IEC TR 61908-2004 – CAMAC Serial Highway: Architecture and Protocol Specifications

1. Introduction and Rationale

IEC TR 61908-2004 defines the serial highway system for CAMAC — a serial communication protocol that enables the interconnection of CAMAC crates over extended distances using a minimum number of transmission lines. While the parallel branch highway (defined in IEC TS 61895) provides high-speed multi-crate interconnection within a single room or building, its 50-meter cable length limitation makes it impractical for plant-wide distributed instrumentation systems. The serial highway addresses this limitation by using serial data transmission over coaxial, twisted-pair, or fiber-optic media, supporting distances of up to 5 kilometers between the serial driver and the farthest crate.

The serial highway operates at a basic data rate of 5 Mbps (bit-serial mode) or 40 Mbps (byte-serial mode), supporting up to 62 crates on a single serial highway — a significant increase from the 7-crate limit of the parallel branch highway. The serial highway uses a message-based protocol where each CAMAC operation is encoded as a serial message frame, transmitted over the serial link, decoded by the serial crate controller, and executed as a standard dataway cycle.

2. Serial Highway Architecture

2.1 Topology and Components

The serial highway employs a unidirectional daisy-chain topology with a return path, forming a loop configuration. The Serial Highway Driver (SHD) at the system controller end transmits serial message frames onto the highway. Each Serial Crate Controller (SCC) receives the serial stream, extracts messages addressed to its crate, executes the corresponding dataway operations, inserts response data back into the serial stream, and forwards the stream to the next crate. After passing through all crates, the serial stream returns to the SHD, which verifies the responses and detects any transmission errors. This loop topology provides inherent message verification — the SHD can compare the transmitted command with the returning response to detect data corruption.

2.2 Transmission Media and Signal Characteristics

The standard specifies three transmission media options. Option A (coaxial cable, 75 ohm) supports distances up to 1 km per segment at 5 Mbps using ECL differential signaling. Option B (twisted pair, 120 ohm) supports up to 500 meters per segment using RS-485 differential signaling, suitable for electrically noisy environments. Option C (optical fiber, multimode 62.5/125 micron) supports up to 5 km per segment with complete galvanic isolation, eliminating ground loop problems and providing immunity to electromagnetic interference — the preferred option for nuclear power plant installations where cable routing through areas with high EMI (such as near large motors or switchyards) is unavoidable.

3. Message Protocol and Frame Structure

3.1 Message Format

Each serial highway message frame consists of: a start delimiter (8 bits), a destination crate address (6 bits, allowing 62 crates), a command byte (8 bits encoding the CAMAC N, A, and F fields), a 24-bit data word (for write operations), a 16-bit CRC checksum, and an end delimiter (8 bits). For read operations, the data field carries the 24-bit read data on the return path. The frame efficiency (ratio of payload bits to total bits) is approximately 50% for write operations and 35% for read operations, with the overhead consumed by addressing, error detection, and framing.

3.2 Crate Addressing and Message Routing

The 6-bit crate address field supports up to 62 addressable crates (addresses 0-61), with address 62 reserved for broadcast messages (all crates execute simultaneously) and address 63 reserved for system initialization and diagnostics. Each SCC continuously monitors the serial stream. When it detects its own address in a message frame, it captures the message, executes the CAMAC command, and inserts the response (Q and X status bits plus 24-bit read data if applicable) into the return data field of the same frame. For broadcast messages, all SCCs simultaneously execute the command and their responses are logically OR-ed onto the return path — the SHD detects conflicts if two or more crates respond differently, which is used as a diagnostic feature.

4. System Configuration and Performance Optimization

When configuring a serial highway system for a nuclear facility, several practical considerations emerge. The loop topology means that a failure or power loss at any intermediate SCC interrupts communication with all downstream crates. To mitigate this, the standard defines a “repeater-bypass” feature: if an SCC detects a loss of its own power or a critical internal fault, it activates an internal relay that bypasses the SCC circuitry and connects the incoming serial stream directly to the output, maintaining loop integrity. For critical applications, dual-redundant serial highways (Highway A and Highway B) are recommended, with each SCC having two independent serial interfaces and automatic switchover on failure detection — achieving effective serial highway availability exceeding 99.999%.

The standard also addresses performance optimization through message pipelining. Since the serial highway introduces propagation delay proportional to the number of crates and the cable length, the SHD can transmit multiple messages before receiving the responses to earlier messages (pipelining). With a loop propagation delay of approximately 5 microseconds per kilometer of fiber plus 1 microsecond per SCC, a 30-crate system on a 3 km loop has a round-trip delay of approximately 45 microseconds. Pipelining allows the SHD to maintain continuous message transmission, achieving effective throughput of approximately 80,000 operations per second in bit-serial mode despite the per-operation latency.

5. Frequently Asked Questions