Quartech 2800 Manual page 6

2800 & 2900 Series PLC Drivers
8 Pin RJ45
RxTxB
RxTxA
SC
Send/Rec
DH485 Network Basics
The DH485 Data Link network uses a multiple master, token passing protocol. For this protocol, the devices on the
network form a logical ring; that is, the devices assume an ordered sequence, with the last device in the sequence
followed by the first. Each device knows the node address of the device following it (its successor). The physical
ordering of the devices on the network is irrelevant and independent of the logical ordering.
Each device on the network has the responsibility of periodically granting an opportunity for new devices to enter the
network. Any time there exists a gap between a given device address and its successor's address, that device must
periodically issue a "solicit successor" message to each potential node address between itself and its successor. The
solicit successor message allows an opportunity for a new device to enter the network.
A total of 32 devices may share the DH485 Data Link network. When only a few devices are sharing the network the
soliciting time can become considerable. To reduce unnecessary soliciting, a maximum poll address may be assigned
in the OIT and most other devices designed for use on the network. The OIT, and other devices utilizing this feature,
will send a "solicit successor" message only to those device addresses that are in the gap between itself and its
successor, and have an address less than or equal to its maximum poll address. Use of the maximum poll address
can relieve the device with the highest node address from the burden of soliciting a large number of unused node
addresses.
Choosing Device Node Addresses
It is imperative that each device on the network is assigned a unique node address. For best performance start node
addresses at zero and increase sequentially, with no gaps between successive addresses. It is not important which
device is assigned which address. The device with the highest address on the network should have its maximum poll
address set equal to its own address.
For example: A DH485 network might include one SLC500, one programmer, and one OIT.
Assigned node addresses could be as shown below.
<
OIT node address = 00
<
Programmer node address = 01
<
SLC500 node address = 02. Max. poll address = 02.
Allen-Bradley SLC500 DH485
Quartech Part Number: 2145-10
SLC500
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1 <
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2 <
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7 <
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5 <
2800/2900
15 Pin Female D-Type
< 12
< 13
< 7
< 8
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< 4
.))))) .)))))
< 5
))) )))
Drain Wire < 1
RxTxB
RxTxA
SC
Send/Rec
RTS
CTS
FG
DM2829_PLC Revision 2
Page 6