Connection to the Modbus-SL Network with IFM Interface

Introduction

Use the Modbus cable to interconnect the intelligent modular units comprising IFM interfaces TRV00210 , supply them with power, and connect them to the Modbus client.

In terms of power supply, to limit voltage drop, the maximum number of IFM interfaces stacked to one IFE server is 11.

In terms of Modbus communication, it depends on the performance requirement. As it takes approximately 500 ms at 19,200 Baud per device to refresh 100 registers, the more interfaces added the longer the minimum refreshment period. The minimum refreshment period depends on the number of IFM interfaces stacked to one IFE server. Multiply the time to refresh one device by the number of devices to find the minimum refreshment period expected in the application. For instance, an installation with eight IFM interfaces stacked to one IFE server at 19,200 Baud would take approximately 4 seconds to be read.

Composition of the Modbus Cable

The following figure shows the Modbus cable:

A	Outer sheath
B	Shielding braid
C	Twisted-pair sheaths
D	Communication pair (white/blue)
E	Power supply pair (red/black)

The characteristics of the Modbus cable are as follows:

Shielded cable with two twisted-pairs:
- One pair with 0.25 mm² (24 AWG) cross-section for the RS 485 signal (D0, D1).
- One pair with 0.5 mm² (20 AWG) cross-section for the power supply (0 V, 24 Vdc).
Shielding braid to be connected to the ground terminal of the 5-pin connector on the IFM interface.
External diameter: 8.7–9.6 mm (0.35–0.38 in).
Color of outer sheath: orange.

The 0 V terminal of the power supply pair is also the Modbus common, that is, the 0 V for the RS 485 signal (D0, D1).

The 0 V cable (Modbus common) must be distributed along the entire length of the network, right up to the Modbus client.

Other Modbus cable part numbers are given in appendix.

Connection of Modbus Cable to the IFM Interface

Each point on the 5-pin connector on the IFM interface has a specific marking to make it easier to connect the Modbus cable.

Connector	Marking	Color	Description	Unsheathed length	Stripped length
	D1	Blue	RS 485 B/B’ or Rx+/Tx+ signal	≤50 mm (1.96 in)	7 mm (0.27 in)
	D0	White	RS 485 A/A’ or Rx-/Tx- signal	≤50 mm (1.96 in)	7 mm (0.27 in)
		–	Modbus cable shielding braid, connected to the local machine ground in the IFM interface	≤20 mm (0.79 in) ⁽¹⁾	7 mm (0.27 in)
	0V	Black	0 V for Modbus common and power supply	≤50 mm (1.96 in)	7 mm (0.27 in)
	24V	Red	24 Vdc for the power supply	≤50 mm (1.96 in)	7 mm (0.27 in)

(1) To ensure that the shielding is effective against high-frequency disturbances, keep the shielding braid between the Modbus cable and the ground terminal as short as possible.

NOTE: Do not connect more than two wires in the same terminal on the 5-pin connector on the IFM interface.

Connection of the 0 V Terminal on IFM Interface to the Functional Ground Terminal Block

The 0 V terminal on IFM interfaces is connected to the functional ground terminal block at only one point of the Modbus line (first stacked IFM interface or at the Modbus client if IFM interfaces are not stacked with IFE server). No other devices must have 0 V connected to ground.

Modbus Line Termination

The Modbus cable communication pair has a typical impedance of 120 Ω. The Modbus cable must therefore be terminated at each end by a Modbus line termination with a 120 Ω impedance.

The Modbus client is at one end of the Modbus cable and usually has a switchable termination impedance. At the other end of the Modbus cable, a Modbus line termination with a 120 Ω impedance must be connected.

To obtain a 120 Ω impedance at high frequency without loading the cable with DC, optimize the Modbus line termination in the form of an RC cell: 120 Ω in series with a 1 nF capacitor and two 10 cm (3.9 in) wires for direct connection (between D0 and D1) to the 5-pin connector on the last IFM interface.

Illustrations	Description	Part number
	Two Modbus line termination (120 Ω + 1 nF)	VW3A8306DRC

General Rules for Modbus Cable Length

The maximum permitted length for the Modbus network (for the trunk cable, excluding tap links) is 500 m (1,640 ft) at 38,400 Baud and 1,000 m (3,281 ft) at 19,200 Baud.

The Modbus cable connecting the IFM interfaces in the ULP system incorporates both the Modbus communication network and the 24 Vdc power supply. Because of the stresses caused by a drop in the supply voltage, more restrictive limitations are imposed:

The voltage drop between the power supply and the furthest point, both on the +24 V wire and on the 0 V wire, must be limited to 4 Vdc (2 Vdc on the +24 Vdc wire and 2 Vdc on 0 V wire).

A minimum supply of 24 Vdc -20% (19.2 Vdc) is thus obtained on the last IFM interface, with a 24 Vdc power supply regulated at:
- +/-3% (23.3–24.7 Vdc) for 3 A power supplies.
- +/-5% (22.8–25.2 Vdc) for 1 A power supplies.
For optimum quality of the Modbus communication, the voltage on the 0 V terminal on each IFM interface (Modbus common) must not vary by more than +/-4 Vdc compared to the 0 V voltage of any other Modbus device in the installation. This restriction further limits length when the Modbus equipment is divided between a number of power supply segments.

The Modbus cable length depends on the architecture in the ULP system.