Sensoray 2600 - Manual (Page 24)

2600 Family Instruction Manual 19 Chapter 4 : Serial Communication Server
Figure 22: RS-485 with termination
4.2.2 Device Networks
When operating as an RS-422 or RS-485 interface, a single
MM com port can be used to connect to a network of remote
devices. A com port’s physical layer type must be configured
to match the requirements of the connected device network.
The physical layer type is configured by means of the PHY
value, as described in
Section 4.4.1.2 .
Some common device network topologies and examples are
discussed in the following paragraphs. All of these examples
assume the MM is acting as a master on the device network.
4.2.2.1 RS-422 with a Single Master
In this case, the MM is the sole master of the device network
and one or more remote slaves reside on the network. No other
devices are permitted to transmit on the MM’s transmit pair. A
network having this topology may be configured as either PHY
type 01 or 11. Figure 23 shows a network of this type with
two slave devices.
Figure 23: Single-master RS-422 device network
Multiple slave devices must never simultaneously drive the
MM’s receive pair. This is typically ensured by assigning
unique addresses to the slaves and programming them so that
they will drive the MM’s receive pair only when responding to
directed command messages from the MM.
4.2.2.2 RS-422 with Multiple Masters
In a multi-mastered device network, the MM must share its
transmit pair with one or more remote masters, and one or
more slave devices may reside on the device network along
with the MM and other master devices. A device network
having this topology should be configured as PHY type 11.
Figure 24 shows an example of this type of network, in which
the device network is shared by two masters (the MM and one
remote master) and one slave.
Figure 24: Multi-mastered RS-422 device network
Care must be taken to ensure that the MM and the remote
masters never drive the shared MM transmit pair at the same
time, as transmitter collision detection is not available in this
configuration.
4.2.2.3 RS-485
RS-485 networks employ a single pair for both transmit and
receive. Only one transmitter is permitted to drive the
common pair at a time. Figure 25 shows an RS-485 network
with one master (the MM) and two slaves, although it is
possible to utilize more than one master on a device network.
Figure 25: Single-master RS-485 device network
RS-485 networks may be configured as PHY type 10 or 11.
Type 10 should be used for half-duplex operation in which no
collisions are possible, and type 11 should be used when
collisions are possible and must be detected (e.g., multiple
masters) by means of full-duplex feedback.
The common pair must be driven by only one transmitter at a
time. In single-master device networks, this is enforced by
assigning unique addresses to the slaves and programming
them so that they will drive the common pair only when
responding to directed command messages from the MM.
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9
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Remote
Device
RS485+
RS485-
MM
COM Port
Connector
GND
Cable
Shield
Remote MM
Slave
Remote
Slave
Master
Remote
Slave
Remote
Master
MM
Master
Remote MM
Slave Master
Remote
Slave