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| Place of Origin | JAPAN |
| Brand Name | FANUC |
| Certification | CE ROHS |
| Model Number | A20B-2902-0490 |
Part Number: A20B-2902-0490
Manufacturer: FANUC Corporation (Japan)
Product Type: HSSB CRT Control Module (High-Speed Serial Bus Daughter Board)
Board Series: A20B-2902
Compatible Systems: FANUC Series 16-C, 18-C and compatible
The A20B-2902-0490 is the HSSB CRT control module for FANUC Series 16-C and 18-C CNC systems. HSSB stands for High-Speed Serial Bus — the communication architecture FANUC used to link the CNC control unit to peripheral devices including the CRT display.
This module is the interface that makes that link work. It plugs directly onto the main CPU board of the Series 16/18 Model C controller and handles the signal path between the CNC processor and the display system.
Without this board, the CRT display function is absent.
The CNC may operate internally, but the operator panel display will not receive data or render the CNC screen correctly. In systems configured with a CRT monitor, this board is the hardware that enables the operator to see what the CNC is doing.
The A20B-2902 series is FANUC's family of plug-in daughter boards for the Series 16/18 main CPU platform.
The -0490 variant specifically handles the high-speed serial bus output to the CRT. It brings display communication capability to the controller without requiring a separate display interface board in the controller rack — the function is integrated into the main CPU board through this plug-in module.
| Parameter | Value |
|---|---|
| Part Number | A20B-2902-0490 |
| Manufacturer | FANUC Corporation |
| Product Type | HSSB CRT Control Module |
| Board Series | A20B-2902 |
| Communication | HSSB (High-Speed Serial Bus) |
| Function | CRT display interface and text/graphic control |
| Compatible Systems | FANUC Series 16-C / 18-C Main CPU |
| Design | Plug-in daughter board, SMD type |
| Origin | Japan |
| Operating Temperature | 0 – 55°C |
| Storage Temperature | −20 – 60°C |
| Humidity | 75% RH max (non-condensing) |
| Condition Available | New / Refurbished / Repaired |
Early FANUC i-series and C-series CNC controllers separated the control electronics from the display and MDI unit.
The two units were connected by a high-speed serial bus rather than the older parallel interface methods. HSSB was the standard for this connection on the Series 16/18 platform.
It allowed a single cable to carry display data, keyboard input, and control signals between the controller and the operator station.
The A20B-2902-0490 provides the controller-side hardware for this bus. On the display unit side, a matching interface board or circuit processes the incoming serial data and drives the CRT. The two ends of the HSSB link must be compatible.
Replacing only the controller-side module while the display-side hardware has a fault will not restore display function, and vice versa.
The serial bus approach reduced wiring complexity compared to parallel display interfaces.
It also provided some electrical isolation between the high-voltage CRT unit and the CNC controller electronics — a benefit in the cabinet environments where machine tool controllers operate.
On Series 16-C and 18-C controllers, the display serves multiple roles. It shows the CNC status, active program, axis positions, parameters, alarms, and graphical tool path displays where the option is installed.
All of this screen content flows from the CNC processor to the CRT via the HSSB link managed by the A20B-2902-0490.
Text display is the baseline.
The CNC continuously sends screen data representing the current display state — menus, parameter values, alarm text, position readouts.
The CRT interface module at the display end renders this data as visible characters and numbers on screen.
Without the HSSB module on the controller side generating this data stream, the display receives nothing.
Graphic functions — if the system is configured with graphical tool path display or other graphic-mode screens — also route through the same HSSB link. The controller-side module handles both text and graphic data output as defined by the CNC software.
The A20B-2902-0490 mounts directly on the main CPU board of the Series 16/18 Model C controller. It uses the A20B-2902 series' standard SMD daughter board interface.
The board connects through a dedicated socket on the main CPU PCB and does not require separate cabling within the controller cabinet.
This integration approach keeps the controller's internal wiring minimal.
The entire HSSB output function is self-contained on the daughter board.
If the board fails, the module is removed and replaced without disturbing other boards in the controller.
The main CPU PCB, the memory modules, the PMC module, and other daughter boards remain undisturbed.
After installation, no software reconfiguration is typically required — the board provides a hardware function that the CNC software already expects to be present in a correctly configured system.
Q1: The CRT display is blank on power-up. All other CNC functions appear normal. Is the A20B-2902-0490 the fault?
A blank display with normal CNC operation is consistent with an HSSB communication failure — either the controller-side module (this board) or the display-side interface has faulted.
Swap diagnosis order: confirm the CRT unit itself powers on (its own indicators or backlight), then check the HSSB cable for damage or loose connections at both ends.
If cable and display unit check out, the A20B-2902-0490 becomes the primary suspect.
Q2: The display shows garbled characters or incomplete screens. Could the HSSB module cause this?
Garbled or partial display output indicates corrupted data on the serial link. This can originate from the transmitter (the A20B-2902-0490), the cable (noise pickup, damaged conductors), or the receiver at the display end.
Start with the cable — HSSB cables in machine tool environments accumulate mechanical wear at the connectors and along flexible sections. A damaged cable produces exactly this symptom.
Replace or test the cable before concluding the module is at fault.
Q3: The module was replaced and the display works, but only in text mode — graphic tool path display is absent. Is this the board?
Graphic tool path display requires the appropriate FANUC option to be loaded and enabled in software.
The HSSB module provides the physical communication path for all display content, but the graphic display option must be separately installed in the CNC's FROM and enabled through parameters.
A missing or disabled graphic option produces exactly this symptom independent of the display hardware.
Q4: Are all revisions of A20B-2902-0490 interchangeable?
Revision suffixes on FANUC daughter boards (e.g., /01A, /02A) indicate production revisions that may address manufacturing or component changes. In most cases, later revisions are backward-compatible with earlier ones.
However, if a specific revision is called out in the machine's service documentation, use that revision unless confirmed otherwise.
The /02A suffix noted in some product listings indicates a specific revision within this part number family.
Q5: The Series 16-C machine has been running fine. The CRT display started flickering and then went blank. What should be checked before replacing the HSSB module?
A display that degrades from flickering to blank is more typically a display unit fault (CRT tube aging, inverter failure, or power supply fault in the display) or a cable fault than an HSSB module failure on the controller.
The HSSB module either works or it doesn't — gradual degradation is not characteristic of digital interface boards.
Check the display unit's power supply and the CRT tube condition first.
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