 on the line and start-stop operation will often be good enough. A somewhat higher character transmission rate could be obtained with synchronous transmission. The other main advantage of synchronous transmission is that the error rate can be less. Extremely good error control can be achieved with high-order error-detecting codes and a buffer in the sending machine SO that retransmission can be automatically requested. The expense of syn- TERMINAL TECHNOLOGY 55 chronous operation is diminishing as the cost of logic circuitry drops. At the same time, the reliability of logic circuitry is substantially in- creasing. All the logic for a buffered, synchronous terminal can now be constructed on one large-scale integration chip, which are low cost when mass-produced. The use of buffers is the only way to guarantee a good response time on a multidrop line, and to make transmission from the buffer as fast as possible, it would be worthwhile to use synchronous transmission. A terminal on a polled line (Fig. 4-3) must recognize its own ad- dress and respond to the polling signals. The logic for this process is contained in the terminal control unit, buffer, plus the ability to code and decode error-detection characters in messages and to re- transmit or request retransmission of errors. The terminal may use full-duplex or half-duplex transmission. It Dial or leased line Modem Modem Modem Up to 32 Modem Polled line Modem Modem Station communications Address processor Low cost modem eliminaters can be used for local connection Fig. 4-3. Typical terminal configurations. 56 TELECOMMUNICATIONS may be designed SO that data can travel in both directions at once, although it is more common for data to travel in one direction while only control signals travel simultaneously in the other. Many ter- minals are not capable of transmitting and receiving at the same time-in which case, there is no point in paying extra for a full- duplex line for them. In some countries a full-duplex line costs the same as a half-duplex line, and SO a t