******************* FILE 3 of 6 FILES ********************** (C) 31 JUL 88 Eric Gustafson, N7CL 2018 S. Avenida Planeta Tucson, AZ 85710 DPLL Derived Data Carrier Detect (DCD) For Filter Based and Single Chip Modems TNC SIGNALS Once you have constructed the DCD circuit, you will have to obtain some signals from your TNC for the new DCD circuit to use. You will also have to arrange for the output of this circuit to be substituted for the normal DCD signal used in the TNC. The signals required for the DCD circuit operation are: 1. A sample of the data recovered by the demodulator in the modem chip. 2. A sample of a clock which has a frequency of either 16 or 32 times the baud rate (X16 or X32 baud clock). 3. The intercepted Carrier Detect (CD) signal from the modem chip. This is the CD generated by the modem chip based on amplitude of the input audio. 4. A source of + 5 volts. If you use all CMOS parts, the current requirements are minimal. The 74HC14 MUST be a CMOS part for the circuit to work properly. 5. Ground There are so many different TNCs to which this circuit can be applied that I cannot give specific interface information for all of them. However, I can provide signal pin numbers for the 2 land line modem chips most frequently encountered and I can help with signal locations in the AEA PK-232 and PK-87, the Kantronics KAM, and the Pac Comm TINY-2 TNCs. The signals of interest on the AMD7910 modem chip are: 1. Receive Data output (RD)-----> pin 24 2. Carrier Detect (CD)----------> pin 25 This signal is negative true for the 7910 chip. The signals of interest on the TCM3105 modem chip are: 1. Receive Data output (RXD)----> pin 8 2. Carrier Detect (CDT)---------> pin 3 This signal is positive true for the 3105 chip. 3. In TNCs which use the TCM3105 chip but do not provide another source of the baud clock, like the Kantronics KAM, you can use the signal at pin 2 of this chip. This signal is very close to 16 times the baud rate (19.11 KHz instead of 19.2 KHz for 1200 baud). TNC INTERFACE If your TNC has provision for a TAPR style modem disconnect header, these signals (including the X16 or X32 baud clock) will be easily located and conveniently interfaced at this header. If it doesn't have this header, you will have to fish around in the circuit of your TNC on your own to locate them. SHAME ON THE MANUFACTURER OF A TNC WITH NO MODEM DISCONNECT HEADER!! The absence of a standard modem disconnect header means you may not CONVENIENTLY use ANY external modem with the deficient TNC. Using a standard disconnect system, the external modem can provide a front panel switch to allow you to select between the external and the internal modem. Modems which you might like to interface without loosing the use of the internal AFSK modem would include the BPSK / MANCHESTER FM modems required for several of the satellites. In any case, the DCD signal currently used in your TNC will have to be disconnected and rerouted through the new circuit. STANDARD HEADER SIGNALS The signal locations on the standard modem disconnect header are as follows: Receive Data is obtained from header pin 18. Carrier Detect is obtained from header pin 2. DataCarrier Detect (DCD) is inserted at header pin 1. Jumper from header pin 1 to header pin 2 is removed. The X16 (TNC-2) or X32 (TNC-1 and possibly TNC-2 clones using an 8530 HDLC controller instead of the Z80SIO) baud clock is obtained from header pin 12. COMMERCIAL TNC SIGNAL LOCATIONS Here is the information you need to find the proper signals in several commercially available TNCs. This is not intended to be a complete list by any means. It merely represents the units which I have had available to apply this circuit to here locally. These are the only TNCs for which I have specific interface information at this time. AEA PK-87 It is relatively easy to interface this new DCD circuit to the PK-87 in spite of the fact that there is no standard modem disconnect header. This is because there is no requirement to switch back to the internal DCD circuit once the modification is installed. If this were an external special purpose modem, you would be forced to open the TNC case and move several jumpers whenever you wished to change the modem being used. However, for our purposes in this modification, the jumpers provide convenient, easily located places to obtain and inject signals. The Receive data signal is obtained from the center pin of JP4. The Carrier Detect signal is obtained from the end of JP5 which connects to the modem chip. The DCD output signal from the new circuit is inserted at the center pin of JP5. Use the NEGATIVE TRUE output. The jumper originally installed at JP5 is removed. The DCD indicator on the front panel will show the action of the new DCD circuit. The X32 baud clock signal is obtained from pin 13 of U20 (a 74LS393 divider). Don't be tempted to get this signal from the "clock" line on J4, the external modem connector, as this is a X1 clock. I see so many manufacturers sending only the X1 baud clock out to an auxiliary modem connector that I have to wonder if they simply don't realize that synchronous modems require a clock which is a multiple of the baud rate. Asynchronous modems can cheaply and easily divide the X16 clock to get X1 but it is hard for synchronous modems to derive a faster clock from the X1 signal. (continued in file #4) EOF