General
The Modbus RTU (Remote Terminal Unit) Master drive.web options (-12, -13) enables a device to communicate with Modbus RTU slave devices using a three-wire EIA-485 physical connection.
EIA-485 (3-wire) Wiring
Labeling of EIA-485 signals varies within the industrial controls industry. Some vendors use an A & B terminology while other vendors use a Data- & Data+ designation. The drive.web products use the A & B terminology as follows:
| Terminal | Designation | Description |
|---|---|---|
| A | Data- | Inverting pin |
| B | Data+ | Non-inverting pin |
| C (S) | Common | Signal Common/Shield |
For a valid EIA-485 network, all of the A's must be connected, all of the B's must be connected, and all of the C's (on the speedy-sp and smarty products labeled S-shield) must be connected.
Ideally, a three conductor shielded cable will be used (Belden 9533 or equivalent) with the shield connected to chassis ground at exactly one point in the system. However, a two-wire plus shield cable will suffice with the shield used as the third conductor connected to the C (or S) (Belden 9841 or equivalent).
LED Indicators
All isolated EIA-485 drive.web devices have light emitting diodes (LED's) indicating receive and transmit status as follows:
| LED Color | Designation | Description |
|---|---|---|
| Yellow | Tx | Device transmitting data |
| Green | Rx | Device receiving data |
The un-isolated EIA-485 drive.web devices do not have LED's indicating transmission status.
Supported Modbus Function Codes
The Modbus RTU Protocol uses serial transactions referred to as function codes to read/write data on the comms link. The Modbus RTU enabled drive.web products support the following subset of Modbus RTU function codes:
| Function Code | Description |
|---|---|
| 03 | Read Holding Registers |
| 06 | Preset Single Holding Registers |
| 16 | Preset Multiple Holding Registers |