The TAPR 'DCD MODS'
Proper operation of Data Carrier Detect (DCD) is imperative for efficient sharing of a packet channel. Many TNC's don't provide optimum DCD operation, and version 2.0 of the AX.25 Level 2 protocol compounds the problem. However, an inexpensive solution is now available to combat the former case and progress is being made in the latter case with proposed changes to AX.25 Level 2 Version 2.1.
Over time, it has become apparent that most modems lack the proper DCD operation. Some work well, some are worse than others, some allow improper operator adjustment, and some have the 'threshold' adjustment set incorrectly. With the release of TNC-2 code 1.1.6 and later, packets that are received without the DCD being active are not passed to the user. This encourages the operator to properly set any DCD threshold control that may be on his TNC. Eric Gustafson, N7CL, has investigated this problem and his findings are available in the 7th ARRL Computer Networking Conference proceedings "Can We Continue to Ignore Level One? ". It has also been published in TAPR's Packet Status Register.
If the DCD decision could be made on the basis of 'information coherence' rather than 'is there some sort of signal or noise present?', LAN operation would improve. This premise has been demonstrated in a number of locations where modifications to TNCs have been made. Unfortunately, these early modifications involved wire-wrapping a fair bit of circuitry, thus preventing widespread adoption of the DCD mods. The TAPR DCD kits were designed to make it a trivial matter to upgrade most TNCs to improved DCD operation with the addition of a simple board. These mods are extremely useful for both VHF and HF operation.
State Machine DCD Upgrade
For TNCs with modems such as:
- PacComm Tiny-2/Micro-2
- PacComm TNC-220
- DRSI PC*PA Type 1
- DRSI PC*PA Type 2
- AIWA APX-25
- AEA PK-87
- AEA PK-88
- AEA PK-232
- AIWA APX-25M
- TASCO TNC-20
- TASCO TNC-20H
- Heath HK-232
- Heath Pocket Packet
If your TNC is not on this list, see the XR2211 DCD Modification (below), which is designed for XR2211 based modems.
The upgrade adaptor for these TNCs adds an EPROM-based State Machine to derive DCD based on lockup of a digital phase-lock loop. It is a small PC board assembly that mounts easily inside the cabinet of most any TNC (NOT the Heath Pocket Packet/TASCO TNC-521).
This upgrade will dramatically improve DCD operation, allowing you to run your radio unsquelched. This alone reduces other stations' TXDelay requirements, improving channel throughput.
Download DCD State Machine Assembly Manual (61K)
The price for the State Machine DCD Modification is:
$ 18 US for members of TAPR
$ 20 US for non-members+ shipping/handling if applicable.
DCD Clock Option
For TNCs with 'software HDLC,' an optional clock generator may be added to provide the necessary clock signal (KPC-1, KPC-2, HF channel of KAM).
The price for the Clock Option (for DCD or 9600b modem) is:
$ 5 US for members of TAPR
$ 5 US for non-members+ shipping/handling if applicable.
XR-2211 DCD Upgrade
For TNCs using the XR-2211 demodulator such as:
- TAPR Beta Board
- GLB PK-1
- GLB TNC-2A
- PacComm TNC-200
- Heath HD-4040
- MFJ 1270
- MFJ 1274
- AEA PKT-1
- AEA PK-80
- DRSI HF*MODEM
If your TNC is not on this list, see the DCD State Machine Modification (above), which is designed for non-XR2211 based modems.
The PC board is tiny and shaped to fit into a TNC-1 or TNC-2. After construction (an hour or so), you simply unplug the XR-2211 chip from its socket, insert it into the socket on the upgrade board, then plug the upgrade board into the IC socket vacated by the XR-2211 chip on the TNC.
This upgrade will dramatically improve DCD operation, allowing you to run your radio unsquelched. This alone reduces other stations' TXDelay requirements, improving channel throughput. The result will be fast-attack, slow decay DCD with a hang time to compensate for temporary fades due to multipath. When all stations sharing a channel have proper DCD action, data flows more efficiently.
If you are into HF operation, provisions are made to connect a 'Threshold' control onto the demodulator.
Download XR-2211 State Machine Assembly Manual (34K)
Sorry, XR2211 DCD Modification is no longer available