NT20S-ST121B EPROM Compatibility: Resolving "Incompatible Hardware" Errors

The Omron NT20S series programmable terminals were widely deployed alongside the CQM1, C200H, and early CS1 PLC families from the mid-1990s through the early 2000s. Two part numbers — NT20S-ST121B and NT20S-ST121B-EV3 — surface repeatedly in field service reports. Although the suffix implies a simple firmware revision, the actual hardware difference is a system EPROM that determines whether the unit accepts a project download from the modern NTST 4.61 Windows support tool. When an installer swaps the EPROMs between units or attempts to load an agyod project into a screen still carrying the older abtod firmware, the support tool returns Incompatible PT hardware and refuses to transfer.

This reference covers how to identify each EPROM revision, configure the NT20S Tool Mode memory switch, swap firmware safely, and bring mixed fleets of NT20S terminals back online with an Omron CQM1 host. The procedures assume a host running Windows 95, Windows 98, or Windows 2000 with a serial transfer cable wired per the NT20S Operation Manual (Cat. No. V020-E1-).

1. NT20S Family Overview

The NT20S is a 5.7-inch STN monochrome programmable terminal with a resistive analog touch overlay. It uses a 4 × 20 or 8 × 20 character display depending on the font selected and supports both text and simple bitmap screens. Two board revisions are encountered in service stock:

The suffix designators are not officially documented as firmware revisions in the operation manual; however, the field-proven convention is that -EV3 units ship with the agyod mask and ST121B units ship with the abtod mask. The hand-written EPROM stickers confirm this — the working EPROM carries the omron agyod label, and the non-working EPROM carries the mitsubishi abtod label (likely a third-party cross-reference, not a Mitsubishi origin).

2. EPROM Identification and What Each Mask Supports

The system EPROM is a 27C256-class 32-Kbyte UV-erasable device soldered into a socket on the NT20S main PCB. Its contents include the bootloader, system menu, PT communication protocol, and the transfer-mode handshake expected by NTST. Two masks are relevant in service:

2.1 Behavior of an abtod EPROM in any NT20S chassis

When the abtod EPROM is installed, the NT20S still powers up, displays the system menu, and accepts navigation through Transmit and Maintenance Mode. However, the moment NTST initiates a project download, the terminal returns a hardware-mismatch NAK to the support tool. The result is the misleading Incompatible PT hardware dialog in NTST, even though the chassis itself is fully functional. The mask simply lacks the modern transfer protocol expected by NTST 4.61.

2.2 Behavior of an agyod EPROM in any NT20S chassis

The agyod mask supports the full NTST 4.61 transfer protocol, including the agyod command set used to query terminal identity, project size, and memory layout. With this EPROM installed the support tool reads the correct hardware ID and allows project upload/download. NTST can also be configured to target the unit as either an NT20S or, for mixed-fleet deployments, as an NT20M when memory size limitations prevent loading a full NT20S project.

3. NTST 4.61 vs. NTSS V2.33 (DOS)

The original NT20S units shipped before 1998 were supported by NTSS V2.33, a DOS-based configuration tool that uses a direct serial transfer at 9600 bps, 7 data bits, even parity. NTST 4.61 (and the related NT-ZJCAT1-EV4 support tool) is the Windows replacement and is the canonical choice for any unit with an agyod EPROM. The transfer parameters used by NTST are:

When migrating an older abtod-mask project originally authored in NTSS, the project file must first be opened and re-saved in NTST 4.61 before it can be transferred to an agyod-mask unit. NTST includes a conversion utility (File → Convert NTSS Project) that translates the older .pts file to the modern .ntc format. If the converted project is loaded without first opening and re-saving it in NTST, the unit again returns Incompatible PT hardware because the project header still carries the old NTSS identification block.

4. Preparing the Host Computer

NTST 4.61 is a 16-bit Windows application. Although it runs on Windows 2000 and Windows XP, NTSS V2.33 only runs reliably on Windows 95, Windows 98, or under a DOS boot disk. The host must provide a true COM port. The following checklist resolves the most common transfer issues observed on Windows 2000 systems:

1. Disable any power-management on the COM port (Device Manager → Ports → Properties → Power Management → uncheck "Allow the computer to turn off this device to save power").
2. Set the FIFO buffers to the lowest setting (Device Manager → Advanced → 1-byte trigger). NTSS V2.33 cannot recover from a 16-byte FIFO overflow.
3. Disable any anti-virus or fax-resident COM monitors that may capture the COM port.
4. Verify the COM port number is ≤ COM4. Some NTST builds do not enumerate higher port numbers correctly.
5. Use a shielded NT20S transfer cable (Omron part XW2Z-200S or equivalent). Cable runs above 15 m require a 422/485 repeater.

5. Entering Tool Mode on the NT20S

The agyod mask exposes a Tool Mode entry in the memory-switch menu that older abtod masks omit. Tool Mode is required for system transfer operations. The exact navigation sequence from the NT20S Operation Manual (Cat. No. V020-E1-) is:

1. Power-cycle the NT20S. Hold the top-left and top-right corners of the touch screen simultaneously during power-up to enter the System Menu.
2. Touch Maintenance Mode.
3. Touch Memory Switch.
4. Touch Tool Mode. The screen displays a confirmation prompt asking whether to enable Tool Mode for the next transfer session.
5. Touch Yes. The terminal reboots into Transfer-Ready state.

Once Tool Mode is enabled, the host's NTST can detect the terminal, read its project memory, and download a new project. If Tool Mode is disabled or unavailable (i.e., the abtod mask is fitted), the System Menu stops at Memory Switch and does not present a Tool Mode option — this is the most reliable field test for EPROM mask identification without opening the chassis.

6. Step-by-Step Procedure to Recover a Mixed NT20S Fleet

The following procedure restores both the older NT20S-ST121B (with abtod) and the newer NT20S-ST121B-EV3 (with agyod) to a working state against an Omron CQM1 PLC. The procedure assumes the user has both EPROMs available and a verified-good programming host.

6.1 Identify the current EPROM in each chassis

1. Power down the NT20S and remove the rear cover (four Phillips screws).
2. Locate the 28-pin EPROM socket at the centre of the main PCB. The label sticker should read either omron agyod or abtod (sometimes with a third-party shop prefix).
3. Record which chassis carries which mask. Mark the chassis with a permanent-marker identifier so future service swaps do not repeat the mismatch.

6.2 Confirm the working EPROM using a swap test

1. Move the agyod EPROM into the ST121B chassis. Power up and confirm the System Menu shows Tool Mode.
2. Connect NTST 4.61 and transfer a small test project. If the project loads, the chassis hardware is good.
3. Return the agyod EPROM to its original chassis.

6.3 Place the agyod EPROM in the unit you intend to program

1. Install the agyod EPROM in the target NT20S.
2. Reassemble the rear cover and apply power.
3. Enter Tool Mode as described in Section 5.

6.4 Transfer the project from NTST 4.61

1. Launch NTST 4.61 and open the converted .ntc project.
2. Select PT → Transfer → Download.
3. Confirm the PT Type matches the target unit. If project memory exceeds the NT20S budget, re-create the project as an NT20M target — the source field-test confirmed this is the cleanest workaround for legacy projects.
4. Monitor the transfer log. A successful transfer ends with the message Transfer Complete. An Incompatible PT hardware message at this stage indicates either an NTSS-format project header was not converted or the EPROM in the chassis is not agyod.

6.5 Connect to the CQM1 host

1. Switch off both the NT20S and the CQM1.
2. Connect the NT20S peripheral port to the CQM1 peripheral port using an Omron XW2Z-200S cable (2 m) or XW2Z-500S (5 m).
3. Power up the CQM1 first, then the NT20S.
4. The CQM1 should auto-detect the PT on its peripheral port (no DIP switch setting required on the CQM1-CPUxx units).
5. Confirm communication by toggling a bit on the CQM1 and observing the corresponding indicator on the NT20S screen.

7. Memory-Switch Settings for CQM1 Host Link

While the unit is in Maintenance Mode, the following memory switches must be set for proper host-link operation with a CQM1:

8. Wiring and Cable Reference

The standard NT20S-to-CQM1 peripheral cable uses the following pin-out on the DB-9 connector at the NT20S end:

The matching peripheral-port connector on the CQM1 is a half-pitch mini-DIN. Use only Omron-supplied cables; off-the-shelf serial cables will not seat correctly and may damage the mini-DIN receptacle.

9. Error-Code and Behaviour Matrix

10. Migrating Older Projects from NTSS V2.33

If the original project was created with NTSS V2.33 on DOS, follow this migration sequence:

1. Boot the host into MS-DOS or Windows 95. NTSS V2.33 cannot run under NTVDM on Windows 2000/XP without specific compatibility settings.
2. Transfer the existing project from the NT20S to the host PC using NTSS (F4 Transfer → Upload from PT).
3. Save the project as a .pts file.
4. Boot into Windows and launch NTST 4.61.
5. Select File → Open and select the .pts file. NTST prompts to convert the project — accept.
6. Verify all screen objects. NTSS used a different tag-addressing convention for bit-mapped alarms; check any alarms referencing HR or AR areas.
7. Re-save as .ntc and transfer to the NT20S fitted with the agyod EPROM.

11. When the EPROM Cannot Be Swapped

If the agyod EPROM is unreadable or lost, there are three recovery paths:

1. Pull a working EPROM from a donor unit. Verify the donor's project is backed up first. After swap, re-upload the donor's project to NTST so both units can be restored.
2. Source a pre-programmed EPROM. Some Omron distributors still stock factory-new NT20S-ST121B-EV3 units with fresh agyod EPROMs. The full part number is NT20S-ST121B-EV3.
3. Replace the entire NT20S with an NS-series terminal. For new installations, the Omron NS5 or NS8 is the supported migration path. Project conversion from NT20S to NS is performed with CX-Designer.

12. Verification Checklist

After the EPROM swap and project transfer, run through the following checks before returning the line to production:

• Power-cycle the NT20S three times consecutively and confirm no watchdog reset message appears.
• Toggle each PLC bit used by the project and confirm the corresponding indicator on the NT20S updates within 200 ms.
• Force a screen switch using the touch overlay. Verify the next screen displays within 1 s.
• Disconnect and reconnect the peripheral cable while the CQM1 is running. The NT20S should re-establish communication automatically within 5 s.
• Power down the CQM1 only; the NT20S should display Communication Error and enter its standby screen. Power the CQM1 back up and verify the NT20S recovers.
• Save the final project to the host PC and label the project file with the EPROM mask used, the PT serial number, and the date.

13. Frequently Asked Questions