TACLog can make use of three kinds of external interface circuitry, either connected to the serial port or the parallel port. The serial interface is capable of keying the key line and push to talk (PTT). The parallel interface can key and PTT, but, also key external voice messages (voice keyer).
----------------------
| None | COM | LPT |
---------------------+------+-----+--------
| Printer port | 0 |*****| # |
---------------------+------+-----+--------
| Interface port | 0 | # | # |
---------------------+------+-----+--------
| Interface mode | 0 | 1 | 2/3/4 |
-------------------------------------------
Where # is port number, and interface mode is as shown below.
The interface uses Data Terminal Ready (DTR) for keying the message, and uses Request To Send (RTS) to key the PTT. The circuitry consists of a dual optocoupler, providing 100% galvanic isolation between PC and radio. Between the RS232-pin and the opto-diode a 4,7 kOhm resistor is inserted. The collector of the optocoupler is the first transistor in a darlington coupling and the combined collectors are the PTT line. Apply capacitors, there is RF in the air - you know!
PC Radio
DTR <--[4k7]--+
(RTS) | --+--> PTT
-- |/ | C (Key)
\/ | |/
-- |\-|
| |\ E
PC GND <---------+ |
= Radio ground
©Bo Hansen, OZ2M (with permission)