Mentec PDP-11

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Mentec International Ltd was founded in 1978[1] and initially focused on the development of monitoring and control software and systems. It was a significant Digital Equipment Corporation (DEC) reseller and OEM in Ireland.[1] Mentec Computer Systems Limited was a subsidiary of Mentec Limited that repackaged PDP-11 processors. Mentec Inc.[2] was a US-based subsidiary of Mentec Limited. In the early 1980s it had a range of remote terminal units based on the SBC/11-21 (Falcon).

Once the DEC J-11 PDP-11 processor chip set became available in 1982 Mentec commenced the design of its first PDP-11 single board computer, the M70.

In 1994 Digital transferred the PDP-11 operating systems to Mentec Inc.[3]

Product Range[edit]


The M70 was developed between 1982 and 1984. It was a quad Q-bus module based on the J-11 chipset incorporating onboard ECC DRAM, bootstrap EPROMs and 4 serial lines implemented using DEC DC319 DLART chips.


The M71 was a version of the M70 intended for process control.[4] It provided for 1/4 or 1 M Byte of ECC DRAM, up to 1/2 MB of EPROM, 4 serial lines (DC-319 DLARTs) and two parallel ports implemented using 8255 chips. It was initially designed by Mentec for use in its own Remote Terminal Units.


The M80 was a further development of the M70 but using parity memory and a slightly higher clock rate. It also introduced software configuration via the bootstrap which all but eliminated wire-wrap configuration.


This was effectively merely a clock tweaked version of the M80.


The M100, a redesign of the 11/93, was the last of Mentec's J-11 based processor boards.[5][6] It ran the J-11 chipset at 19.66 MHz, and had four on-board serial ports, 1-4 MB of on-board memory, and an optional FPU. The M100 was a somewhat tidied up and faster re-design of the M90.

Some late models incorporated a daughter card with a Xilinx part which replaced the DLARTs and implemented a FIFO to prevent overruns for OEM applications. A small number of late models also incorporated an SRAM daughter card which replaced the on-board DRAM.


The M11 was a microcoded implementation of the PDP-11 instruction set done from scratch, Mentec announced, describing it as a processor upgrade board replacement for the M100.[7] It was based around two Texas Instruments TI8832 ALUs and a TI 8818 microsequencer. One of the ALUs was used as the processor ALU while the second was used to implement the memory management unit. An Intel i960 processor was used to load the microcode, perform floating point (in IEEE format) and provide ODT. The 4 DLARTs of the earlier M100 were emulated on a single Xilinx part. All of the memory (both microcode and PDP-11 main memory) was implemented using SRAM. While not of any significant effect in the field it suffered from the fact that it used a large number of microcode controlled drivers onto tri-state buses, which made developing microcode somewhat hazardous.

The M11 design was implemented in VHDL and fully simulated using Mentor Graphics QuickSim II with behavioural language models for both the Q-Bus and console UART. It ran patched versions of the Digital PDP-11/23 CPU diagnostics on the simulator before any hardware was constructed.


The M1 was a processor upgrade board, done as an ASIC re-implementation of the M11's implementation of the PDP-11 instruction set. Despite being an ASIC implementation it was also fully microcoded.[8] The M1 used an Atmel 0.35 μm ASIC.[9]


Q-bus board


Ownership of RSX-11S, RSX-11M, RSX-11M Plus and Micro/RSX was transferred from Digital to Mentec Inc. in March 1994[10] as part of a broader agreement.[11] Mentec Inc. was the US subsidiary of Mentec Limited, an Irish firm specializing in PDP-11 hardware and software support. In 2006 Mentec Inc. was declared bankrupt while Mentec Ltd. was acquired by Irish firm Calyx in December 2006,[12]. The PDP-11 software, which was owned by Mentec Inc. was then bought by XX2247 LLC, which is the owner of the software today. It is unclear if new commercial licenses are possible to buy at this time. Hobbyists can run RSX-11M (version 4.3 or earlier) and RSX-11M Plus (version 3.0 or earlier) on the SIMH emulator thanks to a free license granted in May 1998 by Mentec Inc.[13]

Legal ownership of RSX-11A, RSX-11B, RSX-11C, RSX-11D, and IAS never changed hands; therefore it passed to Compaq when it acquired Digital in 1998[14] and then to Hewlett-Packard in 2002.[15] In late 2015 Hewlett-Packard split into two separate companies (HP Inc. and Hewlett Packard Enterprise),[16] so the current owner cannot be firmly established.[dubious ] New commercial licenses have not been issued at least since October 1979 (RSX-11A, RSX-11B, RSX-11C)[17] or 1990 (IAS),[18] and no one of these operating systems was ever licensed for hobbyist use.


In 1997 Digital and Mentec granted anyone wishing to use RSTS 9.6 or earlier for non-commercial, hobby purposes a no-cost license . The license is only valid on the SIMH PDP-11 emulator. The license also covers some other Digital operating systems. Copies of the license are included in authorized software kit available for download on the official website of the SIMH emulator.[19]


  1. ^ a b Steve Lohr (August 15, 1988). "Irish applaud austerity policies". The New York Times.
  2. ^ "PDP-11 RSX RT RSTS Emulator Osprey Charon". Archived from the original on August 13, 2006. Retrieved August 8, 2010.
  3. ^ "Digital And Mentec, Inc. Announce PDP-11 Software Agreement" (Press release). June 29, 1994.
  4. ^ SBC M71 Single Board Computer Users Guide, Mentec,1987
  5. ^ "Mentec's M100 Brochure". Archived from the original on August 19, 1999.
  6. ^ SBC M100 Series User Manual, Mentec Computer Systems, 1990
  7. ^ "Mentec's M11 Brochure". Archived from the original on August 19, 1999.
  8. ^ Development Project Report
  9. ^ "Development Project Report" (PDF). Archived from the original (PDF) on April 12, 2016. Retrieved April 14, 2014.
  10. ^ Mentec Inc. (1995). "Notice of Copyright Change for PDP-11 Software Products" (PDF). Retrieved December 11, 2017.
  11. ^ Digital Equipment Corporation (June 29, 1994). "Press release: Digital and Mentec announce PDP-11 Software Agreement". Retrieved December 11, 2017.
  12. ^ Mentec Inc. (December 13, 2006). "Calyx buys Mentec". Retrieved December 11, 2017.
  13. ^ Mentec Inc. (May 1998). "Mentec Hobbyist license for PDP-11 operating systems". Retrieved December 11, 2017.
  14. ^ Michael Kanellos (January 26, 1998). "Compaq to buy Digital for $9.6 billion". Retrieved December 11, 2017.
  15. ^ Michael Kanellos (March 10, 2002). "HP to acquire Compag for $25 billion". Retrieved December 11, 2017.
  16. ^ Arik Hesseldahl (November 2, 2015). "Hewlett-Packard splits in two today, now what ?". Retrieved December 11, 2017.
  17. ^ Products no longer appeared on Digital PDP-11 System Software Catalogue issued Oct. 1979: "PDP-11 System Software Component Catalogue" (PDF). Digital Equipment Corp. October 1979. Retrieved December 11, 2017.
  18. ^ "IAS Version 3.4 Software Product Description" (PDF). Digital Equipment Corporation. May 1990. Retrieved December 6, 2017.
  19. ^ See for example this image of RSTS/E 7.0 installation tapes: