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IEC 61690:2000 — Nuclear Instrumentation — CAMAC
CAMAC (Computer Automated Measurement and Control) is one of the most enduring modular instrumentation standards in nuclear physics. IEC 61690:2000 codifies the electrical and mechanical specifications that have enabled CAMAC systems to remain operational in laboratories worldwide for over five decades.
Introduction
IEC 61690:2000, titled “Nuclear instrumentation — CAMAC — Size and type of plug-in units,” specifies the physical dimensions, connector assignments, and mechanical interface requirements for CAMAC modular instrumentation units used in nuclear and particle physics data acquisition systems. Originally developed by the ESONE Committee in the late 1960s, CAMAC (IEEE 583/ANSI N42.3) became the first standardized modular instrumentation system for nuclear data acquisition, predating VME, VXI, and PXI by decades.
Despite its vintage, CAMAC remains in active use at many nuclear research facilities, particle accelerators, and fusion energy experiments worldwide. Its longevity is attributable to the robust design of the dataway backplane, the simplicity of the crate-controller architecture, and the vast library of existing modules covering ADC, TDC, scalers, coincidence logic, and histogramming functions.
While CAMAC is considered a legacy technology, many high-value nuclear experiments continue to rely on it. Understanding IEC 61690 is essential for maintaining, upgrading, or interfacing these systems to modern data acquisition networks.
Scope and Physical Specifications
IEC 61690:2000 defines two standard sizes for CAMAC plug-in units — single-width (17.2 mm) and double-width (34.4 mm) — and specifies the mechanical dimensions, guide rail system, front panel layout, and connector pin assignments. The standard also defines the mounting requirements for the CAMAC crate (rack-mounted chassis) that houses up to 25 single-width stations.
| Parameter | Single-Width Unit | Double-Width Unit | Crate |
|---|---|---|---|
| Width | 17.2 mm | 34.4 mm | 482.6 mm (19-inch rack) |
| Height (front panel) | 221.5 mm | 221.5 mm | 265.9 mm (6U) |
| Depth (PCB + front panel) | 305 mm max | 305 mm max | — |
| Number of stations | 1 | 2 | Up to 25 |
| Dataway connector | 86-pin (two 43-pin rows) | 86-pin | 86-pin backplane |
| Weight limit | 2.5 kg | 5.0 kg | — |
The 86-pin dataway connector is the heart of the CAMAC system. It carries 24 data lines (read and write), 5 address lines (station selection), 4 function codes, 3 timing strobes, and power distribution — all using TTL-level signals on a passive backplane.
Dataway Operation and Protocol
Command-Response Architecture
The CAMAC dataway operates on a master-slave protocol where the crate controller (master) issues commands to individual modules (slaves) occupying specific stations. Each command consists of a station number (N), a subaddress (A0-A4), and a function code (F0-F4). The standard 24-bit read and write data lines support parallel data transfer.
Timing and Handshake
Dataway operations are synchronized by three strobe signals: S1 (strobe 1, commands the module to execute), S2 (strobe 2, commands data transfer), and the Look-at-Me (LAM) interrupt line. The standard specifies timing requirements: S1 must occur within 100 ns after address/function settling, S2 follows S1 by 400-600 ns, and the module must respond within the nominal 1 μs cycle time.
Busy and Response Lines
Each module asserts a Response (Q) line and a Busy (B) line to communicate status back to the crate controller. The Q line typically carries module-specific status (e.g., data ready, overflow, conversion complete). The B line, when asserted by any module, inhibits further dataway operations until the condition is cleared.
| Function Code (F) | Operation | Typical Application |
|---|---|---|
| F(0) | Read register | Read ADC conversion result |
| F(1) | Read and clear register | Read scaler, then reset |
| F(8) | Test look-at-me | Check LAM status |
| F(9) | Clear register | Reset module to known state |
| F(16) | Write register | Set threshold or DAC value |
| F(24) | Disable interrupt | Mask LAM generation |
| F(25) | Enable interrupt | Enable LAM generation |
| F(26) | Execute | Start conversion, begin acquisition |
When designing CAMAC modules, careful attention must be paid to the dataway driver characteristics. Open-collector drivers are used for the Busy and LAM lines (wired-OR configuration), while tri-state drivers are used for the read data lines. Improper driver selection can cause bus contention and permanent damage to both the module and the crate controller.
Engineering Insights for System Integration
1. Crate Controller Selection. The crate controller is the single most important component in a CAMAC system. IEC 61690-compatible controllers range from simple parallel interfaces (for direct connection to a computer’s I/O bus) to intelligent controllers with embedded processors, memory, and Ethernet or USB connectivity. For modern integration, controllers with USB 2.0 or Gigabit Ethernet interfaces are recommended, supporting sustained data rates of 1-2 MB/s across the dataway.
2. Grounding and Noise Immunity. CAMAC systems in nuclear environments must contend with high electromagnetic interference from particle accelerators, pulsed power supplies, and detectors. The standard ensures noise immunity through the dataway’s ground plane design and the use of shielded front panels. Practical implementation should include: dedicated power distribution with separate analog and digital ground returns, ferrite chokes on all external cable connections, and optical isolation for crate-to-computer links exceeding 10 meters.
3. Migration Path to Modern Systems. While CAMAC modules are still functional, spare parts and new modules are increasingly scarce. A practical migration strategy involves using CAMAC-to-VME or CAMAC-to-PCI crate controllers that allow legacy modules to interface with modern DAQ software (LabVIEW, EPICS, MIDAS). The IEC 61690 mechanical specifications remain compatible with many modern crates that accept both CAMAC and NIM modules.
4. Power Distribution Management. The CAMAC crate power supply must provide +6 V, -6 V, +24 V, -24 V, and +12 V (optional) to all stations through the dataway backplane. Total power capacity typically ranges from 100 W to 300 W. When populating a crate, engineers must sum the power requirements of all modules and ensure that neither individual voltage rails nor the total power budget is exceeded. A module exceeding 15 W per single-width station requires forced-air cooling.
Frequently Asked Questions
1. How does CAMAC compare to VME and PXI for nuclear instrumentation?
CAMAC has a slower dataway speed (1 μs cycle time versus VME’s 100-200 ns) and lower data throughput (1-2 MB/s versus VME’s 40-80 MB/s). However, CAMAC’s advantages include better noise immunity due to its conservative TTL logic levels, simpler programming model, and a very large installed base of specialized nuclear instrumentation modules (ADCs, TDCs, discriminators) that have no direct modern equivalents without significant development effort.
2. What is the maximum number of crates that can be connected in a CAMAC system?
The parallel branch highway (IEEE 596) allows connecting up to 7 crates in a multi-crate system, while the serial highway (IEEE 595) supports up to 62 crates. IEC 61690 focuses on the individual crate and plug-in unit specifications, while companion standards (IEC 60516, IEC 60771) address the multi-crate interconnection.
3. Can CAMAC modules be hot-swapped?
IEC 61690 does not support true hot-swapping. The dataway power pins make contact before the signal pins during insertion, but module insertion or removal with power applied is not recommended and may cause dataway bus contention, voltage transients, or damage to the module’s backplane interface drivers. Always power down the crate before module installation or removal.
4. What is the expected lifespan of CAMAC equipment?
CAMAC systems built in the 1970s and 1980s continue to operate reliably due to the conservative design margins specified in IEC 61690. The passive backplane has no active components and essentially unlimited lifespan. Module longevity depends on electrolytic capacitor aging (typical 20-30 year lifespan), connector corrosion, and availability of spare ICs. Many facilities maintain a stock of spare modules and components for continued operation.
