Quick Start Guide > Quick Start - For Instruments using the Modbus Protocol

If the instrument you are using uses Modbus protocol you need to know how the instrument is configured.

If there is a choice between Modbus or J-Bus protocol choose J-BUS

If there is a choice for Data Bits choose 8 (EIGHT)

If there is a choice for Parity choose NONE

If there is a choice for Stop Bits choose 1 (ONE)

The format of the address specification for Modbus is:

AA,PT;O

Where:

AA: Two-digit address of the instrument in decimal.

P: Number of decimal places for user defined items (see Note 1 below).

T: Type of protocol: J for J-Bus, M for Modbus (see Note 2 below).

O: (Optional) Address offset (see Note 3 below).

Examples:

1,1J Address 1, 0 decimal place, J-Bus parameters

2,1J;2 Address 2, 1 decimal place, J-Bus parameters, offset by 2

10,0M 0 Decimals, Modbus parameters

Note 1 (Decimal Places):

0 / 1 / 2 / 3 SpecView will put a decimal place in the position indicated by this setting. The setting affects those variables where the number of decimals required is based on an instrument setting such as the range that has been selected. Some items, such as Output Power are fixed at a given number of decimal places and are not affected by this setting.

AutoDetect may assume 0 decimal places when detecting instruments. This might result in values being 10 or 100 times too large on displays. If this is the case, then edit the address for the instruments from AA,0T to AA,1T (See below for the meaning of this).

Note 2 (Protocol):

If in ANY doubt, do not enter anything for this item. SpecView will add it for you.

There is often confusion between Modbus 'Registers' and 'Addresses'. With SpecView the user does not need to fully understand the difference but the correct setting MUST be made here. When 'J' (for J-Bus) is used the 'Address' that has been pre-defined by SpecView is sent to the instrument. When 'M' (for Modbus) is used ONE is subtracted from the pre-defined address. SpecView pre-defined instruments ALWAYS assume the address and NEVER the register. Unless advised otherwise by SpecView either leave the letter out or use 'J'.

Note 3 (Address Offset):

Some instrument companies use an address structure so that a particular instrument view can be re-used with the appropriate address offset. This feature is also used with SpecView's 'Generic Modbus' instrument views. The addresses for each instance of the instrument are the same, offset by a fixed number. See Generic Modbus for more details. Also, where this feature is required for a specific instrument, refer to the help for that instrument.

The rate at which the Modbus driver will supply the values of variables from the instruments is defined by the driver command WTTL (default=19 tenths of a second). This can be set using the Setup COM Ports menu command on the Setup menu.

For Instruments using Modbus TCP/IP

The address, as detailed above, is prefixed with the full IP address followed by a comma.

For example: 192.168.1.99,1,2J