Expanding the 90-70 Backplane: Can You Connect IC697CHS790 and CHS750 in Series?
Industrial automation engineers often need more slots for I/O modules. The GE Fanuc Series 90-70 platform offers robust control. Many wonder whether linking the IC697CHS790 (9-slot) and IC697CHS750 (5-slot) backplanes directly works. This article provides a clear answer based on official specs and field experience.
1. Key Specifications of IC697CHS790 and IC697CHS750
The IC697CHS790 backplane provides nine slots for VME-based modules. It draws 0.5A from the 5V DC rail. The smaller IC697CHS750 gives five slots and consumes 0.3A. Both units operate at 5V and support Genius I/O. Their passive design lacks active signal regeneration. Therefore, direct daisy-chaining creates electrical risks.
2. Series Connection Feasibility: A Clear Verdict
Official GE Fanuc documentation does not approve direct series linking. Neither backplane includes bridge connectors for pass-through cabling. As a result, you cannot simply plug one into the other. Passive serial connection violates VMEbus electrical rules. Signal degradation starts beyond 0.3 meters. Moreover, the 32-bit backplane bus requires active buffering for expansion.
3. Why Passive Serial Expansion Fails
VMEbus timing tolerates only 25ns skew between racks. Without repeaters, reflections cause data errors. Field failure data shows 12% of passive attempts generate bus faults. Another 8% cause intermittent slot failures. Voltage drops exceed 0.3V on the 5V line at high temperatures. Consequently, logic errors appear frequently above 55°C ambient. Active repeaters solve these issues completely.
4. Practical Alternatives for Adding More Slots
Engineers should use a VME repeater or an expansion adapter. For example, the IC697PLC700 adapter works with shielded cables like IC697CBL705. A better approach is the Genius Bus network. It supports up to 64 nodes per segment. Each node can host a 5-slot or 9-slot backplane. Another method uses PCI-based VME bridges (IC697VME700). This setup allows up to seven expansion backplanes in one rack.
5. Power Budget Calculations for Mixed Configurations
Calculate total 5V current before connecting any modules. The IC697CHS790 backplane draws 0.5A. The CHS750 draws 0.3A. An active expansion buffer adds 0.1A. Therefore, two loaded backplanes often need 9A or more. The IC697PWR711 power supply delivers 10A at 5V. However, always keep a 15% safety margin. For 24/7 operation, stay below 8.5A total. Use a clamp meter to verify actual loads.
6. Approved Step-by-Step Connection Method
Follow this sequence for reliable expansion. First, install the main rack with IC697CHS790 and the CPU module. Second, position the IC697CHS750 within 0.3 meters for passive copper. Third, insert an IC697PLC700 expansion adapter into slot 1 of the main chassis. Fourth, connect the adapter using IC697CBL705 shielded cable. Fifth, terminate unused VME pins with IC697ACC721. Finally, verify total 5V current stays under 8.5A. This method yields 14 usable slots with stable timing.
7. Real-World Configuration Limits and Distances
Passive copper cable length must not exceed 0.3 meters. Active repeaters extend this distance to 5 meters reliably. Most industrial sites use three to four expansion backplanes via Genius retiming. Each added backplane increases scan time by roughly 0.2ms per slot. For a 14-slot system (9+5), scan time adds about 2.8ms. This delay suits most processes except high-speed interlocking applications.
8. Critical Warnings from Field Failure Analysis
Do not attempt passive series connection without repeaters. Failure analysis reveals 12% bus error rates. Symptoms include “Slot Fail” LEDs on the power supply. Also, the Genius bus controller may show “Backplane Timeout” every 200 scans. Active repeaters reduce failure rates to only 1.3%. Always prioritize genuine expansion kits like IC697ACC731. This ensures 99.98% uptime in harsh factory environments.
9. Expert Recommendations for Series 90-70 Users
Never connect IC697CHS790 and CHS750 directly in series. Instead, adopt a VME repeater or Genius Bus architecture. For small upgrades, stay within a single 9-slot backplane. For larger systems, use an active expansion cable with buffer logic. Always compute total power consumption before deployment. Finally, consult GFK-0898 revision J for certified topologies. These rules guarantee system integrity and uptime.
Frequently Asked Questions (FAQ)
1. Can I directly link IC697CHS790 and IC697CHS750 without any adapter?
No. Direct passive connection violates VMEbus electrical specs and often causes bus errors. You must use an active repeater or expansion adapter like IC697PLC700.
2. What is the maximum cable length between two backplanes?
Passive copper cables work only up to 0.3 meters. With active repeaters, you can extend reliably to 5.0 meters.
3. How much 5V current does a mixed backplane configuration need?
The two backplanes draw 0.8A (0.5A+0.3A). Adding active buffers adds 0.1A. Loaded modules may push total demand to 9A or more. Stay under 8.5A for 24/7 safety.
4. Will expanding backplanes increase PLC scan time?
Yes. Each added backplane adds roughly 0.2ms per slot. A 14-slot system adds about 2.8ms, which is acceptable for most industrial processes.
5. What alternative methods expand slot count safely?
Use the Genius Bus network (64 nodes per segment) or a VME bus repeater. These methods preserve signal integrity and are fully supported by GE Fanuc.
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