Icom IC-720A
To
Ham Radio Deluxe
Interface Project
The Icom
720A has a very rudimentary interface. Considering the radio dates from 1980,
we are lucky it has any interface at all. Four parallel lines are used to interface
to the microprocessor in the 720A. Four additional control lines are also used
to control the bus. To interface HRD to the radio, an Icom
725 is emulated in hardware so that HRD believes it is controlling the IC-725.
The data stored in registers in the PIC is then used to communicate the
frequency and mode information to the 720.
Here is a Youtube
video showing the original construction on a protoboard.
Here is a video
showing the final PCB in a nice implementation at ARS WB2FXO as well as the
rotary relay replacement modification.
Shown in Figure 1 below is
the schematic for the interface. It was discovered that once the circuit was
moved over to a PCB, D1 and C2 were no longer needed, so they are not populated
on my board. Figure 2 shows the PCB artwork, and Figure 3 shows the parts
placement information. A PDF file with normal and mirrored PCB images at 100% scale
size is provided below, as well as the Eagle files for the PCB. This interface
can be connected to HRD in one of two ways. Either a serial port connection using the “DB” pins, or an Icom
CI-V interface using a 1/8 inch plug. Connection speed is 9600 Baud. The Molex
connector to interface to the 720A is Digikey P/N WM1219-ND
and the pins are WM3680CT-ND.
There are two functional halves
to the project. One half talks to HRD and the other to the
720A. When initially testing the interface, first establish communication
with HRD. The LED mounted on the PCB toggles between on and off as packets are received from HRD. If all is well with this half, the
connection screen in HRD will show green check marks and then the normal
operating screen in Figure 5. If this shows up and the frequency on the screen
tracks with changes, then the HRD half of the project is working. Once this
half is functioning, move on to verify the radio tracks with HRD. Once the
interface is in use, do not use the controls on the radio to modify frequency
or mode. Doing so will cause the system to lose sync and will have
unpredictable results. If it is desired to switch between HRD control and radio
knob control, it is advisable to add on ON/OFF switch to the interface box.
Place the switch in the 12V line from “M2” in the schematic interrupting the
input to the 78L05 regulator. A power LED is provided if desired and the wires
can be run to the PWR_LED1 and PWR_LED2 pads on the PCB.
Figure 1. Icom 720A to HRD interface
schematic
Figure 2. Icom 720A to HRD interface PCB.
Figure 3.
Parts placement information for PCB
Figure 4 shows a screen
capture of the connection screen in Ham Radio Deluxe. An Icom
725, with CI-V address 28 and com port speed of 9600 must be selected for
proper communication. Figure 5 shows the top part of the HRD screen once a
connection is made. The frequency displayed is the current frequency of the
radio. Since HRD thinks it is talking to an Icom
IC-725, it provides buttons for functions not present in the 720A. The only
functions you can control are Frequency, Mode, and TX/RX state. I have provided
a free bonus, however. The 720A was too old to have memories, but I have now
provided one. If you click the “MW” (memory write) button in HRD, the current
frequency and mode are stored in non-volatile memory in the PIC, and will be
the startup condition of the radio from then on! One final note: There seems to
be a bug in HRD and whenever you first connect to the 720A it is in transmit
mode. Clicking the “TX” button corrects this. If you don’t like this, just
disconnect pin M3 from the Molex connector. Incidentally, TX control only works
when using the serial port because Icom radios do not
support the PTT function in CI-V.
If you are interested in
building this interface, I will program a DIP Package PIC 16F690 for $25 if it
is sent to me by prior arrangement with an SASE.
Figure 4.
Connection Screen in HRD
Figure 5.
Control Buttons in HRD
Schematic and PCB files in Eagle
format can be downloaded here.
PCB images in PDF format at
100% scale can be downloaded here.