TNC Setup

This section deals with some of the basics and some things you should be aware of in setting up your TNC to communicate with a PacketCluster node. The goal of this section is not to explain how to set up any particular TNC--that subject is left to the user. The Cluster attempts to handle all user input regardless of what TNC is being used, however, some parameters must be set correctly to allow use of all PacketCluster features.

The TNC interfaces to your terminal or computer via a digital interface and ASCII data flows between the two. The TNC interfaces to the 2-meter rig via three lines. Audio is fed from the TNC to the radio microphone input, audio is fed from the radio speaker output to the TNC audio input, and a PTT line from the TNC to the radio keys the transceiver. The primary consideration is that your TNC should support AX.25 Level 2 Version 2 Protocol. If it does not, you will not be able to interface with PacketCluster. All TAPR-2 TNCs and clones (MFJ-1270, AEA PK-80, PACCOMM 200s) running version 1.1.2 or higher, all Kantronics running version 2.0 or higher support this protocol. BUT -- There is a command called AX25L2V2 which MUST BE ON. The TNC may come from the factory with this command defaulted to OFF. It MUST be ON.

TNC Modes Although this is not a course in how to communicate on packet, you need to read your TNC manual and make sure you learn the difference between COMMAND and CONVERSE mode. In COMMAND mode you are talking to the TNC from your keyboard and anything you type is interpreted as a command by the TNC. In CONVERSE mode you are talking to the world (in this case, the Cluster) and anything you type is interpreted as text and is sent out over the air as a packet when you hit a carriage return (C/R). You get into COMMAND mode by typing CRTL-C (C/R), and you know you are in COMMAND mode because the TNC sends a prompt that looks like this: 

cmd:.

If you are in COMMAND mode, you get into CONVERSE mode by typing the command CONVERSE (or CONV for short). If you are in CONVERSE mode and you wish to return to COMMAND mode, this is done by typing CONTROL-C (holding the CTRL key down while simultaneously typing a "C"). Remember, if you want to tell the TNC to do something you must be in COMMAND mode, and if you want to tell the Cluster to do something, you must be in CONVERSE mode.

Deviation One of the big problems we have found is that many users feed a signal from their TNC to their radios that is too high in level. The result is overdeviation, distortion, and in general it doesn't work. Most 2-meter radios have limiter diodes somewhere in the audio path, and if your input audio level is too high you will be slamming up against the limiter diodes and feeding square waves to the modulator. On the air it sounds like a demented buzz-saw. Most modern 2-meter rigs that use an electret microphone require an audio level somewhere in the range of 10 to 20 millivolts peak-to-peak. The best way to set it up is to set the deviation to 3.5 - 4.0 khz with a deviation meter, but it's not necessary to have one. TNCs have a "CALIBRATE" or similar command which allows you to key the rig and send a packet tone. Listen to your transmitted signal with another rig, and turn up the level until the perceived volume stops increasing. At this point, you are up against the limiter diodes. Now, back off the input level (level of audio from the TNC to the 2-meter radio) until you hear the perceived volume decrease. Turn it down a bit more and you will be close enough for all practical purposes. The audio should sound clean with no distortion or harshness. Some TNCs have an adjustment pot on the board, some have an adjustment accessible through a hole in the side of the unit, and some have two fixed output levels selectable with a jumper. If one of these does not work, you may have to open up the 2-meter rig and find the mic gain control. If this is necessary, be sure you adjust the mic gain control and NOT the deviation control. The mic gain control is before the limiters, and the deviation control is after the limiters.

 Parameters There are several timing parameters which you need to set properly. In the discussion to follow, the times will be given in milliseconds, but the units of various timing parameters vary according to the TNC. Check your manual for the proper conversion.

TXDELAY is the delay interval between key-up and start of data transmission. Normally 300-400 milliseconds is adequate, but some 2-meter rigs take a bit longer for the phase-locked loop to lock in after the keying line is asserted. If you seem to be having a problem being heard and all else seems OK, try increasing TXDELAY to 400-600 milliseconds.

PacketCluster sends a group of packets to users in a single transmission. The Cluster node (or the NET/ROM repeater, depending on how you connect) immediately repeats those that it has been asked to, but after the carrier drops there can be a problem if everyone tries to acknowledge receipt of the packet simultaneously. The parameters RESP (Response Time) and DWAIT in conjunction with PERSIST and SLOTTIME allow staggering of these acknowledgements. RESP is the time delay between reception of a packet and transmission of an ACK, and DWAIT sets the delay between when activity is last heard on the channel and key-up of your transmitter to send the ACK. You should set values of RESP and DWAIT to the recommended value in milliseconds. Your TNC probably accepts a value in "counts" rather than in milliseconds -- so don't forget to convert by the proper value in order to arrive at the correct timing value in milliseconds. For example, if you have been asked to set DWAIT to 600 milliseconds and the units of DWAIT for your TNC are 10 milliseconds per count, then you would command DWAIT = 60.

Most of the current vintage TNCs contain commands called PERSIST and SLOTTIME, which help enormously in avoiding collisions. PERSIST sets the probability that a packet will be transmitted during a given time interval called a SLOTTIME. The parameter SLOTTIME governs the interval between transmission timing "slots." Initially, PERSIST should be set to approximately 64 and SLOTTIME to a value of about 10, which is equivalent to 100 milliseconds. Basically, PERSIST is the probability that when your TNC needs to transmit, it will transmit in the next time slot -- if it doesn't transmit on this one, then, one slottime later, the same probability is applied. Eventually, the packet is transmitted, but the delay varies--and the reason is so that your TNC doesn't get synchronized with someone else's every time with the result that the node might always hear responses from you both, and detect neither. This causes channel congestion for sure!

FRACK should be set to 6 and RETRY to 10. FRACK sets the number of seconds between retries and RETRY sets the number of times your TNC will try to send a packet and gain acknowledgement of it before it gives up and disconnects -- this is called RETRYING OUT.

Remember to set AX25L2V2 ON, or you will cause a great deal of congestion on the frequency that affects all the users of the system.

If all of this sounds a little demanding, you must remember that when you check into PacketCluster you become part of a multi-state network involving 28 nodes and many users. If your setup is marginal, requiring a lot of retries (retransmission of packets), then you are affecting all the local users and, to a lesser degree, the entire network. It's not quite the same as a keyboard-to-keyboard QSO or logging into a packet bulletin board system where only you and one other entity are affected. The PacketCluster Network is NOT the place to check out your packet system or to learn how to use it. You can always arrange to go off on another frequency with another packet operator for your on-the-job training. You don't have to be a computer expert to successfully operate packet radio, but it does require more commitment than just picking up a 2-meter hand-held and conversing on a voice repeater. 

SUMMARY OF TNC SETTINGS