Schneider Legacy PLC Replacement Guide

Bottom line: for a legacy Schneider PLC, decide first whether you are keeping the platform alive or leaving it. If you are keeping it, buy the exact CPU and rack references as spares. If you are leaving it, budget for rewiring and a program conversion, because TSX Micro, TSX Premium, 984, Momentum and Twido parts do not drop into X80 racks.

The PLC Source PRO catalog still carries TSX Micro, TSX Premium, Momentum, Twido and Modbus Plus hardware for installed-base support. Every SKU below links to a live product page. For the full brand view, see the Schneider Electric collection; for a migration review, use the contact page.

How do you decide between keeping and replacing a legacy Schneider PLC?

Keep the platform when you need a like-for-like spare and the application code is stable. Replace it when spares are scarce, the software toolchain is unsupported, or the plant needs Ethernet diagnostics. The decision is driven by spare availability and software support, not by the age of the controller alone.

Three datasheet facts frame the choice across these platforms:

• Capacity gap: TSX3721001 supports 256 discrete and 64 analog I/O with 96 kB of memory; TSXP574823M supports 1,024 discrete and 256 analog I/O with 448 kB.
• Network continuity: TSXP571634M and TSXP572634M include Ethernet plus two serial links, while the older Modbus Plus segment needs 990NAD23000 junction boxes or a 416NHM30030 PCI card to stay serviceable.
• Power supply match: TSXPSY2600M provides 24 V at 2.5 A, while TSXPSY5520MC provides 24 V at 5 A. Match the supply to the rack load rather than to the nearest higher rating.

Which legacy selection dimensions actually change the part number?

Dimension	What changes across the platforms	Buying consequence
Platform	TSX Micro (TSX37), TSX Premium (TSXP57), Momentum (170/171), Twido (TWD), Modicon 984	Racks, power supplies and I/O modules are not cross-compatible
I/O capacity	256 / 64 on TSX3721001 up to 1,024 / 256 on TSXP574823M	Confirm the existing rack count before choosing a replacement CPU
Memory	96 kB on TSX3721001, 192 kB on TSX3722001 and TSXP571634M, 256 kB on TSXP572634M, 448 kB on TSXP574823M	Program conversion usually needs more memory than the original
Ports	Serial only on TSXP573623M; serial plus Ethernet on TSXP571634M, TSXP572634M and TSXP574823M	Ethernet models are the practical choice for diagnostics
Power supply	TSXPSY2600M (24 V, 2.5 A) versus TSXPSY5520MC (24 V, 5 A)	Do not undersize the supply when adding communication modules
Momentum I/O	170ADI54050 (16 inputs, 90–140 VAC) and 170ADO34000 (16 outputs, 24 VDC)	Momentum I/O pairs with a Momentum processor adaptor, not with a Quantum rack
Network hardware	990NAD23000 junction box and 416NHM30030 PCI card for Modbus Plus	Modbus Plus is a cabling and termination discipline, not just a protocol setting

Which legacy path fits your situation?

• If a TSX Premium line must keep running: replace the failed processor with the same reference, for example TSXP572634M for 256 kB and Ethernet, or TSXP574823M where 1,024 I/O points are required.
• If a TSX Micro machine needs a spare: TSX3722001 supports 512 discrete and 128 analog I/O, while TSX3721001 covers 256 discrete and 64 analog. Confirm the base reference on the existing rack before ordering.
• If a Momentum machine needs Ethernet: 171CCC96030C is the Momentum M1E processor adaptor with a 10/100 Ethernet port, 4,096 kB of RAM and 2,048 kB of program memory.
• If a Twido controller has failed: TWDLMDA20DRT is the modular base controller and TWDLCAA40DRF the compact 40-I/O controller. Treat them as platform spares, not as upgrades to M221.
• If the Modbus Plus segment is failing: 990NAD23000 is a six-connector junction box and 416NHM30030 is a two-channel PCI card. Replace the physical layer before blaming the program.

Which legacy SKUs are in the PLC Source PRO catalog?

All 15 links below returned HTTP 200 on 7 November 2026. Specifications are the datasheet values published on each product page; confirm the exact reference and firmware before ordering a spare for a running line.

Model	Platform and function	Key specification	Product page
TSX3722001	TSX Micro modular base controller	512 discrete / 128 analog I/O, 192 kB memory, 10.7 W	View TSX3722001
TSX3721001	TSX Micro modular base controller	256 discrete / 64 analog I/O, 96 kB memory, 10.4 W	View TSX3721001
TSXP571634M	TSX Premium processor	512 / 128 I/O, 192 kB, 2 serial plus Ethernet	View TSXP571634M
TSXP572634M	TSX Premium processor	512 / 128 I/O, 256 kB, 2 serial plus Ethernet	View TSXP572634M
TSXP573623M	TSX Premium processor	512 / 128 I/O, 192 kB, 2 serial links	View TSXP573623M
TSXP574823M	TSX Premium process processor	1,024 / 256 I/O, 448 kB, 2 serial plus Ethernet	View TSXP574823M
TSXPSY2600M	TSX Premium power supply	100–240 VAC input, 24 V at 2.5 A, 3.5 W	View TSXPSY2600M
TSXPSY5520MC	TSX Premium power supply	24 V at 5 A, 16 W, conformal coated	View TSXPSY5520MC
170ADI54050	Momentum discrete input	16 inputs, 90–140 VAC, 106 mA at 24 VDC	View 170ADI54050
170ADO34000	Momentum discrete output	16 outputs, 24 VDC, 0.5 A per channel, 10 A per module	View 170ADO34000
171CCC96030C	Momentum M1E processor adaptor	10/100 Ethernet, 4,096 kB RAM, 2,048 kB program	View 171CCC96030C
TWDLMDA20DRT	Twido modular base controller	32 I/O, 500 basic instructions, transistor outputs	View TWDLMDA20DRT
TWDLCAA40DRF	Twido compact base controller	40 I/O, Ethernet, relay outputs	View TWDLCAA40DRF
990NAD23000	Modbus Plus junction box	6 female connectors, 1 Mbit/s network	View 990NAD23000
416NHM30030	Modbus Plus PCI card	2 channels, up to 1 MB/s, 32-bit PCI	View 416NHM30030

Which legacy parts are interchangeable, and which are traps?

• TSX Premium versus TSX Micro: TSXP57 processors and TSXPSY supplies belong to the Premium rack family; TSX37 base controllers are a different platform. Racks and modules are not interchangeable.
• Premium versus Quantum: both are legacy Schneider platforms, but the rack, backplane and I/O references differ. A Premium module does not fit a Quantum 140 rack.
• Momentum I/O: 170ADI54050 and 170ADO34000 pair with a Momentum processor adaptor such as 171CCC96030C. They do not plug into a Quantum or X80 rack.
• Twido: TWDLMDA20DRT and TWDLCAA40DRF are Twido controllers. M221 is the successor line, but the base units, expansion references and wiring are not automatically interchangeable; verify every reference.
• Modbus Plus: a 990NAD23000 junction box and 416NHM30030 PCI card keep a Modbus Plus segment serviceable, but cable type, termination and node addressing must match the existing design.
• Power supplies: TSXPSY2600M and TSXPSY5520MC are not interchangeable on current alone; connector and rack compatibility must be checked as well as the 2.5 A versus 5 A output.

Frequently asked questions

Can a TSX Premium PLC be replaced directly by an M340 or M580?

No. TSX Premium uses its own rack, power supply and I/O modules, while M340 and M580 use X80 racks. A migration requires new racks, rewiring and a program conversion. The M580 is the usual target when Ethernet RIO and larger capacity are needed; M340 is the smaller option. Plan the conversion before removing the existing controller.

Is 984 ladder logic portable to Unity or Control Expert?

It is convertible, not binary-compatible. 984 ladder programs can be translated into the newer engineering environment, but the result must be reviewed, simulated and re-validated against the machine sequence. Direct memory and instruction behaviour can differ, especially around communication and special functions.

What replaces a failed Twido controller?

The catalogue replacement is another Twido base controller such as TWDLMDA20DRT or TWDLCAA40DRF, because those keep the existing wiring and program. Moving to M221 is a platform change: confirm I/O references, expansion modules, programming software and panel cut-outs before treating it as a spare-part swap.

Do you supply Modbus Plus network components?

The catalog includes the 990NAD23000 six-connector junction box and the 416NHM30030 two-channel PCI card. Modbus Plus also depends on the correct cable type, terminators and node addressing; replacing a tap without checking the rest of the segment often leaves the fault in place. Send the network drawing with the order enquiry.

Need a legacy spare or migration review?

Send the existing CPU reference, rack photo and program tool version to the PLC Source PRO contact page. We will confirm whether the exact spare is in the catalog and identify the parts that cannot be reused in an X80 migration. For the current platforms, see the M580 buying guide, the M340 and X80 buying guide and the Quantum 140 buying guide.