Saturday, 27 October 2012

The boring stuff

Here is the relay control box I threw together to control all my
various DATV transmitters. The two relay boards and the
FPGA board were bought on eBay from China. The Veroboard
has an RS232 level converter chip on it. The whole thing is
controlled via a serial link. R001 switches on relay 0 
and R150 switches off relay 15 etc. I decided to use RS232 
as the amplifiers are some distance from the computer and
USB2 wouldn't cut it. I have implemented a simple rx UART
in Verilog which runs on the FPGA board. I may add ADC
support later so I can measure temperature, power, VSWR etc.

Using an FPGA seems a bit like overkill in this application
but the boards cost about the same as ready made PIC boards
if you are careful in your purchasing and of course they have
far more I/O on them than a PIC would have.

When I send the commands from my Linux system I can hear the
relays clicking and the right ones come on, so I know it is working.
Next I have to do some rewiring of the transmitter rack
to accomodate the new relay box. I have amplifiers for 70 cms,
23 cms, 13 cms and 9 cms so there is a lot of cabling to add.

DATV-Express is in the hands of the PCB layout guy so I have a
window to sort out my own stuff at the moment. 


  1. Hi Charles,

    I was contemplating I2C instead of RS232 for similar application (temperature and Ibias reads/sets from/to remote and mast-mounted PAs).
    Surprisingly I2C can work up to 50 meters (NXP App Note AN444).
    Benefit is that it is a bus and so can talk to multiple devices. (And that with so many I2C ICs out there functionality (e.g. temp sensor, ADC, DAC, relay control) is added easily and cheaply to a project. Might even think of setting the frequency of a mast-mounted VCO/PLL from within the shack.

    Herman, PE1GTA

  2. Hello Herman,
    I hadn't really thought of using I2C.
    DATV-Express uses I2C to communicate
    between the FX2 controller and the

    I am going to add an I2C slave Verilog module
    inside the FPGA so I can send interpolation
    settings and I/Q correction values to the
    FPGA from the P.C via the FX2 controller on
    the board.

    We already bring the I2C bus out to the edge
    connector on the Express board to allow the
    sort of things you are intending to do.

    - Charles