Rockwell Automation 1761 MicroLogix 1000 Programmable Controllers - Manual (Page 337)

Understanding the Communication Protocols
D–9
Using Modems with MicroLogix 1000 Programmable Controllers
The types of modems that you can use with MicroLogix 1000 controllers include
dial-up phone modems, leased-line modems, radio modems and line drivers. For
point-to-point full-duplex modem connections that do not require any modem
handshaking signals to operate, use DF1 full-duplex protocol. For
point-to-multipoint modem connections, or for point-to-point modem connections
that require Request-to-Send/Clear-To-Send (RTS/CTS) handshaking, use DF1
half-duplex slave protocol. In this case, one (and only one) of the other devices
must be configured for DF1 half-duplex master protocol. Do not attempt to use
DH-485 protocol through modems under any circumstance.
Note Only Series D or later MicroLogix 1000 discrete controllers and all MicroLogix
1000 analog controllers support RTS/CTS modem handshaking and only when
configured for DF1 half-duplex slave protocol with the control line parameter set to
“Half-Duplex Modem”. No other modem handshaking lines (i.e. Data Set Ready,
Carrier Detect and Data Terminal Ready) are supported by any MicroLogix 1000
controllers.
Dial-Up Phone Modems
Dial-up phone line modems support point-to-point full-duplex communications.
Normally a MicroLogix 1000 controller, on the receiving end of the dial-up
connection, will be configured for DF1 full-duplex protocol. The modem connected
to the MicroLogix 1000 controller must support auto-answer and must not require
any modem handshaking signals from the MicroLogix 1000 (i.e., DTR or RTS) in
order to operate. The MicroLogix 1000 has no means to cause its modem to initiate
or disconnect a phone call, so this must be done from the site of the remote modem.
Leased-Line Modems
Leased-line modems are used with dedicated phone lines that are typically leased
from the local phone company. The dedicated lines may be in a point-to-point
topology supporting full-duplex communications between two modems or in a
point-to-multipoint topology supporting half-duplex communications between three
or more modems. In the point-to-point topology, configure the MicroLogix 1000
controllers for DF1 full-duplex protocol (as long as the modems used do not require
DTR or RTS to be high in order to operate). In the point-to-multipoint topology,
configure the MicroLogix 1000 controllers for DF1 half-duplex slave protocol with
the control line parameter set to “Half-Duplex Modem”.