ATL ATU Telex Unit Посібник з експлуатації та встановлення - Сторінка 9

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Flow Control by XON/XOFF

The flow of data in each direction is controlled by the inclusion of

control characters in the data. The characters used are Enable Input

(XON, ASCII DC 1) and Disable Input (XOFF, ASCII DC3). The characters

can be sent at any time and are not restricted to use only during data

transfer.

When the Telex Unit sends (XOFF) the Host must stop sending

characters to the Telex Unit. The Telex Unit will successfully receive a

number of characters (at least 10) before data transfer ceases. If the

Host continues to send data in excess of this Telex Unit assumes a Host

fault.

Flow Control by DSR/DTR

Alternative method of flow control but uses DSR/DTR. The DTR signal

is used to control the flow of data passed from the Host and DSR to

control the flow of data to the host. The signal is set 'low' to disable

input and high to enable; the same rules apply as for XON/XOFF.

Security Checking by use of ENQ/ACK

This method of security checking involves the regular exchange of

control characters between the Telex Unit and the Host. The Telex Unit

sends a Confidence Check enquiry character (ENQ) to the Host and

expects to receive an OK character (ACK) in response. After sending an

(ENQ) the Telex Unit checks to see if an (ACK) has been received. The

Host must reply within 10 seconds (5 seconds in U.K.) otherwise a fault

in the Host is assumed and the line is cleared.

If the Host is not ready to receive data it sends a Not OK character

(NAK) response. In this case the Telex Unit clears any incoming call and

closes the line to incoming calls. This condition is cleared by the Host

sending an (ACK) in response to a later (ENQ).

Security Checking by DSR/DTR

An alternative method of security checking is by use of the DSR/DTR

RS232C interface signals. The Telex Unit will continue to operate

provided the DTR signal is 'high'. If the DTR is low for longer than 10

seconds (5 seconds in U.K.) the Telex Unit will assume there is a Host

fault.