The TAPR 'DCD MODS' ------------------- Background -- 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. The Problem -- 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. It has also been published in TAPR's Packet Status Register. Solution -- 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: KPC-1 PacComm Tiny-2/Micro-2 KPC-4 DRSI PC*PA Type 1 KPC-2400 DRSI PC*PA Type 2 KAM AIWA APX-25 AEA PK-87 AIWA APX-25M AEA PK-88 TASCO TNC-20 AEA PK-232 TASCO TNC-20H PacComm TNC-220 Heath HK-232 Heath Pocket Packet 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 stationsU TXDelay requirements, improving channel throughput. DCD Clock Option ---------------- For TNCs with 'software HDLC,' an optional clock generator may be included to provide the necessary clock signal (KPC-1, KPC-2, HF channel of KAM). XR-2211 DCD Upgrade ------------------- For TNCs using the XR-2211 demodulator such as: TAPR Beta Board GLB PK-1 TNC-1 GLB TNC-2A TNC-2 PacComm TNC-200 Heath HD-4040 MFJ 1270 & 1270B AEA PKT-1 MFJ 1274 AEA PK-80 DRSI HF*MODEM Note: MFJ 1270C does not require the XR2211 DCD Upgrade 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. If you are into HF operation, provisions are made to connect a 'Threshold' control onto the demodulator. 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. ========================================================================== Tucson Amateur Packet Radio Corp. 8987-309 E Tanque Verde Rd #337 * Tucson, Az * 85749-9399 Phone: (817) 383-0000 * Fax: (817) 566-2544 TAPR is a Non-Profit R & D Corporation ==========================================================================