Flowserve DDC-100 Modbus Direct-to-Host User Manual download pdf (Page 19)

32 DDC-100 Direct-to-Host Programming Guide FCD LMAIM4019-00 FCD LMAIM4019-00 DDC-100 Direct-to-Host Programming Guide 33

4.2.3 Single-Line Multi-drop

The single-line multi-drop topology is shown in Figure 4.3. Single-line multi-drop is wired by

cabling from the Host and connecting to the ﬁrst ﬁeld unit port A1. The cable to the next ﬁeld unit

is attached to the same terminals on the ﬁrst ﬁeld unit and then run to port A1 on the second ﬁeld

unit. This continues until the last ﬁeld unit is connected.

A single-line multi-drop topology can simplify installation (especially when pre-existing wiring

is used), but it does not offer the extra reliability of a looped communication path. A line break

prevents communication with ﬁeld units beyond the break, and a line short will cause a loss of

communication with all ﬁeld units.

The maximum number of ﬁeld units that can be accommodated by the single-line multi-drop

network is 28 units and the maximum distance between the Host and the last ﬁeld unit is 1800

feet (550 m) without the use of repeaters. Note that all Limitorque ﬁeld units can be wired to act as

repeaters by using ports A1 and A2.

Flowserve recommends use of the single-ended loop topology in lieu of single-line, multi-drop for

the MX-DDC.

NOTE: The single-line multi-drop topology requires the removal of termination resistors and bias

voltage jumpers from all but the last ﬁeld unit. See the appropriate ﬁeld unit manual or contact

Flowserve for assistance.

Figure 4.3 – DDC-100 Single-Line Multi-Drop Network

RS-232

RS-485

Diagnostic Note:

Earth Ground Note:

MOV-2

A1*

TB5

MOV-26

N/C

A1*

TB5

MOV-28

D-M

D-M*

D-S

D-S*

A1*

TB4

TB3

TB5

Notes:

Legend

MOV

D-M

D-M*

D-S

D-S*

N/C

Data terminal is positive with respect to data* terminal

RS-232 to RS-485

converter with

surge suppression

Data 1

Earth Ground

(See Note 4)

See Note 5

1) Belden 3074F, 3105A, or 9841 shielded cable is recommended.

2) Correct polarity for ﬁeld unit and network controller

connection is necessary for proper operation.

3) Connections shown are typical. The number of

MOVs shown may not indicate true system size.

4) Earth ground: ground rod

5) Earth ground: ground rod or lug in

actuator if actuator is grounded.

Polarity and level of the network’s data connection can

be checked by measuring voltage between data and

data* terminals. This voltage should be greater than

+200 mVDC with network controller network ports

disconnected.

If low impedance earth ground is not available at

each actuator, contact engineering for alternative

earth ground surge protection strategies.

Host

RS-232

PORT 1

Network Port A

N/C

D-M

D-M*

TB3

TB4

D-M

D-M*

TB4

TB3

Ground 3

Data* 2

- Motor-Operated Valve

- Data A1 (UEC-3-DDC)

- Data A1*(UEC-3-DDC)

- Data A2* (UEC-3-DDC)

- Shield

- No Connection

- Data A2 (UEC-3-DDC)

MOV-1

A1*

TB5

See Note 5

D-M

D-M*

TB3

TB4

MOV-3

A1*

TB5

See Note 5

D-M

D-M*

TB3

TB4

MOV-27

A1*

TB5

See Note 5

D-M

D-M*

TB3

TB4

N/C

4.3 Network Polling

Network polling is a Host-generated systematic request for information from each ﬁeld unit on

the serial communication network. This systematic process updates the Host Data Table (Poll

Table) with each complete network scan. By utilizing this sequential update sequence, the Host can

operate more efﬁciently because the Data Table always contains up-to-date information.

In systems in which there is no up-to-date Data Table, the Host must check the status of a ﬁeld unit

every time it needs to actuate a valve. This check is required to determine if the actuator is capable

of movement prior to the issuance of the command to move. The continuous poll process should

be enabled at all times to allow peak network performance.

Safeguards are built into the Limitorque ﬁeld units that operate in the event of a disruption in the

polling process. The ﬁeld unit network watchdog timer will start a ﬁeld unit reset process if the ﬁeld

unit does not recognize that it has been polled in a speciﬁed time interval (default 60 seconds). This

ﬁeld unit reset process is designed to clear any errors in the ﬁeld unit that may prevent successful

communication to a Host device.

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