APRS Digipeatersoftware DIGI_NED
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Serial Modems (Baycom)

BayCom-compatible modems can be used without changes. In rare cases there could be problems which are caused by the more stable supply of power to the modem in AX25_MAC compared BayCom. In AX25_MAC the TXD line remains steady at approximately +12V, the BayCom solution has a pulse signal on this control line. Therefore the supplied voltage to the modem is a little higher, the voltage on pin 7 of the TCM3105 may not have the ideal voltage anymore. In this case a re-adjustment of the voltage on pin 7 is required (see modem documentation).

Tip: this voltage is also very important for the correct functioning of the software DCD, you can use the Soft DCD indicator to adjust the voltage.

Additionally there is a option of connecting the modem (e.g. from DigiCom) to a LPT-Port. When using this, 6 data output lines are switched to a stable 5V which could be used as voltage supply to the modem (Use at your own risk).

Here are the connections for the Modem-Ports

COM-Port

Signal 25pol.9pol.Meaning
DTR20
4
Transmit data +/- 12V
RTS
4
7
PTT, High active, -12V=RX, +12V=TX
CTS
5
8
Receive data
GND
7
5
Signal ground
TXD
2
3
+12 V for Baycom-Modem

LPT-PortSignal

Signal 25pol.Meaning
DATA1-62-7 constant 5V approx. for Modem
DATA7 8
Transmit data, TTL-Level
DATA8 9
PTT, High active, 0V=RX, 5V=TX
BUSY 11
Receive data
GND 18-25 Signal ground

Modems using the AM7911 can also be used. You may have to increase the TXTAIL-parameter (command @TA) a little for this. At this point I like to point out you need different modems for higher baudrates.

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