Fldigi Users Manual  4.1.17
PSM Power Signal Monitor


PSM was originally introduced when the KISS IO interface was added and is specifically designed for the unattended operator portion of the Amateur Radio Bands; there are however other possible uses.

PSM was originally called KPSQL and the name was change to better differentiate it's function over the function of SQL.

To configure see Configure PSM.

CSMA Carrier Sense Multiple Access

It's function is to monitor for signals within the bandpass of the modem and place a TX inhibit anytime a signal was present. This feature is reminiscent of the hardware TNC's during the heydays of packet radio. Hardware TNC's with TNC2 firmware supported a feature called CSMA. CSMA stands for Carrier Sense Multiple Access. CSMA would monitor for a signal and inhibit transmit when a signal is present. But CSMA does more then just monitor for signals it controls the aspect of how and when a transmit will occur. To assist in the prevention of transmit collisions the transmit schedule included a short random delay in the transmit process. Additionally CSMA provided other parameters to control the frequency or how aggressive the transmits are allowed and the duration of quite periods so other stations have the opportunity to transmit. As you can tell this is designed to allow multiple stations to cooperate on a single frequency. CSMA is not exclusive to TNC's either in hardware or software. The communications provided by your ISP using Ethernet is another example.

Busy Channel

Busy Channels is an extension to the CSMA methodology by incorporating programming code to consider gaps in the transmission. This is either the results of QSB or other broken/intermittent transmissions like the gaps between RSID's and the data that follows. The side effect of this feature is, it can be used to allow casual operators the use of a busy forward and store BBS frequency. When enable the frequency would have to be clear for the given set time before BBS operations would be allowed to continue.


Histogram operation is pretty much experimentally at this stage. The basic intent is to keep the detection threshold just above the noise level. In doing so, the triggering of a transmit is highly aggressive, as far as software is concerned. The actual aggressiveness is pretty slow when compared to hardware detection used in TNC's. This option should only be used with FM modulation and is not compatible with Busy Channel.

WARNING Histogram: Experimental
You may not make many friends if you use this on FM AFSK packet frequencies. The Author tested this on APRS frequencies and it fundamentally works. But wasn't left on to instigate a riot.

Possible Use(s)

As stated above it's primary designed for unattended operations. However, it's reasonable to assume this might work during a digital net operation when multiple stations are involved. Since PSM (KPSQL) has been moved out of KISS IO interface, it's functionality is available to keyboard, KISS, ARQ, and XMLRPC interfaces. This has the potential to cooperatively transmit between multiple stations while using FLDIGI's suite of tools. Namely, FLMSG, FLAMP, etc, and third party programs that makes using of the aforementioned interfaces.

PSM Characteristics

For those who have never used a CSMA device directly this may take a little getting used to. Without PSM enabled you have direct control as to when the transmit occurs. The human mind is pretty good at telling (by monitoring the waterfall) when the best time to transmit is, far better then the current implementation use in FLDIGI (detection wise). Patience is a PSM virtue, a prerequisite for cooperative activities. Not to say operators don't have this attribute, it's just PSM is more consistant in it's application.

With PSM enabled the operator can only instruct FLDIGI that transmit has been requested. CSMA decides when the appropriate time to do so. How and when CSMA transmits is dictated by adjustable parameters located on the PSM configuration panel (Configure Power Signal Monitor (PSM)). Expect delays in the transmitting of data. The amount of delay will vary with each TX/RX cycle.

Be mindfull of the adjustments made. The human nature side of this story revolves around the impatient side of us. As an example during the packet heydays (again) it's wasn't unusual for an operator to adjust CSMA parameters to gain an unfair advantage and capitalize the limited bandwidth available with our "slow" bitrate modems. You can not expect "high" throughput on a frequency used by multiple operators. This doesn't even include the other restrictions with limited bandwidth radio's and the rule of law.

The correct CSMA parameters other then the defaults for any given condition is to varied, ie, number of users, band conditions, etc. Experimentation is the only recourse. Dont be afraid of tweaking the values, just be mindful of the outcome and it's effect on the other operators using the frequency.

Adjustment of PSM Threshold and Options

In order to adjust PSM theshold enable the following:

  1. From the PSM configuration panel. Enable "Show and enable Power Signal Monitor button (PSM)" checkbox.
  2. Enable PSM button located in the lower right hand side of FLDIGI's main window.

When PSM button is enabled on FLDIGI's main window. The Signal Level indicator and SQL threshold slider are now active for PSM. A green box on the PSM button indicates an active transmit inhibit. A yellow box indicates the possibility of a transmit. Transmit may or may not occur depending on the set parameters located on the PSM configuration panel.

Moving the SQL slider up changes the internal threshold reference, requiring a higher signal level to trigger a detection that will inhibit transmit. Setting the SQL slider lower allows a weaker signal to inhibit transmit, which if set to low, means that inhibit may be triggered by the base noise level. To determine where detection/non-detection threshold resides monitor the PSM button's square colored indicator. When this boundary is reached the color will toggle between green and yellow. It's easier to determine the theshold point when Busy Channel is disabled.

When Busy Channel is enabled and set to 3 seconds (for example) each time the signal crosses the set threshold of the SQL slider position a 3 second transmit inhibit is activated. If a signal crossing the threshold occurs again while inhibit is active the internal timer is reset to 3 seconds and the inhibit continues until the timer expires or reset again by another signal. This ensures there will be a 3 second period of "silence" before the transmitter is allowed to transmit.

Enabling CSMA adds a randomness to the transmit event and is controlled by the Persistance parameter. The values for Persistance ranges from 0-255 which is translated to a percent value as seen on the PSM configuration panel. When set to "63" this equates to a 25% chance of transmitting each time a random number is generated and compared to the persistance value. Higher values results in the greater likelihood transmit will occur. Slot Time is the no transmit period between tranmissions and is measure in milliseconds. Transmit data delay is used to send idle data before the relavent information is sent. This feature is only available if the modem selected has an idle transmit state.

Deselecting PSM either by the configuration panal or by the PSM button provided on FLDIGI's main window returns the SQL slider and signal level indicator back to the SQL function. This enables the user to make SQL threshold adjustment to inhibit character quality receptions.
PSM Configuration
Configure PSM

Return to Top of Page
Return to Main Page