Rockwell Automation 1761-HHP-B30 MicroLogix 1000 with Hand-Held Programmer (HHP) - Manual (Page 433)

Appendix D
Understanding the Communication Protocols
D–7
With the addition of DF1 half-duplex slave protocol, the controller clears the
file ownership if no supported commands are received from the owner within
the timeout period. If the file ownership were not cleared after a download
sequence interruption, the processor would not accept commands from any
other devices because it would assume another device still had file
ownership.
If a download sequence is interrupted due to noise caused by electromagnetic
interference, discontinue communications to the controller for the ownership
timeout period and restart the program download. The ownership timeout
period is set to 60 seconds as a default for all protocols. However, if you are
using DF1 half-duplex, and the poll timeout value is set to greater than 60
seconds, the poll timeout value is used instead of the ownership timeout.
After the timeout, you can re-establish communications with the processor
and try the program download again. The only other way to clear file
ownership is to cycle power on the processor.
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.
Important: 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, is 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.