ProSoft Technology MVI46-DNP - Manual (Page 109)

Reference MVI46-DNP ♦ SLC Platform
Master/Slave Communication Module
ProSoft Technology, Inc. Page 109 of 143
August 23, 2007
5.9 DNP Collision Avoidance
WHEN REQUIRED:
Collision avoidance should be enabled whenever the following conditions are all
true:
ƒ
A multi-point network is used (that is, master unit is communicating with
several slave units on same physical link). This excludes a dial-up modem
network where the master only communicates with one slave at a time in a
point-to-point physical link. Will only operate on two-wire, half-duplex
communication networks.
ƒ The user layer supports unsolicited messages. Unsolicited messaging is used
where asynchronous, spontaneous messages may be generated by any
node on the network.
ƒ Any network where the physical layer does not implement a collision
avoidance scheme and permits several nodes to communicate at one time
(that is, some radio networks).
IMPORTANT NOTE: A system should either be configured to operate in a polled-only
mode, or in a report-by-exception mode. The Data Link was never designed to handle
the traffic load of frequent scheduled polls by the master interspersed with unsolicited
messages from slave devices.
The collision avoidance scheme in DNP was designed for half-duplex
communications only. It requires the use of DCD and RTS control lines by every
device on the link. A device will assert RTS before transmitting data on the
circuit. The RTS signal is an indication that the line is busy.
Any device that wishes to transmit a frame does so according to the following
rules.
RULES:
ƒ
RTS controls DCD line on all other units.
ƒ No stations transmit while DCD line is high except the one that has the RTS
line high.
ƒ After DCD line drops low, slaves wait variable time before attempting to
transmit.
ƒ Master has smallest delay (can be set to 0)
ƒ Slaves have higher delays (fixed delay (that is, slave#*20 milliseconds) +
random delay (20 to 50 milliseconds))
ƒ Only supported in 2-wire half-duplex mode. Not supported in 4-wire half-
duplex mode.
ƒ This option is set in the configuration of the module with the port type.
CABLE:
This is the cable required for direct connection of the module to a remote unit:
DB-9 DB-25
DCD RTS 7 4
RX TX 3 2
TX RX 2 3
GND GND 5 7
RTS DCD 1 8