Asynchronous Terminals and Printer Processes Programming Manual

TERMINAL PROGRAMMING CONSIDERATIONS

Types of Modems

Many types of modems are manufactured by many different vendors.

Most of them are described as being compatible with or equivalent

to devices manufactured by the Bell system. The following types

of modems are supported by ATP6100 and TERMPROCESS: 103, 113,

202, 212, and other full-duplex, asynchronous modems.

Bell 202-type modems are used on half-duplex lines. They require

request to send (RTS) to be dropped when switching from transmit

to receive. Secondary data carrier detect (SDCD) must be high

to the DCE when receiving. This is accomplished by raising the

secondary request to send (SRTS) signal. If the reverse channel

is present, SDCD must be high from the DCE before transmitting

can begin. BREAK is detected if SDCD goes low from the DCE while

transmitting.

Bell 212-type modems are dual speed. SDCD is the speed

indicator. If SDCD from the DCE is high, the speed is 1200

baud. If SDCD is low, the speed is 300 baud. Data terminal

ready (DTR) is not raised until the I/O process issues a request

to enable the connection (CONTROL 11). DTR is not dropped unless

the I/O process issues either a request to disable the connection

(CONTROL 12) or a stop line request.

Data set ready (DSR) is always monitored.

The carrier (CD) is monitored only if autodisconnect is enabled.

Modem Connections

There are two types of modem connections: leased and switched

(dialed over the public telephone network).

Leased Line Connections

With leased line connections, the modem is indeed transparent.

Your application interacts with the remote terminal with file

system procedure calls, such as OPEN, CLOSE, READ, and WRITE.

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