Modbus RTU Addressing: Configuring T3000 for MK5 Guide

The critical integration rule is to convert the Atlas Copco MK5 register list from one-based addressing to the zero-based Modbus address expected by the Siemens T3000 interface: subtract 1 from every published MK5 relative address. Keep start, stop, run, trip, and motor-current signals hardwired; use Modbus RTU for measurements, status, counters, and settings. This split preserves essential control if the communication path fails while adding detailed DCS monitoring.

Architecture and control split

The installation integrates three instrument-air compressors, four service-air compressors, and three denitrification-air compressors. Their original MK4 panels had operated for nearly 20 years and were obsolete. The upgrade replaces the panels with MK5 controllers and connects their data to the T3000 DCS.

Route each signal according to its operational consequence:

Path	Signals	Purpose
Hardwired through AP020	Start and stop commands; run and trip feedback; compressor current	Maintains deterministic control and essential indication independently of serial communication
Modbus RTU	Measured values, operating states, accumulated values, and settings	Expands monitoring without installing duplicate sensors or field wiring

The communication chain is:

The topology description calls the MK5-to-GATEWAY link “CA,” while the hardware description identifies it as CAN. Confirm the interface designation and wiring in the MK5 and GATEWAY manuals before commissioning; it does not change the Modbus address conversion on the DCS side.

Hardware constraints that affect integration

The MK5 works with an IO2 module to collect PT1000 temperature, pressure, and digital-input signals and to issue digital outputs. PT100 temperature acquisition requires the additional IO34 expansion module.

The GATEWAY converts the controller-side protocol to Modbus RTU or Profibus DP. The documented installation uses 24 VAC power and permits as many as 30 compressors per gateway. Three gateways separate the service-air, instrument-air, and denitrification-air systems, limiting a gateway or bus problem to one compressor group.

Each supports asynchronous RS232, RS422, and RS485 interfaces. With ordinary multimode fiber, its specified communication distance is 4 km and its maximum rate is 460 kbps. The unit can mix two serial interfaces, uses terminal-block serial connections, and accepts SC, ST, or FC optical interfaces. This installation uses ST connectors for mechanical strength and dust resistance.

The CS3000 provides two RJ45 production-network ports and four DB9 RS232 ports for third-party equipment. It runs the serial protocol interface and exposes the resulting data to T3000 over Profinet.

Why MK5 values shift by one register

Modbus RTU transmits a one-byte slave address, one-byte function code, a variable-length data field, and a two-byte check field. The transmitted register address is an offset; labels such as an input-register or holding-register number are documentation conventions and are not sent in that form.

The MK5 list uses one-based relative addresses, but the T3000 configuration and the Modbus request use zero-based offsets. Apply this conversion:

T3000 Modbus address = MK5 listed relative address − 1

MK5 register class	MK5 listed address	T3000 request address
Input register	Controller temperature at relative address 2	1

This is the primary correction for data that appears shifted to the adjacent point. Preserve the register class while changing the offset: discrete inputs and input registers are read-only, while coils and holding registers represent output or setting data. Do not move a point between classes merely to obtain a plausible value.

Each Modbus slave on a shared bus requires a unique address from 1 through 247; 0 is the broadcast address. A duplicate address can produce collisions or intermittent responses even when every register offset is correct.

Serial and physical-layer diagnostic sequence

1. Confirm that the targeted compressor belongs to the intended gateway group and that its slave address is unique.
2. Match baud rate, data bits, parity, and stop bits at both ends of each asynchronous serial segment. A mismatch in any one field invalidates complete frames.
3. On RS232, cross TXD and RXD and connect signal ground. RS232 is point-to-point and full duplex.
4. On RS485, verify A (485+) and B (485−) polarity. RS485 is differential and half duplex in this installation.
5. Install 120 ohm termination at the beginning and end of the RS485 trunk. Do not treat every device connection as a bus end.
6. Confirm the receiving CS3000 DB9 port is the configured RS232 channel and that the corresponding runtime container and hardware agent are active.
7. Poll one known MK5 point, subtracting one from its listed address. Verify the raw response before scaling or assigning it to a graphic.

If no slave responds, troubleshoot power, physical wiring, fiber path, port selection, serial format, and slave address first. If the slave responds but values belong to neighboring points, correct the address base. If only selected points are wrong, compare their register class, data representation, word allocation, and scaling against the MK5 register list.

T3000 configuration procedure

1. Enter exactly the same baud rate, data-bit count, parity, and stop-bit count used by the field gateway.
2. Define each compressor with its assigned Modbus slave address.
3. Select the register class stated in the MK5 list: coil, discrete input, input register, or holding register.
4. Convert every MK5 relative address using listed address − 1. For example, enter 1 when the MK5 input-register list identifies controller temperature as relative address 2.
5. Apply the data type, word handling, engineering-unit scaling, and read/write permission documented for that point. Read settings first; enable writes only for parameters intentionally controlled from the DCS.
6. Map essential commands and feedback to the hardwired AP020 signals rather than making serial communication their only path.
7. Build alarms and graphics only after the raw register values have been validated against the local MK5 display.

Verification and recurring pitfalls

Verify the integration at three levels. First, compare each hardwired start, stop, run, trip, and current signal with the local panel. Second, compare representative Modbus values with the MK5 display while the compressor changes operating state. Third, interrupt the communication path and confirm that loss of serial data cannot issue an unintended command or remove essential hardwired indication.

The service-air implementation exposes 18 analog values, including low-pressure-stage discharge temperature, high-pressure-stage discharge temperature, and outlet pressure. It also exposes 15 status points, 10 accumulated values such as running time, loaded time, and motor-start count, plus five settings including loading pressure 1 and unloading pressure 1. Use these point counts as an acceptance checklist for that compressor group.

Recurring errors include entering the MK5 address without subtracting one, confusing a display-style register label with the transmitted offset, assigning the wrong register class, duplicating slave addresses, reversing RS485 polarity, terminating branch devices instead of the two trunk ends, and allowing stale communication values to look valid. Add communication-quality indication and mark dependent values invalid when polling fails.

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