Field Engineer’s Notes (From the Trenches)
The “Gotcha” is 512KB Memory Limit, Battery & No Redundancy.
Memory Limit (512KB): This is the #1 “gotcha” when migrating a program from a 772 (1MB) or 774 (2MB) to a 731. If the original project is >512KB (large symbol tables, many rungs, big %R arrays), it will not fit — PME will error “Insufficient Memory” on download.
Fix: Trim unused logic/symbols, reduce %R array sizes, or upgrade to IC697CPU772 (1MB). Never assume a 731 is a drop-in for a 772 without checking program size.
Battery (IC697ACC701): Same as RX7i batteries. “BAT LOW” LED → replace within 30 days (typical). Replace with power ON to preserve RAM. If power is lost with dead battery → program gone.
Mitigation: After major logic changes, perform “Store to Flash” if your firmware supports it (some 731 revisions do; check PME options). On cold start with dead battery, it auto-loads from Flash if available.
No Embedded Ethernet: If you need the PLC on the plant Ethernet for HMI/SCADA, you mustadd an IC697CMM742 (10Mbps Ethernet Interface Module) in a spare I/O slot. The 731 itself cannot talk TCP/IP. For new designs requiring Ethernet, consider migrating the rack to an RX7i (IC698CPE010/CPX772) which has embedded 10/100 Eth.
Scan Time: At 25MHz, complex math (floating point, PID loops) will be noticeably slower than a 772 (50MHz) or 774 (100MHz). For simple sequencing this is irrelevant; for turbine protection or fast motion, use the faster CPUs.
Real-World Applications
- Coal Handling Belt Feeder (90-70 Legacy): An in a small MCC controls 3 belt feeders (start/stop/interlocks), reads 24 DI (pull-cord, zero-speed, chute jam), drives 16 DO (starter coils, alarm horns). Program is 180KB. The three serial ports connect to: (1) LM90 laptop for maintenance, (2) Cimplicity HMI via RS-232, (3) local alarm printer. No Ethernet needed. The 731 has run 15 years with one battery change.
- Ash Sluice Pump Station (Retrofit): Replaced a failed IC697CPU772 with a 731 after verifying the program was 410KB. Added a CMM742 later when the plant DCS was upgraded to require Ethernet. The 731’s simplicity matched the low-complexity logic (pump lead/lag, valve interlocks, high-level alarm).
High-Frequency Troubleshooting FAQ
Q: “RUN” LED OFF, “FLT” ON (Solid Red). “BAT LOW” was on last week. Program lost?
A: RAM Cleared (Battery Dead + Power Loss).
- If you performed “Store Project to Flash” previously: Power cycle the rack. The CPU auto-loads from Flash (MS/7-seg blinks during load → RUN comes on in ~30 sec). Wait.
- If noFlash store was done: You must Download the project from Proficy Machine Edition (or Logicmaster 90-70). Connect via Serial Port 1 (SNP protocol, 9600-19200 baud typically). Target → Connect → Download.
- Prevention: Always “Store to Flash” after logic changes. Replace battery (IC697ACC701) with power ON when BAT LOW appears.
Q: CPU goes to RUN but some I/O modules show “Not Responding” or “Config Mismatch”.
A: Hardware Detect / Rack Power Issue.
- Open PME Hardware Config. Right-click the 90-70 rack → Detect Hardware. Does it see the modules you physically have? If mismatch (e.g., config says IC697MDL753 but physical is IC697MDL754), update config → re-download.
- Check the Power Supply (IC697PWR710/711) in Slot 0: “PWR” LED Green? Measure 5V at P/S test points. If 5V sags <4.85V with load, the backplane may not reliably poll I/O.
- For expansion racks: Ensure IC697ETM001 is installed in a spare I/O slot and expansion cable is seated. Expansion rack must have its own P/S (IC697PWR710) in Slot 0 of the CHS392.
A: Only if the program fits in 512KB.
- Check your current project size (in PME: Project → Properties → Memory Usage). If User Memory Used ≤ 512KB → Yes, you can substitute.
- If >512KB → you must use IC697CPU772 (1MB) or IC697CPU774 (2MB). Forcing a 731 with a 1MB program results in “Download Failed — Insufficient Memory.”
- The 731 is simplex only — if the original 772 was part of a Redundant pair (IC697CRM001), the 731 cannot participate in redundancy. You’d need another 772/774 for the standby CPU.
Please note: The listed price is not the actual final price. It is for reference only and is subject to appropriate negotiation based on current market conditions, quantity, and availability.
