Controlling Delta VFD Inverters via RS-485 Modbus RTU using Delta DVP Series PLCs
1. Why Modbus RTU Digital Control Outperforms 0-10V Analog Wiring
Traditional VFD wiring requires separate hardwired relays for Run/Stop, directional contacts, and shielded analog cables (0-10V / 4-20mA) for frequency references. These analog runs are highly susceptible to IGBT switching EMI noise, causing speed jitter and requiring costly analog expansion cards. Employing the PLC's built-in RS-485 Modbus RTU port requires only a single shielded twisted-pair cable (D+, D-) to govern up to 31 inverters. Beyond simple motion commands, it grants complete two-way diagnostic telemetry: real-time motor current (A), DC bus voltage (V), and active trip fault codes.
2. Essential Delta Inverter Parameter Settings (VFD-M / VFD-EL)
| Parameter | Value Setting | Functional Description |
|---|---|---|
| Pr.00 (Frequency Source) | 03 or 04 | Accepts frequency references via RS-485 Serial Interface |
| Pr.01 (Operation Command) | 03 or 04 | Accepts Run/Stop/Reverse logic via RS-485 Serial Interface |
| Pr.88 (Communication Address) | 01 (or 02, 03...) | Unique Slave Node ID in the multidrop network |
| Pr.89 (Baud Rate) | 01 (9600 bps) | Baud rate transmission speed matching PLC master |
| Pr.92 (Data Framing) | 03 (7, E, 1 Modbus RTU) | 7 Data bits, Even parity, 1 Stop bit Modbus RTU format |
3. Primary Delta VFD Modbus Register Addresses
- Register 2000H (Control Word): Commands drive state (H0012 = Run Forward, H0022 = Run Reverse, H0001 = Ramp Stop, H0002 = Coast to Stop).
- Register 2001H (Frequency Setpoint): Target frequency with 2 decimal places (e.g. K5000 = 50.00 Hz).
- Register 2101H (Output Frequency): Real-time operating frequency currently generated to motor windings.
- Register 2104H (Output Amperage): Instantaneous motor load current telemetry.
4. WPLSoft / ISPSoft Ladder Logic Architecture
// 1. Initialize COM2 Serial Interface during first scan (M1002 Initial Pulse)
// Value H86 = 9600 bps, 7 data bits, Even parity, 1 stop bit (Modbus RTU)
[ M1002 ] -------- [ MOV H86 D1120 ]
[ M1002 ] -------- [ SET M1120 ] (Retain Communication Settings)
[ M1002 ] -------- [ SET M1143 ] (Select Modbus RTU Protocol Mode)
[ M1002 ] -------- [ MOV K100 D1129 ] (Communication Timeout = 100ms)
// 2. Write 50.00 Hz Frequency Command to Slave Station 1 (Register 2001H)
// X0 pulses trigger frequency load into D10 and dispatch MODWR
[ X0 ] ----------- [ MOV K5000 D10 ]
[ X0 ] ----------- [ MODWR K1 H2001 D10 ]
// 3. Dispatch Forward Run Command to Address 2000H
// H0012 executes forward operation
[ X1 ] ----------- [ MOV H0012 D20 ]
[ X1 ] ----------- [ MODWR K1 H2000 D20 ]
// 4. Dispatch Controlled Decel Stop Command
// H0001 executes ramp deceleration to zero
[ X2 ] ----------- [ MOV H0001 D20 ]
[ X2 ] ----------- [ MODWR K1 H2000 D20 ]Transmission Line End-of-Line Termination Resistors
On cable trunks exceeding 15 meters or connecting multiple daisy-chained drives, transmission line impedance mismatches trigger high-frequency signal reflections resulting in framing corruptions. Countermeasure: Install a 120 Ω 1/4W resistor bridging terminals D+ and D- at the final physical device on the RS-485 bus to terminate standing reflection waves.