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 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://k1nu.home.comcast.net/k1nu/Products/ K1NU interfaces]
[http://www.cssincorp.com/prod-cable.htm Creative Services Software interfaces]
[http://home.att.net/~n8st/rigcontrol.html Donner Digital Interfaces]
[http://home.comcast.net/~n4vas/wsb/html/view.cgi-home.html-.html N4VAS interfaces]
[http://g4zlp.co.uk/unified/StandardIcomCAT.htm ZLP Electronics CT-17 replacements]
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.
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