Connecting an Icom Transceiver to your PC
PC control of an Icom transceiver is accomplished via a bidirectional CI-V bus, which uses an asynchronous protocol communicated via TTL voltage levels. Thus an interface between your PC and your transceiver is required; the Icom CT-17 provides this function.
The basic function of the interface is voltage level conversion from the RS-232 levels used in a standard serial port (+12 and -12 VDC) and the TTL levels used by your transceiver (+5, and 0 VDC). The interface itself requires power, which can be provided either externally, or with some circuits by using one of the serial port modem control signals as a power source.
There are two decisions that will drive your choice:
1. do you want to use one of your PC's serial ports, or one of its USB ports?
2. would you prefer to build something or buy it?
Being able to choose #1 is a relatively new phenomenon. Previously, controlling a transceiver via USB port meant purchasing a USB-to- serial-port adaptor (e.g. from Belkin or ByteRunner) and connecting it to your interface's serial port. Now, however, there are off-the- shelf USB interfaces; see, for example http://www.microham.com/USB% 20interfaces.html . You can also build one of these yourself, as described in http://www.eham.net/articles/8192 ; if you go this route, be sure to read the comments, as the circuit as presented contains an (easily correctable) defect.
If your PC has a spare serial port, or you'd prefer an outboard USB- to-serial-port converter, then you'll have many more choices with respect to the interface - down both the "buy it" and "build it" paths.
[http://www.microham-usa.com/Products/USB.html microHAM USB Interface II] (recommended)
[http://www.cssincorp.com/prod-cable.htm Creative Services Software interfaces]
[http://home.att.net/~n8st/rigcontrol.html Donner Digital Interfaces]
[http://k1nu.home.comcast.net/k1nu/Products/ K1NU interfaces]
[http://home.comcast.net/~n4vas/wsb/html/view.cgi-home.html-.html N4VAS interfaces]
[http://www.piexx.com/index.php?main_page=product_info&cPath=7&products_id=18 Piexx USB to CI-V interface]
[http://g4zlp.co.uk/unified/StandardIcomCAT.htm ZLP Electronics CT-17 replacements]
[http:http://www.radioshack.com/sm-buy-the-usb-scanner-programming-cable-on-http-wwwradioshackcom--pi-2177609.html Radio Shack USB Scanner Programming Cable, part number 20-047]
You'll find product reviews of the above, along with more alternatives at http://www.eham.net/reviews/products/53 .
Building a interface is a nice little project that can be accomplished in an afternoon, including the trip to Radio Shack and back. This circuit was published many years ago in QST; its very reliable, and you may already have the necessary parts laying around:
Another nice circuit,
uses the MAX-232 IC for level conversion rather than discrete transistors. It also includes a circuit to control your 706's PTT circuit using one of the serial port modem control signals, which you'll find convenient if you're planning to operate digital modes.
This circuit is small enough to build into a DB9 connector shell:
This module could be used as the foundation of a homebrew USB interface; be sure to drive the CI-V bus with an open-collector driver like the 7417:
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