How to Make the CAS Modbus Scanner Read A Modbus 6 Digit Address (JBUS)
In many legacy industrial automation environments, Modbus implementations were restricted by a 5-digit addressing convention. This often limited the number of objects of each type to 9,999. For example, a Holding Register was traditionally capped at 49,999, and an Input Register at 39,999.
Modern requirements often exceed these legacy boundaries. While the Modbus protocol itself supports up to 65,535 objects per type, older systems imposed a validation limit that "choked" the available address space. The CAS Modbus Scanner removes these restrictions, allowing you to access the full 6-digit address range.
Understanding 5-Digit vs. 6-Digit (JBUS) Addressing
The shift from 5-digit to 6-digit addressing (often referred to as JBUS) is essential for modern SCADA and PLC integration. The CAS Modbus Scanner handles this by focusing on the Protocol Data Unit (PDU) offset rather than arbitrary legacy limits.
Addressing Comparison Table:
| Type | Legacy 5-Digit Range | Extended 6-Digit Range |
|---|---|---|
| Holding Registers | 40001 – 49999 | 400001 – 465536 |
| Input Registers | 30001 – 39999 | 300001 – 365536 |
Step-by-Step Configuration Example
To read a 6-digit address, such as Holding Register 512,345 (which corresponds to an offset of 12,345 in PDU terminology), follow the request setup in the scanner.
Image 1: Adding a 6-digit address request in the CAS Modbus Scanner.
Once the request is sent, the CAS Scanner provides a comprehensive display. The tool is designed to show both the 5-digit legacy reference and the modern 6-digit address simultaneously to ensure data validation is accurate.
Image 2: Verification of 6-digit register data capture.
FAQ – Modbus 6-Digit Addressing
1) Why do some systems limit Modbus to 9,999?
This was a software-imposed limitation based on the 5-digit display format (4xxxx). It was not a limitation of the protocol itself, but rather a validation "choke" in older hardware and software drivers.
2) What is JBUS?
JBUS is a variant of Modbus that uses the same frame format but inherently supports the 0-65535 address range without the legacy 5-digit constraints.
3) How does the CAS Scanner handle PDU offsets?
The scanner maps the user-entered address directly to the PDU offset. For example, entering address 512,345 instructs the scanner to request the register at offset 12,345 from the target device.
4) Is a firmware update required to support 6-digit addresses?
If you are using the CAS Modbus Scanner, 6-digit addressing is supported natively. If you are using an older gateway, you may need to verify if the internal driver supports extended addressing.