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.