Overview: FX1 and FX1S in the MELSEC-F Family
The Mitsubishi MELSEC-F series spans several generations of compact PLCs. The FX1 and FX1S are frequently confused because their model codes are similar and both are listed as legacy products on the manufacturer's support pages, but they belong to different product generations with different CPUs, instruction sets, programming software, and serial-port architectures. Selecting the wrong driver, cable, or HMI protocol when migrating from one to the other is one of the most common field failures when connecting a third-party HMI such as a Pro-face GP or LT series panel.
This reference consolidates the published specifications, port pinouts, and HMI compatibility information required to connect an FX1S-class CPU to a Pro-face operator panel via the built-in RS-422 programming port, including the DTR/DSR handshaking mismatch that surfaces when an RS-232C cable is wired directly to a Mitsubishi vendor converter.
FX1 Series: Generation and Position
The FX1 is an early MELSEC-F compact PLC superseded by the FX2 and subsequently the FX2N. The FX1 uses the original Mitsubishi programming environment (MEDOC / FX-DOS / early GPP) and the original FX instruction set. It is no longer supported with current programming tools and is documented only as a legacy part.
Key characteristics of the FX1:
- CPU architecture pre-dates the FX2/FX2C generation.
- Memory: small EEPROM (typically 800–2,000 steps depending on variant).
- Built-in programming port: 25-pin D-sub (RS-422) on most FX1 base units.
- No native high-speed counter or pulse-train outputs equivalent to the FX1S.
- Programming software: MEDOC, DOS-based GPP, or MELSEC MEDOC plus.
FX1S Series: Generation and Position
The FX1S Series is positioned by Mitsubishi as a low-cost entry-level compact PLC designed for fixed I/O applications where the I/O count is known and expansion is not required. It is part of the post-FX2N generation and uses the modern FX instruction set plus the modern programming environment.
Key characteristics of the FX1S, drawn from the FX1S Series Programmable Controllers Hardware Manual (JY992D83901):
- Power supply: 100–240 V AC or 24 V DC variants, selected by suffix in the model code.
- Total I/O points: 10, 14, 20, or 30 (fixed; no expansion rack).
- Inputs: 24 V DC, sink/source selectable by S/S terminal wiring.
- Outputs: relay (R) or transistor (T), sink or source by suffix.
- Program memory: 2,000 steps EEPROM, battery-less retention.
- Built-in high-speed counters: 60 kHz single-phase, 30 kHz two-phase on designated inputs.
- Pulse outputs (transistor variants): up to 100 kHz.
- Real-time clock: integrated in the AC-powered variants with the -B suffix; absent on -B-less DC units.
- Programming software: GX Developer (FX) or GX Works2 / GX Works3 in FX compatibility mode.
FX1 vs FX1S Specification Comparison
| Parameter | FX1 (legacy) | FX1S (current legacy) |
|---|---|---|
| Family generation | Pre-FX2 | Post-FX2N, compact class |
| Program memory | ~800 steps (8 kbit) | 2,000 steps EEPROM |
| Total I/O | Modular, with base + extension | 10 / 14 / 20 / 30 (fixed, no expansion) |
| Programming port | 25-pin D-sub, RS-422 | Mini-DIN 8-pin, RS-422 |
| Optional serial | RS-232C / RS-485 via adapter | FX1S-CNV-BD RS-232C, FX1N-485-BD RS-485 |
| Pulse outputs | Not available | Up to 100 kHz (transistor output) |
| High-speed counters | Limited | 60 kHz single-phase / 30 kHz two-phase |
| Programming software | MEDOC, DOS GPP | GX Developer, GX Works2, GX Works3 |
| Real-time clock | Optional cartridge | Built-in on AC/-B units |
| Expansion modules | Limited (FX1 extension only) | None (FX1S is a fixed-configuration CPU) |
| Status (as of FX1S legacy notice) | Discontinued | Legacy / replacement via FX5S or FX3S |
HMI Compatibility and Pro-face Driver Selection
Pro-face (Schneider Electric GP-Pro EX and legacy GP-PRO/PBIII) lists Mitsubishi FX CPUs under distinct driver names. A Pro-face project created for an FX1 driver will not bind registers to an FX1S CPU even though both are "FX" to a casual observer. The HMI configuration must be changed to the FX1S-equivalent driver, typically labelled:
-
Mitsubishi FX1S (CPU)in GP-PRO / PBIII for Windows -
Mitsubishi Electric Corporation / MELSEC FX1S (Ethernet/Serial)in GP-Pro EX (refer to the device list shipped with the project's Device/PLC import) - Generic
Mitsubishi FX Series (CPU)as a fallback (operates on the MC-protocol A-compatible 1E frame over the RS-422 port)
If the older FX1 driver is forced, the panel reports a timeout on the first read because the FX1S replies with a 1E-frame response envelope that the FX1 driver does not decode. Always re-bind the project to the FX1S driver before downloading to the panel.
0x608E → 9 600 bit/s, 7 data bits, even parity, 1 stop bit). Custom parameter changes require a matching D8120 write from ladder logic on the FX1S side.FX1S Serial Port Architecture
The FX1S base unit carries one built-in mini-DIN 8-port that exposes the RS-422 programming channel. There is no RS-232C port on the CPU body; RS-232C is added through the FX1S-CNV-BD option board, and RS-485 is added through the FX1N-485-BD option board. The mini-DIN 8-port pin assignment is:
| Pin | Signal | Direction (PLC relative) | Notes |
|---|---|---|---|
| 1 | RX+ (RD+) | Input | Receive data, positive |
| 2 | RX- (RD-) | Input | Receive data, negative |
| 3 | SG | — | Signal ground |
| 4 | TX+ (SD+) | Output | Send data, positive |
| 5 | TX- (SD-) | Output | Send data, negative |
| 6 | DSR | Input | Data Set Ready, handshaking |
| 7 | DTR | Output | Data Terminal Ready, handshaking |
| 8 | +5 V | — | Used by SC-09 and similar programming cables |
The DTR/DSR pair is exposed at the mini-DIN 8 connector because Mitsubishi's SC-09 RS-422-to-USB programming cable uses the +5 V pin for power and references DTR/DSR for RS-232C-style handshaking inside the cable's adapter. This is the origin of the field-reported DTR/DSR mismatch with Pro-face panels.
Pro-face HMI COM Port Pinouts
Pro-face GP, ST, and LT panels expose RS-232C and, on selected models, RS-422 on a DB9 male (COM1) connector. The DB9 pinout used by Pro-face for RS-232C is:
| DB9 pin | RS-232C signal | Direction (HMI relative) |
|---|---|---|
| 2 | RD (RXD) | Input |
| 3 | SD (TXD) | Output |
| 4 | ER (DTR) | Output |
| 5 | SG | — |
| 6 | DR (DSR) | Input |
| 7 | RS (RTS) | Output |
| 8 | CS (CTS) | Input |
Pro-face HMI firmware uses ER/DR (DTR/DSR) for hardware handshaking by default on Mitsubishi FX drivers. The PLC side, however, presents DTR/DSR over RS-422 on the mini-DIN 8 connector. A direct DB9-to-DIN8 cable will leave the HMI waiting for DR assertion and the PLC will not see the DTR/DSR loop closed.
Cable Construction: RS-422 Active Converter vs Passive Wiring
Two cable approaches are field-proven for FX1S-to-Pro-face integration.
Approach A — Passive RS-422 Cross-Cable
Wire the four RS-422 data pairs and signal ground between the panel and the PLC. Loop DTR/DSR on the PLC side (pin 6 tied to pin 7 inside the mini-DIN 8 shell) so the FX1S always sees DTR asserted. This works for Pro-face projects where the driver's handshaking is set to "None" in the HMI side.
- PLC mini-DIN 8 pin 1 (RX+) → HMI DB9 RS-422 RX+ (terminal block A on GP-2500/2600 or per manual)
- PLC pin 2 (RX-) → HMI RX-
- PLC pin 3 (SG) → HMI SG
- PLC pin 4 (TX+) → HMI TX+
- PLC pin 5 (TX-) → HMI TX-
- PLC pin 6 (DSR) → PLC pin 7 (DTR), shorted at the connector
Approach B — Active RS-422-to-RS-232C Converter with Loop-Backed DTR/DSR
The vendor-supplied Mitsubishi cable (the "SC-09 style" cable with the internal PCB) is an active RS-422-to-RS-232C converter. It pulls +5 V from the PLC's pin 8, level-translates the differential pair, and exposes DTR/DSR on the DB9 side. When a Pro-face panel connects to this cable, the converter's DTR/DSR lines must be looped at the DB9 end (DB9 pin 4 tied to pin 6) to satisfy the HMI's handshaking. Many factory-built SC-09-style cables already do this internally; field cables often do not.
Communication Parameter Configuration (D8120)
The FX1S serial channel is controlled by data register D8120. The default value 0x608E decodes to:
- Baud rate: 9 600 bit/s
- Data bits: 7
- Parity: even
- Stop bits: 1
- Protocol: dedicated protocol (MC-protocol 1E frame for FX-series CPUs)
- Header: not used
- Terminator: not used
To match a Pro-face driver that expects 19 200 bit/s, write a new value to D8120 from the ladder program before initiating the HMI link, for example MOV H6092 D8120 to set 19 200 / 7 / E / 1. The HMI COM1 settings must mirror this exactly; a mismatch on baud rate or parity is the most common reason for a one-way or no-response link.
D8120 while a programming tool is connected to the same port will drop the GX Developer session. Make the change with the programming tool disconnected, or transfer the value through an RS-232C option board on a different channel.Wiring Diagram (SVG)
Commissioning Procedure
- Confirm the FX1S CPU variant. The suffix determines whether the unit accepts AC mains or 24 V DC, and whether the outputs are relay or transistor.
- Select the Pro-face driver:
Mitsubishi FX1S (CPU). If unavailable in older GP-PRO versions, useMitsubishi FX Series (CPU). - Set the HMI COM1 parameters: 9 600 bit/s, 7 data bits, even parity, 1 stop bit (matching default
D8120 = H608E), header disabled, terminator disabled, handshaking ER/DR enabled. - Build the cable per the RS-422 wiring table above. Loop DTR/DSR on the PLC connector for passive cables; verify the loop on the DB9 for active converter cables.
- Power the PLC. With no program running, the panel should display a successful "PLC link established" or equivalent status within 2–3 seconds at 9 600 bit/s.
- From the panel, force one coil (e.g. M0) ON and confirm the change in GX Developer online monitor. This validates both directions.
- Read register D0 from the panel and confirm the value. If the value is always 0 or always 65535, suspect a baud-rate or parity mismatch on
D8120.
Troubleshooting Matrix
| Symptom | Likely root cause | Corrective action |
|---|---|---|
| Panel reports "no response from PLC" with Pro-face FX1 driver | Wrong driver selected; project still references FX1 | Re-bind to FX1S or generic Mitsubishi FX Series (CPU) driver; redownload to panel |
| Panel times out, but GX Developer connects successfully | Handshaking mismatch (DR/ER on HMI side, DTR/DSR on PLC side) | Loop DTR/DSR at PLC mini-DIN 8; or set HMI handshaking to None and loop ER/DR at the DB9 |
| One-way communication (panel reads, never writes) | RX pair reversed or TX pair shorted | Verify RX+ and RX- are crossed correctly to the panel's TX pair |
| Garbled data, occasional characters lost | Baud rate mismatch | Compare HMI COM1 baud to D8120 value; both must agree |
| Vendor SC-09 cable works with GX Developer but not with HMI | Cable's internal DTR/DSR loop not present | Replace with cable that loops DB9 pin 4 (ER/DTR) to pin 6 (DR/DSR) on the HMI end |
| Link works, then drops after a few minutes | RS-422 termination missing on long runs (>10 m) | Add 120 Ω termination across the receive pair at the far end |
| Panel logs checksum error | Parity setting mismatch | Confirm both sides set to "even" parity; some Pro-face drivers default to "none" |
| OPC server communicates, HMI does not | OPC server using a different serial channel (RS-232C option board) | Confirm HMI is wired to the same channel as the OPC server, or rewire the HMI to the active channel |
Replacement and Migration Notes
The FX1S is a legacy product with a defined migration path to the FX5S or FX3S platform. When migrating, the HMI driver must be changed again: FX3S and FX5S use the MC-protocol 3E or 4C frame and require a Pro-face driver labelled FX3S or FX5, respectively. The mini-DIN 8 RS-422 port is retained on the FX3S but the FX5S uses an RJ45 connector with a different pinout, so the cable must be rebuilt rather than re-used.
FAQ
Are the Mitsubishi FX1 and FX1S the same CPU family?
No. The FX1 is a pre-FX2 generation compact PLC discontinued long ago. The FX1S is a post-FX2N low-cost fixed-I/O compact PLC. They have different instruction sets, programming software, and serial port pinouts. They are not interchangeable for HMI driver binding.
Which Pro-face driver should I use to connect to an FX1S?
Use the driver explicitly labelled "Mitsubishi FX1S (CPU)" in GP-PRO/PBIII, or "Mitsubishi Electric / MELSEC FX1S (Serial)" in GP-Pro EX. The older "Mitsubishi FX (CPU)" driver works on the same RS-422 1E frame as a fallback but is not officially validated for every firmware revision.
What is the default serial configuration of the FX1S programming port?
The built-in RS-422 mini-DIN 8 port is configured by D8120 = H608E: 9 600 bit/s, 7 data bits, even parity, 1 stop bit, dedicated protocol, no header, no terminator. Both the HMI and any OPC server must match this exactly.
Why does my SC-09-style programming cable not work with a Pro-face HMI?
SC-09-style cables expose RS-422 on the mini-DIN 8 end and an active RS-232C on the DB9 end, but they expect the host to provide a DTR/DSR loop on the DB9. A Pro-face panel asserts ER (DTR) and waits for DR (DSR). If pin 4 and pin 6 are not looped inside the cable or at the DB9, the HMI never sees the response and the link times out.
Can the FX1S be expanded with extra I/O modules like the FX2N?
No. The FX1S has a fixed I/O count of 10, 14, 20, or 30 points and no expansion bus. For modular I/O, migrate to the FX3S (with adapter) or FX5S platform.