ATL Telecom Telex - Manual (Page 9)

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.
9