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The $10 Icom Computer Interface

from Howard Groveman, KD6UU on July 16, 2007
View comments about this article!

"Editor's Note: Due to the popularity of some of eHam's older articles, many of which you may not have read, the team has decided to rerun some of the best articles that we have received since eHam's inception. These articles will be reprinted to add to the quality of eHam's content and in a show of appreciation to the authors of these articles."

The $10 Icom CI-V Computer Interface

Utilizing No External Power and all parts from

Click on the image to see a larger version


Note: On the miniature electrolytics, the negative lead is the

shorter lead and is marked with "0" in a white band



DB9 Serial Port Connections

The DB9 serial port pin 2 is the RxD and is applied to the LTC1383 pin 14

The DB9 serial port pin 3 is the TxD and is applied to the LTC1383 pin 13

The DB9 serial port pin 5 is a common ground in the circuit

The DB9 serial port's DTR (pin 4) and RTS (pin 7) supply voltage to the input lead 1 of the 78L05 5 volt regulator. You should be able to measure +5 volts output between lead 3 (output) and lead 2 (ground) of the regulator. Voltage is applied to pin 16 of the LTC1383.





Parts List



RS part #


Total cost


Linear LTC1383 5v RS232 to TTL converter IC





Ferrite Bead Pack of 25





22 uf 16 volt electrolytic caps





0.47 uf electrolytic capacitors





1N5818 diodes





1uH RF chokes





100pf capacitor





78L05 5 volt regulator





DB9 Female Connector

See text




1/8" miniature phone plug






Optional parts all from Radio Shack retail store: 16 pin DIP socket, aluminum case, metal standoffs, IC PC Board material (276-159B), grommets - all about $6 additonal.

Theory of Operation

The ICOM CT-17 RS232 computer level converter is both costly and requires external power. By using a cheap, readily available ultra low power TTL to RS232 converter IC, the Linear LTC1383, one can make a self-contained unit, which derives its power from the serial port itself. Although the IBM PC serial port was designed without a DC power pin, early mouse designers figured out a way to squeeze some low-current DC power from RTS or DTR lines (or from BOTH in the case of the design shown above). The LTC1383 draws only about 220uA at 5 volts - no more than most mouse circuitry. The above circuit should work with just about any serial port, including some otherwise finicky PC laptop ports. The LTC 1383 IC does all the work and requires only 4 capacitors to charge the internal pump circuits. Although one can use tiny 0.1uf non-polarized caps for the 4 charge caps, I chose some 0.47uf miniature polarized radial electrolytics to be extra sure of speedy response times in the heat of DX J . All parts are available at the time of this writing online from, but might be hard to find at the retail stores. I have no affiliation to Radio Shack, but enjoy their excellent website and variety of fairly cheap parts.

I chose to use some pre-punched general purpose DIP circuit board, a 16-pin DIP socket, 4 standoffs, 2 grommets and a tiny aluminum case to wire the circuit - which would add about $6 to the cost. Anal-retentive hams can etch and drill their own circuit boards! I'm sure some industrious hams will figure out a way to package the whole circuit inside a DB9 shell!

Instead of purchasing a DB9 female connector, I pilfered the connector and cable from an old Microsoft™ serial mouse. I not only got a "free" DB9 connector, but I found that the 4 conductor shielded cable used by the mouse already had my 5 connections made to the right pins! I even used a couple of feet of the same cable (after cutting off the 3 unused leads) as a single conductor shielded cable to run from the aluminum box to the 1/8 inch miniature phone plug for my IC-746.

IMPORTANT: If you want to use a DB25 serial connector instead, remember that the ground pin is 7, DTR is pin 20, RTS is pin 4, TxD is pin 2 and RxD is pin 3! This would certainly give you more room in the casing to house the circuitry if you decide to go that route. I understand that the same type of circuit can be used for a Kenwood interface if you use separate leads to IC pins 11 and 12 (not tested).

Questions, comments, photos of your creations, etc. are welcome to Although I built a working unit from these instructions, I make no claims or warranties about the suitability or correctness of these circuits.

I am not an engineer and I don't play one on TV.

Copyright, 2000, Howard Groveman KD6UU

Member Comments:
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Parts discontinued by Rat Shack  
by AB0RE on July 16, 2007 Mail this to a friend!
Great article! It is very well written and this would be a very useful project that even a beginner could do.

However, it appears that many of the parts listed above have been discontinued by Radio Shack. Specifically:
- Linear LTC1383 IC - discontinued
- Ferrite Bead Pack of 25 - discontinued
- 22 uf 16 volt electrolytic caps - avail, diff p/n
- 0.47 uf electrolytic capacitors - discontinued
- 1N5818 diodes - discontinued
- 1uH RF chokes - discontinued
- 100pf capacitor - avail, diff p/n
- 78L05 5 volt regulator - only avail in to220 style
- DB9 Female Connector - avail, diff p/n
- 1/8" miniature phone plug - avail, diff p/n

So as you can see the project no longer has the appeal of parts that are available locally at Radio Shack. It's a shame that Radio Shack has discontinued so many parts since this article was originally posted.

Perhaps somebody would take it upon themselves to update the post with current Mouser or Digikey part numbers?

Thanks & 73,
Dan / ab0re
RE: Parts discontinued by Rat Shack  
by N4CQR on July 16, 2007 Mail this to a friend!
Agreed, good article. I noticed the R/S part (900-xxxx) availibilty when I first read the posting. Then I noticed the article was dated 2000.

RE: Parts discontinued by Rat Shack  
by K0BG on July 16, 2007 Mail this to a friend!
Maybe RS doesn't have the parts any longer, but certainly does. If you're into SMD and etch your own, you can build the device INSIDE the DB9 connector.

Alan, KØBG
Get the parts somewhere else  
by N7UQA on July 16, 2007 Mail this to a friend!
I built one of these 7 years ago when this article was first published. The only difference with mine is that I included four jacks so I could control more than one radio at a time. Using a discarded mouse cable is fine except I would recommend using a snap on ferrite choke at the RS-232 connector to reduce RFI. Other than that I still use it to this day, it's certainly a frugal way to go if you don't really need Icom's CT-17. I would also recommend getting the parts from another suppler than Radio Shack, they are more of a Cellular Shack than anything now a days. Digi-key, Frys or Mouser would be a better choice.

Craig - N7UQA
USB to CI-V cables work well  
by N1ESE on July 16, 2007 Mail this to a friend!
The $7 USB to CI-V cables on eGay work pretty well and allows me to keep my serial port free for keying the rig for PSK31 and RTTY.
RE: Parts discontinued by Rat Shack  
by K5LXP on July 16, 2007 Mail this to a friend!
It doesn't get much simpler than the 2-transistor interfaces such as the one found here:

I've built both through hole and SMT versions of these that fit in a DB-9 shell and have yet to find a serial port they don't work with, even laptops I've tried. All of the parts for this simple interface *are* currently available from CellPhone Shack, and as common and non-critical as the parts are, they can probably be salvaged from just about any junk equipment you may have around. I was able to kit these interfaces including DB9 connector, PCB and 3' cable for less than $10 with all new parts, so remote rig control does not have to be complex or expensive.

Mark K5LXP
Albuquerque, NM
RE: Parts discontinued by Rat Shack  
by KB1LKR on July 16, 2007 Mail this to a friend!
Here's some P/N's for AB0RE
22uF 140-MLRL16V22-TB
0.47uF 140-MLRL50V.47-TB
100pF 140-50P2-101K-RC
1N5818 625-1N5818-E3
1uH 542-78F1R0
L7805 (TO-92) 511-L7805ACZ (or 511-L7805ABZ)
L7805 (TO-220) 511-L7805CV (or 511-L7805ACV) bigger case

Linear Technology
LTC1383CN (buy direct),C1,C1007,C1016,P1055

The $10 Icom Computer Interface  
by N6JSX on July 16, 2007 Mail this to a friend!
eHAM how about saving this "base" article to a PDF file allowing readers to download/print and use?
RE: The $10 Icom Computer Interface  
by KB1LKR on July 16, 2007 Mail this to a friend!
revised Mouser P/N:

LM78L05 512-LM78L05ACZ (small TO-92 case)

I had searched for "7805" not "78L05" before.

Note too: 22uF at in and output are overkill, 1uF on the input and 0.1uF on output should be plenty.

0.1uF or 0.22uF should be sufficient for the 4 charge pump caps on LTC1383 too.

An alternate to the Linear Technologies LTC1383 is Maxim's MAX3221E or MAX3222E.

In any case it's a nice design, w/ the ultra low power converter IC, simpler than the 6 transistor versions, and potentially more reliable/robust than the two transistor versions.
RE: The $10 Icom Computer Interface  
by ICR71A on July 16, 2007 Mail this to a friend!
While you are looking about for discontinued or non-stocked parts at RatShack, you might just pick up a model # 20-047 USB Scanner Interface for $30. This is exactly the same chipset and circuitry used in the $60 Piexx model. Unlike the Piexx model, it comes enclosed in a case. The only real difference between the two is that the Piexx has slightly longer cables that are detachable from the circuit board. I can not see where this feature would come in handy, or is worth the additional $20+. Here is a link to the product:

The RatShack version ships with the FTDI drivers, and when the flow control on your port (set under Windows in the hardware manager) is set to "8-1-N" "9600" and "Hardware", behaves identically to the Piexx USB interface. This interface works with all Icoms (maybe others that use the CI-V commands as well) and all rig control software that looks for a serial port.

73 de N4PRT
RE: The $10 Icom Computer Interface  
by NV7F on July 17, 2007 Mail this to a friend!
I can convert the article and the comments for the new part #'s to pdf just by printing to my cups pdf filter. I will do it if eHam says it's okay.

The $10 Icom Computer Interface  
by AA8X on July 17, 2007 Mail this to a friend!
We all know Radio Shack has become a cell phone and toy store. Don’t waste your time going there, they are out of parts business.
RE: The $10 Icom Computer Interface  
by W0NFU on July 17, 2007 Mail this to a friend!
The CT-17 is drastically overpriced. Back in the 90s, I homebrewed one from the diagram which comes with the CT-17. As an experiment in manufacturing pricing, I built mine from all new parts with no searching for lowest price available - I bought each part when first found. I completed the project for around $11 and that was when the MAX232 was expensive. Icom was asking $99 when I built mine. Talk about a big return on something.

Today, the MAX232 is much cheaper. I suspect the CT-17 can be built for around $8 and Icom is currently asking $129.95 in my latest HRO catalog.

I have the diagram available. I'm willing to send it to those who send me an email.

73 - Larry WØNFU
RE: Parts discontinued by Rat Shack  
by KC0RSW on July 18, 2007 Mail this to a friend!
The folks over at Allied Electronics ( I am in no way associated with them ) Are great about converting old and discontinued RatShack part numbers to current Allied Inventory, and are usually very good on pricing.

Just FYI...
The $10 Icom Computer Interface  
by AA6YQ on July 18, 2007 Mail this to a friend!
The Icom CI-V bus is specified as "open collector" meaning that bus drivers should not include an active pullup transistor.

Does the LTC1383 meet this requirement?


Dave, AA6YQ
RE: The $10 Icom Computer Interface  
by K4FX on July 19, 2007 Mail this to a friend!
For those who haven't the inclination or time to homebrew, there is a Icom PC "single rig" interface that most of the dealers keep hidden on their web sites, when I got my 756 I was looking for used CT-17's and looking into homebrew options, until I ran acoss this 20'ish dollar option. Everything is built in the DB9 connector and it works like a champ.

So there is another commerical option other than the way overpriced CT-17

The $10 Icom Computer Interface  
by N2RJ on July 19, 2007 Mail this to a friend!
Nice homebrew.

However, you can get CI-V cables on eBay for $20 or less and they do USB too.

Also, QST of March I think it was had a USB CI-V interface as well.

Seems as though I can't find a decent computer these days that has a serial port.
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