Key Technical Specifications
| Parameter | Specification |
|---|---|
| Product Type | BASIC Communication Module |
| Manufacturer | ProSoft Technology, Inc. |
| Chassis Compatibility | Rockwell Automation ControlLogix 1756 (single-slot, backplane-powered) |
| Primary Function | Migration of legacy 1747-BAS / 1771-DB BASIC programs to ControlLogix platform |
| Serial Ports | 3 ports: DH-485, PRT1, PRT2 |
| Supported Protocols | ASCII, DF1 (full-duplex or half-duplex slave), DH-485 |
| User RAM | 24,065 bytes available for BASIC program storage |
| Storage | CompactFlash (CF) card slot for program backup and transfer |
| Data Exchange | Ladder logic required for data transfer between module and ControlLogix processor; sample ladder files included |
| Typical Applications | Interface to modems, printers, barcode readers, scales, and other ASCII devices; offload math algorithms from the processor |
| Operating Temperature | 0°C to 60°C (32°F to 140°F) |
| LED Indicators | 4 LEDs: Module Operation (STATUS), Backplane Transfer (BP), Application (APP), Serial Activity (ACT) |
| Power Source | Draws power directly from ControlLogix backplane (no external power supply required) |
| Power Draw | Exact mA @ 5V DC — verify with OEM datasheet or module nameplate |
Product Introduction
I’ve walked into more than one plant upgrade where the integrator said “we’ll just rewrite the old SLC code in Studio 5000” and then spent six months debugging string parsing logic that the original 1747-BAS handled in three lines of BASIC. The MVI56-BAS exists for exactly that scenario. It’s a single-slot module that drops into a ControlLogix 1756 chassis and runs your legacy BASIC programs natively, so you can migrate off the old SLC/PLC-5 hardware without rewriting every line of serial communication code.What makes this module worth keeping in your spare parts cabinet is the combination of three serial ports and 24,065 bytes of user RAM. The DH-485 port handles legacy Allen-Bradley networking. PRT1 and PRT2 handle ASCII, DF1 full-duplex, or DF1 half-duplex slave — which covers most serial devices you’ll encounter in the field: barcode scanners, label printers, modems, lab scales, and legacy instrumentation. The CompactFlash slot means you can pull the program off the card and back it up to a laptop in seconds. One thing I’ll flag: this module does NOT talk to the ControlLogix processor on its own. You need ladder logic to move data in and out of the module’s memory map. The sample ladder files ProSoft includes are a starting point, not a finished solution. Budget time to adapt them to your tag structure.
Quality SOP & Tech Pitfalls
The Lab Report (SOP): Here’s our pre-shipment protocol for every MVI56-BAS. Visual inspection first — we’re checking for burnt traces, swollen capacitors, counterfeit markings, and corrosion on the backplane connector pins. Then it goes into a 1756 test chassis. We apply backplane power and verify all four LEDs (STATUS, BP, APP, ACT) illuminate in the correct boot sequence. We insert a known-good CF card with a test BASIC program and verify the program loads without errors. We loop back each serial port (DH-485, PRT1, PRT2) and send/receive ASCII strings to confirm all three ports are functional. We check the ladder data exchange by reading and writing to the module’s I/O image table. We log the firmware revision. Finally, it gets sealed in anti-static packaging with the CF card included. If any port fails the loopback test, it doesn’t ship.The Engineer’s Warning (Pitfalls): Two things will cause downtime with this module. First, the ladder logic data exchange. I’ve seen technicians install the module, see the STATUS LED go green, and assume it’s working — only to find three weeks later that no data was ever moving between the module and the processor because the ladder rung that copies the I/O image table was never added to the main routine. The module and the processor don’t share memory automatically. You must explicitly move data. Second, the CF card. I had a site where the original CF card corrupted after a power surge. The technician replaced the card but didn’t have a backup of the BASIC program. The module powered up fine, but it had no program to run. Always, always keep a backup of the .BAS file on a laptop and on a spare CF card. The CompactFlash format is old — modern cards don’t always work. Use the same capacity and brand as the original if possible.
Installation & Configuration Guide
- Pre-Installation: ⚠️ Lock out and tag out power to the ControlLogix chassis. Wait 30 seconds for backplane capacitors to discharge. Verify zero energy. Photograph the existing module’s LED status, CF card label, and any wiring on the serial ports. If the old module is still communicating, upload the BASIC program from the CF card to a laptop as a backup. Download the current ladder logic from the ControlLogix processor.
- Removal: Label every serial cable at the connector, not just the wire. Note which port (DH-485, PRT1, PRT2) each cable connects to. Disconnect serial cables first, then release the top and bottom mounting screws. Pull the module straight out of the chassis slot. Keep the old module and its CF card — do not discard until the replacement is verified operational.
- Installation: ⚠️ This step prevents 90% of startup failures: Before inserting the new module, verify the CF card contains the correct BASIC program. If the new module ships without a CF card, transfer the program from your backup or from the old module’s card. Insert the CF card into the new MVI56-BAS. Slide the module into the same chassis slot as the old unit. Tighten the top and bottom mounting screws. Reconnect serial cables to the correct ports using your labels — DH-485, PRT1, PRT2. Verify the ladder logic in the ControlLogix processor includes the data exchange rungs for this module’s slot location.
- Power-On & Testing: Restore power to the chassis. Watch the LED sequence: STATUS should flash during boot, then go solid green when the module is running. BP should indicate backplane communication is active. APP should indicate the BASIC program is loaded and running. ACT will flash when serial traffic is present. Open RSLogix 5000 and go online with the processor. Monitor the tags associated with the module’s I/O image table. Send a test command through one of the serial ports and verify the response appears in the correct tag. If the STATUS LED is red or flashing red, check the CF card and the BASIC program for errors. If BP is not active, verify the module is in the correct slot and the ladder logic references the right slot number.
Compatible Replacement Models
- ✅ Drop-in Replacement: MVI56-BAS (same firmware revision). If your existing unit is an MVI56-BAS, this is a direct hardware swap. Same footprint, same ports, same BASIC interpreter. Transfer the CF card or load your backup program, verify the ladder logic slot reference, and you’re done. Zero reprogramming of the BASIC code needed.
- ⚠️ Software Compatible: MVI56E-BAS (Enhanced version). Same physical module and BASIC program compatibility, but with updated firmware and potentially different configuration tool requirements. The BASIC program itself should run without modification, but you may need to update the ProSoft Configuration Builder software and re-download the module configuration. Budget 1-2 hours for verification.
- ❌ Hardware Mod Required: Any other MVI56 module (MVI56-MCM, MVI56-MNETC, MVI56-PDPS, etc.). These are entirely different protocols — Modbus, Ethernet/IP, PROFIBUS. They cannot run BASIC programs. Do not attempt to substitute a different protocol module unless you are rewriting the entire communication strategy and the downstream device interfaces.
Frequently Asked Questions (FAQ)
Can I hot-swap this module in a live chassis?
The ControlLogix backplane supports RPI (Remote Power Interface) and some modules can be swapped under power, but I don’t recommend it for the MVI56-BAS. You’ll lose serial communication to any devices connected to it during the swap, and if your ladder logic doesn’t handle the module-absent condition gracefully, you can fault the processor. Schedule a maintenance window.Will my old 1747-BAS BASIC program run without changes?
In most cases, yes. The MVI56-BAS was specifically designed to run legacy 1747-BAS and 1771-DB programs with minimal modification. However, any hardware-specific calls (like direct memory addresses or port configurations unique to the SLC chassis) will need to be updated to match the MVI56-BAS port structure. Test thoroughly before putting it into production.What happens if the CF card fails?
The module will power up and the STATUS LED may go green, but the APP LED will indicate an error because no program is loaded. You won’t get any serial communication. This is why I keep a spare CF card with a known-good program image in the spare parts cabinet at every site that uses these modules.Does this module support Ethernet communication?
No. The MVI56-BAS is a serial-only module. It has three serial ports (DH-485, PRT1, PRT2) and no Ethernet port. If you need Ethernet-based ASCII communication, look at the MVI56E-MNETC or a different module in the MVI56 family.How do I back up the BASIC program from the CF card?
Remove the CF card from the module and insert it into a CF card reader on your laptop. The BASIC program file will be visible as a .BAS file. Copy it to your laptop and to a backup location. You can also edit it in any text editor — it’s plain-text BASIC. ProSoft’s configuration tools can also read and write the program through the serial port if the card reader approach doesn’t work.What’s the maximum baud rate on the serial ports?
The OEM datasheet doesn’t consistently publish a single maximum baud rate across all three ports. DH-485 operates at fixed Allen-Bradley network speeds. PRT1 and PRT2 support standard ASCII/DF1 baud rates. Verify the exact supported rates against the module nameplate or the official ProSoft technical documentation for your specific firmware revision.
