Welcome to the Model III/4/4D/4P System Expander (M3SE) Project!


The M3SE (Model III System Expander) is an add-on device for the vintage Radio Shack TRS-80 Model III, Model 4, Model 4D, and Model 4P microcomputers. It connects to the buffered expansion port, and provides these benefits:

The AC adapter (US-style plug, UL-listed) needed to power the M3SE, and the ribbon cable to attach the M3SE to the TRS-80, are both included.

M3SE works with both "slow" (2MHz) and "fast" (4MHz) CPU clock rates.

Minimum system requirements: TRS-80 Model 4/4D/4P or Model III with 32KB RAM, and at least one floppy drive. Blank floppy diskettes for making backups of the boot floppy.

Suggested for Model III: 48KB RAM.

To fully utilize M3SE: SVGA capable monitor and VGA cable, CAT5e or CAT6 Ethernet cable and network connection, Atari 2600 compatible joystick.

To reprogram the M3SE FPGA with updates: USB Blaster or Byte Blaster II (parallel port), and free Quartus II Web Edition software from Altera installed.

Product Pricing

Price list


December 18, 2022: From now on, M3SE units will be shipped with single-sided IDE/CF adapters, because I can no longer obtain the reverse-side CF sockets. This means that you can have one CF card acting as a primary IDE drive, but no secondary IDE drive. The preferred backup solution is to dump a raw image of your CF card to your PC using a CF card reader/writer device, and restore the image directly to the CF card should it become necessary.

April 29, 2021: I finally got around to adding information about auto-boot EPROMs to the web pages here. Also added some LS-DOS related manuals to the manuals and data sheets page.

May 18, 2020: I'm now selling M3SE 1.2 units, which have only minor changes from 1.1. A PCB layout issue related to the VGA output has been corrected (it required a simple workaround during assembly.) The real-time clock-calendar chip has been changed to surface-mount. A couple of unnecessary resistor packs have been removed. Other than the updated splash screen and version registers, that's about all.

September 27, 2018: M3SE 1.1 is now available! It is functionally identical to M3SE 1.0, but sports a stealthy appearance with black solder mask and mitered corners.

September/October 2017: MISE, M3SE, and MIRE at VCF Midwest 12 and Tandy Assembly!

3 February 2017: Auto-Boot EPROM chips for Models III, 4, and 4D are now available. Note that all EPROMs, including those for the for the Model 4 Gate Array and Model 4D are 24-pin EPROMS and do not require an adapter board or wiring.

24 December 2016: I'm happy to announce a price decrease on M3SE units, due to lower production costs.

10 September 2016: M3SE and MISE at VCF Midwest 11!

18 June 2016: A new FPGA programming file provides M3SE auto-boot on Models III, 4, and 4D when the corresponding FreHD-style ROM update has been installed. Not needed to autoboot the 4P, but won't hurt anything either.

17 June 2016: A new FPGA programming file provides M3SE auto-boot on the Model III, if a FreHD-style ROM chip C has been installed. I'll probably start offering preprogrammed ROM chips in the near future as a M3SE option.

7 June 2016: The FTP client and the Ping utility have been added to the regular file archives and will be included with all future M3SE deliveries. These programs are currently for LDOS only; LS-DOS versions are in process.

March 20, 2016: I am pleased to announce the availability of LS-DOS 6.3.1 for the Model 4/4D/4P on M3SE. See the instructions here for more information.

20 December 2015: A demo video of M3SE in action is now available. Now I can just sit back and wait for the Academy Award nominations.

Here's a demo video of M3SE in action:


Setting up the M3SE

Using LS-DOS with M3SE

Auto-boot EPROMs for Models III, 4, and 4D

Network Configuration

Network Applications

Network Demos

Compact Flash Hard Disks

Using the VGA display

Using the real-time clock/calendar

Using the joystick port

256-byte general-purpose EEPROM

M3SE feature control and identification registers

Utility programs

Installing FPGA updates

M3SE serial number

Hardware Errata


Obtaining Software

M3SE comes with LDOS-5.3.1 pre-installed, including Matthew Reed's "Year 2012" patches. Roy Soltoff has made LDOS freely redistributable. It is being provided free of charge.

Other software packages you may wish to obtain are copyrighted, so I can't distribute them with M3SE. Recommended ones include:

There are many places to obtain this and other software. The following are good places to look:

Planet Emulation

Tim Mann's TRS-80 Pages for Misosys software

Classic Computing

Once you've obtained the software, simply use the FTP server to copy it to your TRS-80.


The MISE Yahoo group, also the main community support location for M3SE.

The TRS-80 Yahoo group.

The Floppy Days Vintage Computing Podcast, which often discusses the TRS-80. Pay special attention to Episode 20!

The TRS-80 Trash Talk Podcast.

The Vintage Computing Forums' Tandy/Radio Shack Forum.

arcadeshopper is selling a 3d printed M3SE enclosure.


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