Description

This PCB consists in three separate parts.

• 1750Hz access tone decoder
• Watchdog that limits the link-radio transmissions time if it hangs.
• Watchdog that automatically restarts the RLM-computer if the software fails.
  

If you don't need all this tree functions you can skip the components for that functions. For example, if you just need the Reset watchdog you just have to put components that relate to this function on the PCB.

1750Hz decoder

This is a simple access-tone decoder that is based on the PLL- tone decoder LM567. The circuit has one input and one output. You connect the input to the sound output on the repeater receiver and the output connects to the RLM interface, pin 5 (acc. tone IN).

With the trimpot you adjust the frequency to 1750Hz. The low and high frequency is adjustable by replacing the capacitors but is with the default components the detect frequency between 1720 - 1780 Hz.

Link Watchdog

If you are using the RLM software for linking to other repeaters it is undesirable that your link-transmitter hangs in transmission to a repeater. This could happen if the PC is hanged or if the LPT port or the interface fails. If then the repeater is difficult to access then this protection circuit is handy. The function of this link-watchdog is that the link-transceiver only can transmit for about 5 minutes continuously and then it is shut off.

This circuit has one input and one output. The input connects to outport 2 in the RLM interface (LINK-TX) and the output is connected to the transmit-line on the transceiver (PTT).

If you want to change the cut-off time you must alter the capacitors C6 and R8.

Reset Watchdog

Every one second a pulse from the RLM interface outport 5 is going high and low.

If this pulse is stopped the software has hanged. It is this pulse the circuit is detecting. The circuit has one input and 2 outputs. The input is connected to outport 5 in the RLM interface(watchdog-pulse).

Output 1 can be optionally connected to a secondary repeater controller who is operating when the RLM system is down. This temporary repeater controller can be as simple as a capacitor and a relay that only holds the repeater in transmission for some seconds when the Squelch is opening.
The output 1 is HIGH when RLM is working normally.

The output 2 is connected to the computers reset connector. This connector is different on all kind of computers and you may have to add some extra components for this to work properly.

RLM is a very reliable program and does not hang it selves, but you always have the human factor to take into count. For example the computer has mechanical parts that may fail, the temperature in the repeater site may go to high or to low with the result that the harddisk fail or after a power failure the computer doesn't startup properly, and so on.

The time between the pulse has gone and that the computer resets is 1 minute and 20 seconds. If you want to change this you must alter the values on R2 and C2.

Building the circuits

Start with carefully checking the PCB for some missing holes or shortcuts.

I recommend that you use sockets for the IC:s for easy replacement. If you think it is hard to see how the components are mounted, use the schematics for verification.

Nearly all the resistors are mounted in a upright position.

Components for the multicard

/Good Luck

Peter Hansson, Norra pitholmsvägen 14, 941 46 PITEÅ

http://www.metronic.nu/rlm

Version 990208