How to put into use.

The decoder needs just one adjustment. Probably everything will work directly, but in case of troubles here is a step-by-step put-into-use description.

The receiver of Sam Elsdon has inverted output. By connecting pin 9 and 10 of CON2 (forcing pin 30 of IC1 low) the HRPT-input will be inverted, compensating the Eldson-receiver inversion.
Note that in that case, if you use the HRPT-generator as described here, this generator's output has to be inverted too!

Note: Make sure that you insert the FPGAs the right way into their sockets!!!

Test the decoder/generator part.

For this part you don't need IC4 and IC5 (the elastic buffer part). (If you did already add them: No problem.)

Test decoder solo.

  1. Don't connect the decoder to receiver or PC.
  2. Connect power.
  3. Connect 5V to pin 17 of the parallel port (decoder still not connected to PC!) Now the hardware switches from decoder to generator.
  4. Remove 5V from pin 17. ==> hardware switches back to decoder.
  5. Trim C2.
  6. Check PLL.
  7. Connect separate HRPT generator to input.

Test connection between decoder and PC.

If the decoder FPGA works as described above the second FPGA may now be added.
  1. Make sure that the parallel port is in EPP mode. If the parallel port is an old one with open-collector outputs this is also OK.
  2. Cable between PC and decoder should not be too long (1 meter is fine). A bad and/or too long cable could show crosstalk between the wires. This can cause spickles at certain positions of the picture, or spontaneous on/off switching of channels.
  3. Connect PC to decoder and switch on decoder.
    The alarm LED should go on after a few seconds (buffer overflow because PC doesn't read data). (Time depends on how many channels are selected "by accident" at power-up.)
  4. Start software. Make sure that you select the right port.
  5. Channel LEDs should go off if they were on.
  6. Select Record in menu.
  7. Try to select channels by clicking on H/1...5 in the record menu.
  8. Select generator (click on Gen).
  9. Try again to select channels by clicking on H/1...5.

Some power-currents.

These currents are measured with all components (including LEDs) mounted, and PC connected. If the PC is disconnected current will be a few mA lower. If 2 numbers are given the first is with all channels off, second all channels on.


If pin 14 is low, and at the same time the parallel port is in output mode, a dangerous conflict situation exists resulting in high currents! There are a few PC types which come into this situation during boot. After boot the situation may not dissappear because of latchup. Eventually, this can destroy your decoder and/or parallel port!

Watch the power-consumption of your decoder. If it is much higher than 500mA (during and after boot) while this is not the case if you dont connect the decoder to the PC, add resistors of 22 ohm in series with pin 2-9 of the parallel port. This is not an ideal situation because the resistors will make the wires more sensitive to crosstalk between wires. An other possibility is to add some logic to pin 14 which makes this pin high during boot time (e.g. 1 minute).

If everything is OK you can try to make your first recording.

Note that you can read pictures to the screen without saving it into a file. Click on Save in file; becomes Don't save. In this way you can play without making huge files. Read the manual for more details.
You can also show the state of all parallel port pins by clicking on Test. Note that the alarm LED will go on because updating the screen is relatively very slow (slower than the data rate).