Flowserve DDC-100 Modbus Direct-to-Host - Manual (Page 17)

28 DDC-100 Direct-to-Host Programming Guide FCD LMAIM4019-00 FCD LMAIM4019-00 DDC-100 Direct-to-Host Programming Guide 29
Key Specs
• Resistance/1000 ft = 18 AWG (7x26)
6.92 ohms each conductor (13.84 ohms for the pair)
• Capacitance/ft = 14 pF (conductor-to-conductor)
• Capacitance/ft = 14 pF (conductor-to-shield)
Belden 3105A Specifications
• Total cable length between repeaters or nodes with repeaters: up to 19.2 kbps: 4500' (1.37 km)
For loop mode, this is the total length between operating field units. If a field unit loses power,
relays internal to the field unit connect the A1 Channel to the A2 Channel, which effectively doubles
the length of the cable (assuming a single field unit fails). If you need to assure operation within
specifications in the event of power failure to field units, then this consideration must be added.
Example: To assure operation within specification when any two consecutive field units lose power,
then the maximum length on cable up to 19.2 kbps: 4500' (1.37 km) per every four field units.
Key Specs
• Resistance/1000 ft = 22 AWG (7x30)
14.7 ohms each conductor (29.4 ohms for the pair)
• Capacitance/ft = 11.0 pF (conductor-to-conductor)
• Capacitance/ft = 20.0 pF (conductor-to-shield)
Belden 9841 Specifications
• Total cable length between repeaters or nodes with repeaters: up to 19.2 kbps: 3500' (1 km)
For loop mode, this is the total length between operating field units. If a field unit loses power,
relays internal to the field unit connect the A1 Channel to the A2 Channel, which effectively doubles
the length of the cable (assuming a single field unit fails). If you need to assure operation within
specifications in the event of power failure to field units, then this consideration must be added.
Example: To assure operation within specification when any two consecutive field units lose power,
then the maximum length on cable up to 19.2 kbps: 3500’ (1 km) per every four field units.
Key Specs
• Resistance/1000 ft = 24 AWG (7x32)
24 ohms each conductor (48 ohms for the pair)
• Capacitance/ft = 12.8 pF (conductor-to-conductor)
• Capacitance/ft = 23 pF (conductor-to-shield)
4.1 Field Unit Network Communication Channels
Before proceeding with discussions of network polling, network error monitoring, and network
control, it is necessary to explain some network terminology with respect to field unit communica-
tion channels, ports, and hardware features.
All Limitorque DDC-100 Field Units are provided with a dual port network communication channel.
This channel is designated Channel A and the ports are designated ports A1 and A2. Both ports are
capable of bi-directional serial communications and comply with the EIA RS-485 standard.
In addition, some field units can optionally be ordered or retrofitted with a second dual port network
communication channel. This channel is designated Channel B and the ports are designated ports
B1 and B2. Channel B is implemented by adding a Granddaughter Board and an additional Network
Board to the standard field unit (not available with MX-DDC).
The optional channel, Channel B, is used in network topologies that feature dual redundant
communication paths. Please contact Flowserve for further information about dual redundant
communication paths with the DDC-100 Network.
4.1.1 Field Unit Network Bypass Relays
Every Limitorque DDC-100 Field Unit is equipped with a set of network bypass relays designed
to isolate the individual field unit from the network in the event of a field unit power failure. These
normally closed relays are energized on field unit power-up allowing the network data transmis-
sions to enter the field unit repeater circuitry and UART (Universal Asynchronous Receive Transmit
circuit). When the field unit power is turned off, these relays close, shorting the signal through the
field unit network board so the signal may pass to the next field unit. This isolates the powered-
down field unit from the network while allowing the remainder of the network to function normally.
Care must be taken in the design of the DDC-100 network cable routing. This is necessary so
maintenance programs or other group power outage conditions do not add more physical cable
distance between two functional field units than the RS-485 electrical signal can support.
NOTE: The MX-DDC Field Unit will not energize the network bypass relays until the field unit is fully
operational and ready to communicate. UEC-3-DDC Field Units revision 1.57 and greater will “fast
start” the UART, preventing network communication disruption during the field unit reboot cycle.
4.1.2 Field Unit Repeater Circuits
Every Limitorque DDC-100 Field Unit communications channel contains a repeater circuit that
recenters, reclocks, and amplifies the incoming signal and sends it out the opposite port. The circuit
is bi-directional, so it will repeat the received signals from port A1 out A2 or from port A2 out A1.
When the optional Granddaughter Board (not available with the MX-DDC) is added, the repeater
also recenters, reclocks, and amplifies the incoming signal received from B1 out B2 or from B2 out
B1. Although the circuit resends the entire message, there is less than a one-bit time delay before
retransmission out the corresponding port.
4.2 Network Topologies
Three network topologies are commonly used and supported by Limitorque—redundant loop,
single-ended loop (or half loop), and single-line multi-drop. The recommended cable types for
all three topologies are Belden 3074F, Belden 3105A, or Belden 9841. Other individually shielded,
twisted-pair cables with electrical properties within 5% of Belden 3074F, Belden 3105A, or Belden
9841 may be used but have not been performance-tested with the DDC-100 Network.
4.2.1 Redundant Loop
The redundant loop topology requires two serial communication ports on the Host device.
Because each field unit can be accessed by two ports, redundant access paths are supported. The
connections from the serial ports to the field units and between field units are made with shielded,
twisted-pair cable in a loop configuration. This topology tolerates a single-line break or short
while maintaining communications to all field units. Figure 4.1 shows the redundant loop network
topology.
With redundant loop topology, serial data is transmitted from Host port 1 through an RS-232 to
RS-485 converter to port A1 of the first field unit. The field unit passes the data that comes in
port A1 out through port A2 to the next field unit A1 port. Each subsequent field unit receives data
through its A1 port then passes the data out through its A2 port to the next field unit. The looped
communication continues in this manner until the last field unit port A2 relays the serial data to port