Introduction
My intention is to interface my IC-720A to common QSO-logging / contest programs like
N1MM or WinTest. I am not familiar with Ham Radio Deluxe (HRD) and honestly I didn’t miss it over
the last years. Goal is to enrich the IC-720A with an USB interface to allow remote control.
Recently I received the IC-720A bus protocol description (tnx KA6BFB!) so there is hope : -)

Sequencing / timing
When contesting with amplifiers or using seperate RX-antennas, timing is of major
importance to prevent e.g. ‘hot keying’ or detrimental damage due to RF-spikes etc.

Besides the PTT switch on a hand mike, many rigs lack a ‘/PTT input’.
That is, a low current input to switch the rig into TX-mode by making the PTT line low (<– /PTT).

/PTT in
Below the rear side of the IC-720A is depicted. The ‘MEMORY’ cinch connector under the power
supply molex connector looked very tempting to serve as /PTT in.

In my previous posting an internal /PTT point was already addressed. Seemingly the most
easiest way to implement /PTT on the ex-MEMORY cinch connector is to wire an internal /PTT point
to this connector? No . . . !

Nuisance with /PTT’s  (i.e. active low) is that this ‘low’ state is relative towards a ‘high’ state.
Depending on equipment used these ‘high’ states may vary significantly. Therefore ‘paralleled’
/PTT lines may result in detrimental damage! Perhaps the Heathkit SB-200 may serve as an example.
In ‘high’ state I measured around 120V on the /PTT input connector (!) Imagine what would happen
if you simply join the PTT of your rig with the PTT input of the SB-200 . . .

Mostly this nuisance is overcome using relays as isolation instrument. However, I am allergic
towards relays (perhaps besides the RX/TX (coax) relay) and prefer solid state solutions.

Grounding the PTT line I measured around 80 mA current, which is too much to be called ‘low current’.
My initial idea was to use an optocoupler where the switching transistor side had to serve as
/PTT switch. Well, that didn’t work. Despite having enough current through the
LED side of the optocoupler I heard the RX/TX-relay switching, the ‘TRANSMIT’ light weakly
burned but no RF output power was present in RTTY mode or ‘key down’ in CW.

Perhaps my /PTT wasn’t ‘low’ (or ‘strong’) enough?

I took another approach which works flawlessly, see picture below.

‘/PTT in’ is wired inside to the ex-’MEMORY’ cinch connector and ‘/PTT out’ wired to the
grey wire on the MAIN UNIT connector used for the relay accelerator (cf. previous posting).