Description
The Allen-Bradley 1785-CHBM is a specialized Coaxial Cable Attenuation Module designed for the PLC-5 controller platform. It functions as a hardware-matching interface that introduces controlled attenuation into standard Data Highway Plus (DH+) or Universal Remote I/O (RIO) coaxial cable transmission lines. This module is primarily deployed in systems transitioning from legacy twinaxial cable networks to high-frequency coaxial media topologies, ensuring signal integrity by preventing receiver saturation and minimizing signal reflections over shorter cable lengths.
Commercial Availability & Terms
- Condition: Refurbished (Fully Bench Tested, Cleaned, and Inspected) or New Surplus available from legacy manufacturing asset inventories.
- Warranty: 12-Month Functional Warranty on all components.
- Lead Time: Typically ships within 1 to 2 business days from warehouse stock.
- Pricing Note: This hardware is discontinued by the manufacturer. Price fluctuates based on global surplus availability. Contact our procurement desk for a formal, firm quotation.
Technical Specifications
The following table details the physical and electrical performance criteria for the 1785-CHBM hardware module.
| Parameter | Value |
| Manufacturer | Rockwell Automation / Allen-Bradley |
| Model / Line | PLC-5 Communication Accessories |
| Part Number | 1785-CHBM |
| Network Compatibility | Data Highway Plus (DH+), Universal Remote I/O (RIO) |
| Media Type | Coaxial Cable (typically RG-6 / RG-11 configurations) |
| Function | Signal Attenuation and Impedance Matching |
| Form Factor | In-line / Modular DIN-rail or Panel Mount Option |
| Enclosure Protection | Open Type / IP20 when completely mounted |
| Operating Temperature | 0 to +60 °C (+32 to +140 °F) |
| Storage Temperature | −40 to +85 °C (−40 to +185 °F) |
| Relative Humidity | 5 to 95% non-condensing |
Field Integration & Network Engineering Rules
1. Attenuation Requirements
The 1785-CHBM is required in coaxial industrial networks when the physical distance between trunk nodes is short enough that the signal amplitude overloads the transceiver circuits on PLC-5 communication modules (such as the 1785-KE, 1785-KA, or standard PLC-5 processors). Inserting this module prevents signal clipping and subsequent cyclic frame check sequence (FCS) errors in the data log.
2. Network Topology and Coaxial Cable Layout
When implementing a coaxial RIO or DH+ network, ensure the system matches the overall design rules specified in Allen-Bradley Publication 1770-6.2.1. The 1785-CHBM must be mounted in-line between the trunk cable drop and the localized station drop tap. Do not drop multiple taps directly from the attenuation module without proper impedance termination.
3. Termination Resistors
A standard 3-pin phoenix or BNC layout must maintain correct characteristic impedance across the segment. Ensure that standard network termination resistors (150\ \Omega or 82\ \Omega, depending on the specific baud rate and transceiver configuration) are present at the absolute physical ends of the coaxial segment. The 1785-CHBM does not substitute for an end-of-line terminator.
System Constraint: Never attempt to alter, splice, or wire the 1785-CHBM while network traffic is active on the DH+/RIO segment. Disturbing the coaxial signal shield mid-operation can cause localized network noise, dropping whole racks of remote I/O nodes and inducing an immediate CPU fault.
Frequently Asked Questions
When exactly do I need to install a 1785-CHBM module?
You need to install the 1785-CHBM if your network engineering calculation reveals that the total attenuation of your short coaxial run is lower than the minimum threshold required by the PLC-5 transceivers. If you observe continuous “Frame Error” counters incrementing in your RSLogix 5 programming software on short networks, signal saturation may be the cause, requiring an attenuation module.
Can the 1785-CHBM be utilized on standard EtherNet/IP or ControlNet networks?
No. The 1785-CHBM is engineered exclusively for the legacy 57.6 kbps, 115.2 kbps, and 230.4 kbps signaling frequencies used by Allen-Bradley DH+ and Universal Remote I/O systems over coaxial media. It will not function on ControlNet (which utilizes 1786-type taps) or modern Ethernet networks.
What maintenance does this module require?
The 1785-CHBM is a passive hardware component and requires no software or firmware updates. Maintenance is restricted to verifying that the physical coaxial connections remain tight, free from oxidization, and isolated from external structural grounding loops inside the cabinet panel.
