. By dropping one of the communication links, transmission speed can be boosted to 1200 BPS. This allows the data to be transmitted at a much faster rate, thereby improving the throughput.
Because of the lack of the other data link, however, the control of the communications flow becomes much more difficult. Since there is only one data link available, only one modem may transmit at a time (half duplex). This means that some sort of protocol must be decided upon to determine which modem may transmit. One of the more common ones is to send an ASCII "EOT" as the last character. This tells the receiving device that transmission is over and it may turn on its modem.
The operation of the control signals is very similar to the 103 type modem. Therefore only the differences will be discussed:
|PIN 4||RTS||Request to send.
This signal is used to turn the transmitter on and off.
|PIN 5||CTS||Clear to send.
This signal is generated from RTS, DCD, and an optional time delay and indicates when a valid transmission link has been established, and transmission may begin.
|PIN 6||DCD||Data carrier detect.
This signal is used to indicate that a carrier is being received.
|PIN 12||SDC||Secondary data carrier detect.
Optional reverse channel detector signal is present when reverse channel is present.
(On some BELL 202's this is PIN 11)
|SRTS||Secondary request to send.
This signal turns the reverse channel carrier on and off (1 = on; 0 = off). If this PIN is tied high, then the reverse channel is controlled by the request to send (when request to send is on, reverse channel is off, and vice versa).
The reverse channel option is normally used as a circuit assurance or interrupt channel since, as long as the reverse channel is present, the transmitting modem can be assured that the data link is being maintained, and the receiving modem can use it to request an early termination of the transmission by turning it off.
The 202, like the 103, is quite flexible in its operation. It can be used at any speed up to its maximum allowable speed. It is transparent to most protocols as long as they are asynchronous. Some means must be provided, however, for turning the transmitter on and off.
Another modem that is becoming popular is the 212 modem. This modem combines the features of the 103 and 202. It has two data links (full duplex operation), yet can operate at 1200 BPS. It is, however, very limited in its operation. The transmission protocol is fixed, and the speed must be 1200 BPS exactly. For this you get full duplex operation, which means no transmitter control is required. Also, because of the transmission method used, it is inherently more difficult to build (i.e. more expensive). Transmissions between the two linked modems occur as DIBIT synchronous, and are then converted to asynchronous for transmission to and from the connected equipment. The PIN out of the 212 is the same as the 103 type modem. The operational mode (103 or 212) is determined by the signal applied to PIN 23 (0 = 103; 1=212).