Rockwell Automation 1770-HT8 SMART TRANSMITTER - Manual (Page 54)

Installing the Smart Transmitter Interface
Products
Chapter 2
2-29
Modem Connections
The Communications Controller can be connected to a modem via a direct
25-pin to 25-pin cable, which you must construct using Belden #8723 (or
equivalent) cable.
Important: The length of the cable must not exceed 50 feet, and the cable
shield must be connected to chassis ground (using Pin 1) at the Communications
Controller end only.
Figure 2.27
Connection between a Communications Controller RSĆ232C Serial Port and a Modem
1
TXD 2
RXD 3
RTS 4
CTS 5
DSR 6
GND 7
DCD 8
DTR 20
MODEM
2 RXD
3 TXD
4 RTS
5 CTS
6 DSR
7 GND
8 DCD
20 DTR
Shield
90000
Communications Controller
The Communications Controller can be connected to any standard
asynchronous dial-up modem.
Important: Some modems are designed to respond to the DTR signal by
answering the phone whether it is ringing or not. Since the
Communications Controller asserts DTR at all times except during the
hang-up sequence, the phone would always appear to be busy. Do not use
the Smart Transmitter Interface with any type of modem that answers the
phone as soon as DTR is asserted.
The types of dial-up network modems that you can use are:
Manual: these are typically acoustically coupled modems. The
connection is established by human operators at both ends, who insert
the handsets into couplers to complete the connection.