Flowserve DDC-100 Modbus Direct-to-Host User Manual download pdf (Page 6)

6 DDC-100 Direct-to-Host Programming Guide FCD LMAIM4019-00 FCD LMAIM4019-00 DDC-100 Direct-to-Host Programming Guide 7

Queries are used to send requests from the Modbus master (Host) to the Modbus slave (DDC-100

Field Unit), and the slave must respond with an appropriate response or an error message.

The Modbus function codes that are supported in the DDC-100 Network are a subset of the

complete Modbus function codes and are listed below:

01 Read Coil Status Reads the ON/OFF status of discrete outputs (coils) in the ﬁeld units.

02 Read Input Status Reads the ON/OFF status of discrete inputs in the ﬁeld units.

03 Read Holding Registers Reads the binary contents of holding registers in the ﬁeld units.

04 Read Input Registers Reads the binary contents of the input registers in the ﬁeld units.

05 Force Single Coil Forces a single coil to either the ON or OFF state.

06 Preset Single Register Presets a value into a single-holding register.

08 Diagnostics Provides communication tests and checks for internal error conditions in the

ﬁeld units.

15 Force Multiple Coils Forces multiple coils to either the ON or OFF state.

16 Preset Multiple Registers Presets a value into multiple holding registers.

NOTE: All data in Modbus messages are referenced to zero. The ﬁrst occurrence of a data item is

addressed as item number zero. This includes Coils, Inputs, and Registers. For example, coils 1-8

would be addressed as 0-7, inputs 1-16 would be addressed as 0-15, and registers 1-16 would be

addressed as 0-15.

3.2 Modbus

The Modbus protocol was developed by A.E.G. Modicon for communicating to various networked

devices. The relationship between these devices and a central controller is called a master-slave

relationship in which the master (Host device) initiates all communications. The slave devices (ﬁeld

units in the actuators) respond to the queries from the master. Modbus only permits one master to

communicate at any given time (simultaneous communication is prohibited) for assuring process

control integrity.

The controlling device (master) must conform to the Modbus protocol as deﬁned in the Modicon

Modbus Protocol Reference Guide PI-MODBUS-300 Rev. G and support Modbus function codes

01 through 06, 08, 15 and 16. These function codes are a subset of the complete protocol and are

deﬁned in Table 3.2.

Table 3.2 – Modbus Function Codes Supported

Function

Code

Name

Bit/Register

Addressing

Extended

Addressing Range

01 Read Coil Status Bit 0,000 - 9,999

02 Read Input Status Bit 10,000 - 19,999

03 Read Holding Register Register 40,000 - 49,999

04 Read Input Register Register 30,000 - 39,999

05 Force Single Coil Bit 0,000 - 9,999

06 Preset Single Register Register 40,000 - 49,999

08 Diagnostics N/A N/A

15 Force Multiple Coils Bit 0,000 - 9,999

16 Preset Multiple Registers Register 40,000 - 49,999

Note: MX-DDC does not support Modbus function code 02.

Modbus function codes 15 and 16 are supported in:

UEC-3-DDC Modbus Firmware 2.00 and greater

MX-DDC Firmware 02/01.00 and greater

The choice of which query to use in a particular situation can signiﬁcantly affect the efﬁciency of the

network. As an example, consider the situation where the Host requires the status of the coils, the

status of the digital inputs, the status of the faults, and the status of the timers and analog chan-

nels. This information can be obtained by using the 01 - Read Coil Status query, the 02 - Read Input

Status query, and 04 - Read Input Register query. To obtain this information, the Host would have

to send three separate queries, and the ﬁeld unit would have to respond to each query separately.

A more efﬁcient way to accomplish this same request for information would be through the use of

the 03 - Read Holding Register query. The Host would issue the 03 query (specifying the registers

to read), and the ﬁeld unit would respond with one response that would contain all of the requested

information. The latter approach would generate considerably less network trafﬁc than the former

approach, improving network capacity and response times.

In the strict sense, all transmissions from the Modbus master are called commands. In this manual,

a request for information, however, may be referred to as a query. Usually the term query will only

be used in conjunction with function codes (01), (02), (03), (04), and (08), which typically request

data. Commands are used in conjunction with function codes (05), (06), (15) and (16), which

typically initiate ﬁeld unit action.

Examples

• The coil known as “coil 1” in the eld unit is addressed as coil 0000 in the data address eld of a

Modbus message.

• Digital input 129 decimal is addressed as digital input 0080 hex (128 decimal).

• Holding register 40001 is addressed as register 0000 in the data address eld of the message.

The function code ﬁeld already speciﬁes “holding register” operation. Therefore the reference

“4XXXX” is implicit.

• Holding register 40009 is addressed as register 0008 hex (8 decimal).

3.2.1 Modbus Function Code 01 (Read Coil Status)

This function code is used to read the coil status in the DDC-100 Field Unit. There are nine coils

available to be read on DDC-100 Field Units as shown in Table 3.3. For the MX/DDC or UEC-3-DDC

Field Unit, Coil 1 indicates CLOSE contactor and is interlocked with Coil 2, Coil 2 indicates OPEN

contactor and is interlocked with Coil 1. When the I/O Module is used in non-MOV (motor-operated

valve) mode, relays 1 through 6 or coils 3 through 8 are available for user conﬁguration.

1 2 3 4 5 6 7 8 9 10 11 ... 32 33

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