Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 41)

Installation
Chapter 3
3-17
After detecting DCD, the module continues to monitor the DCD line. If
DCD goes off, the module restarts the timeout. If DCD is not restored
within the timeout, the module initiates the hang-up sequence. This
feature allows the remote station to redial in the event that the connection
is lost through a fault in the phone network.
This handshaking is necessary to guarantee access to the phone line. If
this handshaking protocol is defeated by improper selection of modem
options or by jumpers at the connectors, the modem may still answer a
call. But if the connection is lost, the modem will not hang up. It will
then be impossible for the remote station to reestablish the connection
because it will get a busy signal.
Character Transmission
The module sends data serially over the asynchronous interface, one 8-bit
byte at a time. The transmission format conforms to ANSI X3.16,
CCITT V.4, and ISO 1177, with the exception that the parity bit is
retained while the data length is extended to eight bits.
The transmission format may be summarized as follows:
start bit
data bit 0
data bit 1
data bit 2
data bit 3
data bit 4
data bit 5
data bit 6
data bit 7
even parity bit (optional)
one stop bit
For communication rate see “Asynchronous Communication Rate and
Diagnostic Commands” and for parity settings refer to “Asynchronous
Link Features.”