Resolving TP177B to Mitsubishi A1S RS422 Communication Failure

Resolving Siemens TP177B to Mitsubishi A1S RS422 Communication Failure

Integrating a Siemens SIMATIC TP177B HMI with a Mitsubishi MELSEC-A1S PLC over RS422 is a common retrofit requirement on older Japanese-built lines. Field reports consistently show that direct connection attempts using a generic null-modem-style cable fail silently, even when the WinCC Flexible connection wizard reports parameters correctly. This reference provides a complete diagnostic path: hardware identification, electrical interface matching, protocol selection, cable construction, and verification. It is written for automation engineers performing brownfield panel replacements such as swapping an aging Beijer MAC50 for a TP177B.

1. Problem Definition and Field Symptoms

The classic failure mode is described as follows: a TP177B is wired to an A1S station that previously communicated with a Beijer MAC50 over RS422. The technician opens WinCC Flexible, creates a new connection, selects "Mitsubishi" as the PLC vendor, sets Protocol 4, and enters baud rate, parity, and station number. The project is compiled and transferred. The TP177B then displays a "Connection Error" or a system alarm in the status bar, and the value fields stay at "######" or "-".

Comparable symptoms appear under the following conditions:

• TP177B boots, runtime starts, but the connection status line shows red.
• Data tags never update, even though the cable passes a continuity test.
• FX series CPU communicates correctly with the same TP177B over PG protocol, but the A1S does not respond.
• A Beijer MAC50 or Mitsubishi GOT panel can read/write the same A1S without error.

Each symptom points at a different layer of the stack. Resolving the issue requires inspecting the physical layer (cable and interface mode), the data-link layer (protocol family and frame format), and the application layer (WinCC Flexible driver mapping).

2. Hardware Identification

2.1 Siemens SIMATIC TP177B Interfaces

The TP177B is a 5.7-inch STN/LCD panel available in two main variants:

Key specification from the Siemens Industry Online Support documentation: the IF1B port is a 9-pin sub-D female connector configured for RS485 only. There is no RS422 (4-wire full-duplex) capability on the panel itself. The pinout is:

The TP177B does not support separate TX/RX pairs. The same two pins (3 and 8) handle both transmit and receive, which is the textbook definition of 2-wire RS485. The WinCC Flexible runtime is therefore limited to half-duplex polling.

2.2 Mitsubishi MELSEC-A1S Serial Communication Modules

The A1S CPU itself does not provide an RS422 port. Communication is achieved through a dedicated serial communication module installed in the A1S base rack. Common options are:

The four-channel pairs in the C24/UC24 modules are configured via hardware DIP switches for RS422 (4-wire) or RS485 (2-wire) operation. The default for the R4 suffix is RS422. This is the first place to look when a TP177B is being attached.

3. Root Cause Analysis

When a TP177B cannot reach an A1S, the failure is almost always at one of three layers. The first two are the most common.

3.1 Electrical Interface Mismatch (Most Common)

The TP177B exposes RS485 2-wire. The A1SJ71C24-R4 channel is typically factory-set to RS422 4-wire. If a straight-through cable is used with all four data lines tied in parallel, the A1S receiver will not see a valid differential signal because the driver pair is not being driven as RS422 full-duplex. Conversely, if only two wires are run and the A1S is still in RS422 mode, the transmit pair of the A1S floats and is not terminated for half-duplex arbitration.

Verification: measure the A1S module DIP switches. Switch bank SW1 or SW2 on the C24/UC24 module selects RS422/485 mode per channel. Set the channel to RS485 2-wire mode to match the TP177B.

3.2 Protocol Selection Error (Second Most Common)

WinCC Flexible ships with several Mitsubishi driver families. The relevant ones are:

Selecting the FX driver against an A1S causes silent failure: the driver sends a 1C-frame-format request with a syntax that the A1SJ71C24 cannot parse (the C24 module expects 2C/3C/4C frame formats by default). The driver receives a non-response and the HMI times out.

3.3 Cable Construction Error

Even with correct protocol and mode selection, an incorrect cable keeps the link down. The standard A1S-to-HMI cable sold for GOT panels is wired for RS422 4-wire (separate send/receive pairs). That cable will not work against a TP177B without modification. The A1S side must be flipped to RS485 2-wire mode and the cable reduced to a single twisted pair plus ground reference.

4. Cable Construction and Pinout

4.1 Required Cable (RS485 2-Wire, Half-Duplex)

Build a 3-wire shielded cable, 1.5 m to 15 m long, with 120 Ω characteristic impedance. Use a twisted pair for the data lines and a separate drain wire for the reference ground.

For an A1SJ71C24-R4 module, the connector is a 25-pin D-sub (female on the module, male on the cable). The standard pinout for Channel 1 in RS485 2-wire mode is:

• Pin 10: SDA / RDA (data A)
• Pin 11: SDB / RDB (data B)
• Pin 12: SG (signal ground)

Add a 120 Ω termination resistor across pins 10 and 11 at the A1S end. The TP177B IF1B port has an internal termination that is enabled via the WinCC Flexible connection configuration ("Enable bus termination").

4.2 Cable Construction Steps

1. Strip 40 mm of jacket from a shielded twisted-pair cable (e.g., Belden 3106A or equivalent, 24 AWG).
2. Identify the white/blue pair. These will carry data A and data B.
3. Tin the conductors and attach a DB9 female connector to the TP177B end: pin 8 to white, pin 3 to blue, pin 2 to the drain wire and shield.
4. Attach a DB25 male connector to the A1S end: pin 10 to white, pin 11 to blue, pin 12 to the drain wire and shield.
5. Install a 120 Ω 1/4 W resistor across pins 10 and 11 of the DB25 male, tucked inside the backshell.
6. Clamp the shield to the metal backshell at both ends to provide 360° shield bonding.
7. Verify continuity with a multimeter. Confirm no shorts between any data pin and the shield.

5. WinCC Flexible Configuration

5.1 Creating the Connection

1. Open the TP177B project in WinCC Flexible 2008 SP5 or later.
2. In the project tree, right-click Connections and select New Connection.
3. From the Communication driver dropdown, choose Mitsubishi - A-series Link. Do not select FX or Q-series.
4. Set the HMI device to TP177B and the interface to IF1B.
5. Set the baud rate to 19200 bps for first commissioning; this is the highest reliable rate for half-duplex RS485 with 15 m of cable. Lower to 9600 if errors persist.
6. Set parity to Even, data bits to 8, stop bits to 1. This matches the A1SJ71C24-R4 default.
7. Set the station number to a unique value (e.g., 0 for the master; the A1S will be station 0 by default if the C24 module's station number DIP switch is FF, meaning "use parameter").
8. Enable bus termination in the connection properties if the TP177B is at the end of the bus segment.

5.2 Mapping Tags to A1S Memory Areas

The Mitsubishi A-series Link driver in WinCC Flexible supports the following address areas, using a 5-character hexadecimal format. Examples:

For a 16-bit word read, use the prefix and a 4-digit hex address, e.g., D0042. For a 32-bit value, prefix with D- or read the high word first and low word second depending on the driver version. Real-time trending of D-registers requires WinCC Flexible to be configured with a poll rate that does not exceed the bus capacity: at 19200 bps with 11 bits per character, a single 4-word read takes about 5 ms, allowing roughly 40 tags per second per polled group.

6. A1SJ71C24-R4 Module Configuration

6.1 DIP Switch Settings

The A1SJ71C24-R4 has two DIP switch banks, one for each channel. For RS485 2-wire mode, the typical settings are:

Refer to the Mitsubishi Electric FA Document Library for the exact switch positions of the installed module revision. The settings above are valid for A1SJ71C24-R4 modules with the 240V / 24V / 24V family ID and serial number prefix typical of units shipped after 2002.

6.2 Buffer Memory Setup via GX Developer

After setting the DIP switches, verify the channel parameters using GX Developer (or GX Works 2) connected to the A1S programming port. In the parameter tree, navigate to PLC Parameter > I/O Assignment > Switch Setting. Confirm the slot number matches the C24 module and that "Type" is set to "Intelligent". The station number should match what is configured in WinCC Flex.

6.3 LED Status Interpretation

7. RS422 to RS485 Conversion Considerations

If the A1SJ71C24-R4 is already wired to a Beijer MAC50 over RS422 4-wire and the technician prefers not to disturb the existing wiring, a converter is an alternative. A 4-wire-to-2-wire RS422/RS485 converter is required. Industrial units from Phoenix Contact (PSM-ME-RS485/RS485-P), Weidmüller (WAVE-analog), or HMS Anybus are field-proven.

Key converter requirements:

• Automatic direction switching (no RTS control needed from TP177B).
• Baud rate up to 19200 bps with no propagation delay exceeding 1 character time.
• Galvanic isolation between the two segments to break ground loops.
• Fail-safe bias resistors (typically 1 kΩ pull-up to 5V on B, pull-down on A) to define a stable idle state.

A converter is a workaround, not a recommended path. The native 2-wire mode on the A1SJ71C24-R4 is preferable because it eliminates the converter as a point of failure and preserves the original module's diagnostic LEDs.

8. Alternative Solutions

When the TP177B cannot establish reliable communication, three alternative paths exist:

8.1 Replace the HMI with a Mitsubishi GOT

GOT2000 (e.g., GT2103-PMBDS, GT2310-VTBA) connects natively to the A1S with the A-series driver, no protocol gymnastics required. This is the lowest-risk path and is the solution chosen by most engineers in the field when a TP177B is part of a multi-vendor machine that includes a Mitsubishi A1S as the controller. Mitsubishi GOT panels also support GOT Mobile for tablet-based monitoring.

8.2 Add an Ethernet Module and Use MC Protocol over TCP

The A1S does not have a built-in Ethernet port. Installing an A1SJ71E71-B2 or A1SJ71E71-B5 module provides a 10BASE-T or 100BASE-TX interface and runs the same MC Protocol over TCP port 5000. The TP177B can then be paired with a "Mitsubishi A-series Ethernet" driver in WinCC Flexible, which avoids the RS422/RS485 entirely. This is the most robust long-term solution but requires re-wiring the panel side from RS485 to RJ45.

8.3 Use a Protocol Gateway

Devices such as the Red Lion CR3000 or the ProSoft PLX31 convert A-series MC Protocol to Modbus RTU/TCP or to EtherNet/IP. The TP177B can then poll the gateway as a Modbus master. The gateway introduces a third-party device that must be commissioned and maintained, which is acceptable in a long-life plant but excessive for a single-cell retrofit.

9. Verification Procedure

1. Power the A1S and verify the C24 module CPU LED is green, ERR is off.
2. Connect the cable. With the HMI powered, the C24 RD and SD LEDs should flicker within 2 to 3 seconds as WinCC Flex runtime starts polling.
3. On the TP177B, navigate to the diagnostic screen: Main Screen > Diagnostics > Connection. The A-series connection should show "OK" with a green indicator.
4. Create a test tag pointing to a known data register, e.g., D0042 = 1234. Force a value into D0042 from GX Developer and confirm the HMI updates within one poll cycle.
5. Toggle an input X0000 and verify the corresponding HMI indicator changes.
6. Write a value from the HMI to D0050 and confirm the change is visible in GX Developer's buffer monitor.
7. Run a 30-minute soak test with 5-second logging to confirm no timeouts occur under normal scan load.

10. Specification Reference

10.1 TP177B IF1B Electrical Characteristics

10.2 A1SJ71C24-R4 Electrical Characteristics

10.3 Recommended Settable Parameters

11. Frequently Asked Questions