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w11 I/O emulation setup

All UNIBUS peripherals which exchange data (currently DL11, DZ11, LP11, PC11, DEUNA, RK11, RL11, RPRH, and TM11) are currently emulated via a backend process. For details see w11 architecture description. The communication between the FPGA board and backend server can be via

  • Serial port

    • via an integrated USB-UART bridge
      • on Arty A7, Basys3, Cmod A7 and Nexys A7, and Nexys4 with an FT2232HQ, allows up to 12M Baud
      • on Nexys3 with an FT232R, allows up to 2M Baud
      • for all FTDI USB-UART it is essential to set them to low latency mode. That was the default for Linux kernels 2.6.32 to 4.4.52. Since about March 2017, one gets kernels with 16 ms default latency again, thanks to kernel patch 9589541. On newer systems, it is essential to install a udev rule which automatically sets low latency, see the documentation in tools/sys.
    • via RS232 port, as on Nexys2 and S3board
      • using a serial port (/dev/ttySx) is limited to 115 kBaud on most PCs.
      • using a USB-RS232 adapter was tested up to 460k Baud.

  • Direct USB connection using a Cypress FX2 USB controller

    • is supported on the Nexys3 and Nexys2 FPGA boards
    • much faster than serial port connections (see below)
    • also allows configuring the FPGA over the same USB connection

  • Notes:

    • A 12M Baud connection, like on a Nexys A7, gives disk access rates and throughputs much better than the real hardware of the 70's and is well suitable for practical usage.
    • In an OS with good disk caching like 2.11BSD the impact of disk speed is actually smaller than the bare numbers suggest.
    • A 460k Baud connection gives in practice a disk throughput of ~20 kB/s. This allows to test the system but is a bit slow for real usage.
    • USB-RS232 cables with an FTDI FT232R chip work fine, tests with Prolific Technology PL2303 based cable never gave reliable connections for higher Baud rates.

Recommended setup for best performance (boards ordered by vintage):

Board Channel/Interface nom. speed peak transfer rate
Arty S7 USB-UART bridge 12M Baud 1090 kB/sec
Arty A7 USB-UART bridge 12M Baud 1090 kB/sec
Basys3 USB-UART bridge 12M Baud 1090 kB/sec
Cmod A7 USB-UART bridge 12M Baud 1090 kB/sec
Nexys A7 USB-UART bridge 12M Baud 1090 kb/sec
Nexys4 USB-UART bridge 12M Baud 1090 kb/sec
Nexys3 Cypress FX2 USB USB2.0 speed 30000 kB/sec
Nexys2 Cypress FX2 USB USB2.0 speed 30000 kB/sec
S3board RS232+USB-RS232 cable 460k Baud 41 kB/sec