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Chapter 7

Chapter 7:  The KIO3

7.1    Introduction

The designers of the K3 have thoughtfully included an I/O interface, the KIO3, through which you can control the radio with a computer with no need to build or by equipment to convert signal levels, receive and send audio for digital modes such as AFSK, RTTY and PSK-31, and use external devices such as band decoders for controlling antennas, rotators, and other station equipment.

This chapter will show you the circuits involved and how you can connect your computer and other station equipment to the K3.


Figure 71.  The KIO3 interface.

7.2    The K3 RS232 Serial Port

The K3 RS232 Connector and Cable

The RS232[1] serial port in the K3 is part of the KIO3 module. A 9-pin, DE9S connector is mounted on the K3’s back panel; Table 7‑1 shows the pin connections[2]. Your computer or USB-Serial adapter likely has a DE9P connector so you will need a straight-through cable with a DE9S connector on the computer end and DE9P on the K3 end. See Figure 7‑2. This cable has the minimum set of connections required, and each plug pin  is connected to the same socket pin, hence the name straight-through. The other kind of cable is called a Null Modem and you don’t want that kind.

If your computer does not have a real RS232 COM port, you might consider buying one.  A board that can plug into a desktop computer with four RS232 ports costs about $50 US.  You can also purchase RS232 serial ports that can plug into a laptop’s PCI card slot. Otherwise, if your computer has only USB connectors, you will need a USB-to-Serial adapter.

USB-to-Serial Adapters

Getting USB-to-serial adapters to work is a perennial problem. Whenever you change your computer’s operating system, the drivers for your adapter may have to be updated. In general, the adapter available from Elecraft will work with the K3 and the K3 Utility program. If you change operating systems or wish to use the adapter with another program, you may have to visit the adapter manufacturer’s website to update the driver. See Appendix B for more discussion about problems with the USB-Serial adapters and for troubleshooting hints.

. . .

The K3 Serial Port Data Rate

. . .


. . .

Connecting Your Computer to the K3

. . .

Setting up the K3 with a Logging Program

. . .

RS232 Interfacing Problems

Connecting RS232 serial devices can be frustrating at times. If you are having problems, here are a few checks you can make:

·            Check that the data rate (Baud), number of data bits and stop bits are set the same on the computer and the radio.

·            Check that a straight-through cable (not a null modem cable) is in use. This cable must have at least the connections as shown in Figure 7‑2.

·            Check to see if the K3 Utility program can communicate with the K3. If it can, the problem is with the other application. If not, the problem is likely in the serial cable or the USB-Serial adapter, or the data rate is not set the same on the computer and the radio.

See Appendix B for further discussion of RS232 interfaces in general and more in-depth troubleshooting procedures.


7.3    The K3 Accessory I/O (ACC)

The 15-pin DE15S connector[3] (Figure 7‑5), which looks like a VGA video connector but is not, gives us variety of inputs and outputs for various purposes as shown in Table 7‑3. In the following sections we briefly describe the electronic circuits, what you can do with each of these signals, and how to do it.

Table 73.  K3 ACC accessory I/O socket.




Frequency Shift Keying (FSK) input


AUXBUS Input/Output. A microcontroller-to-microcontroller bus


BAND1 Output (Band B[4])


PTT Input (in parallel with front panel mic PTT)


Ground (RF isolated)


DIGOUT0 output


K3 On (output) or TX INHibit (input)


POWER ON input to turn the K3 on


BAND2 Output (Band C)


KEYOUT_LP (10 mA keying output)


DIGOUT1 output




BAND0 Output (Band A)


BAND3 Output (Band D)


EXT ALC input

KIO3 Ground Connections

The K3 and the KIO3 module schematics show four different types of ground connections. In Figure 7‑6(a), Earth/Chassis Ground refers to a ground that is used for zero potential reference and electrical shock protection. It is used for most signals that are entering the K3 from outside and which may have electrical shock potential. The chassis is connected to this ground.

The Digital/Common Ground in Figure 7‑6(b) refers to the ground traces on the PC boards.

The Frame Ground, Figure 7‑6I, refers to the DE9 RS232 and DE15 ACC connector shells. These are connected to the chassis through the DE9 and DE15 connector screws.

The RF Isolated Ground, Figure 7‑6(d), uses a 100 mH RF choke to isolate any RF on the signal line from ground. All grounds are ultimately connected to the rear panel GROUND lug.


Figure 76.  K3 ground connections.



Frequency Shift Keying (FSK) Pin 1

. . .

Figure 77.  FSK input.



. . .


BAND3 – BAND0 OUT Pins 14, 9, 3, 13

. . .


Figure 7‑9 shows the K3’s BAND OUT circuit. The open-drain is pulled up to +5 v by a 2.2 KW resistor. This was a change from the original design that simply presented an open-drain transistor and required all external band decoder devices to supply the pull-up resistor to generate a logic-high. The change was included on all K3s shipped after December 10, 2008 and now the BAND outputs are “TTL” compatible[5].

Caution: If you attempt to use the BAND outputs to drive a device that uses a pull-up to a higher voltage than +5 v and which shares its ground with the K3, current may flow from the external device back into the K3’s +5 v supply. See Figure 7‑10. This may or may not cause a problem. To be safe, if you are driving a device pulled up to higher than +5 v, you should remove the 2.2 KW resistors or install a diode in series with the resistor.


Figure 79.  BAND OUT.


Figure 710.  A problem with BAND OUT pull-ups.


Ground Pins 5, 12

. . .


. . .

Figure 712.  DIGOUT0.


K3 ON or TX INH Pin 7

. . .

Figure 713. K3 ON or TX INH.


. . .

Figure 714.  Power on.



. . .

Figure 715. Keyout – LP.


DIGOUT1 Pin 11

. . .

Figure 716.  DIGOUT1.

EXT ALC Pin 15

. . .

Figure 717.  EXT ALC (positive input).

7.4    Audio Input and Output

. . .

Line In

. . .


Figure 719.  Line In.


Line Out

. . .

Figure 720.  Line Out.



 Figure 7‑21 shows the SPRKS jack used for external stereo speakers. In early versions of the K3 (those shipped before about December 23, 2008), the two 470 ohm resistors shown were not included in the speaker output circuit. They were added to protect the K3’s audio amplifier against a short circuit or very low impedance load, and they protect the audio amplifier if you plug or unplug a speaker with a monaural plug but have set CONFIG:SPKRS 2. They work by holding the 470 mF coupling capacitors at ground potential and eliminating any voltage spike that would result from shorting the speaker outputs to ground. 

The CONFIG:SPKRS menu item allows you to choose either one or two speakers. Two speakers are needed to enjoy the binaural audio effects of AFX and when using a Sub receiver. However, you may use one speaker if you set CONFIG:SPKRS 1.

Warning: Even with the 470 ohm resistors you MUST NOT use a monaural plug in the stereo speakers jack with CONFIG:SPKRS 2. The resistors do not protect against a prolonged short on the right channel audio. See Appendix D for a list of “safe” adapters to use with monaural speakers.


Figure 721.  Speakers.


Rear Panel MIC

. . .

Figure 722.  Rear panel MIC input.


Rear Panel Headphones

. . .



Figure 723.  Rear panel headphones.


7.5    Connecting the K3 to Your Computer

When connecting the K3 to a computer sound card for audio input and output, there are three items of concern. These are (1) the cabling that is required, (2) making sure the audio signal levels are set properly, and (3) bounding all external components together with a heavy ground braid.


K3-to-Computer Cables

In general, unless you are sure that the electronics associated with a stereo signal source are well designed, you should never plug a monaural plug into a stereo jack. Doing so will short to ground the stereo ring connection. On the other hand, there is no hazard with connecting a stereo cable between two monaural jacks. The ring connection and wire in the cable will simply be grounded at both ends. Thus, when connecting the KIO3 audio signals to your computer’s sound card jacks, use stereo cables in all cases.


Computer Sound Card Connections

Table 3‑19 shows common sound card connections and Table 7‑7 the K3’s KIO3 jacks. All jacks are 3.5 mm stereo (1/8”, tip-ring-sleeve), even the monaural Mic connector on the computer. The ring circuit on the computer Mic connector often provides a bias signal for lectrets condenser microphones. The colors shown in Table 3‑19 are typical but you may find some sound cards with different color schemes, particularly in older computers. Line In and Line Out signal levels are normally designed to have a constant gain and the output signal is not dependent on a volume control as a speaker or headphone output is. Most sound cards have driver software that allows you to set the line out level. Level outputs of computer and consumer audio equipment are typically about 1 v peak, which is well suited for connection to the K3’s Line In and Line Out jacks. See Figure 7‑24 for a typical KIO3 to computer cable set connecting the Line In and Line Out signals. See Chapter 3 for further discussion on connecting to a computer and setting levels for digital modes.




Table 76.  PC sound card audio signals.



Typical Symbol

Stereo, Line In

Light Blue

Stereo, Line Out, amplified speakers


Stereo, Center speakers, sub woofer


Stereo, Front Speakers


Stereo, Rear surround speakers


Stereo, Mid surround speakers


Stereo, Mic In


Stereo, Speakers

Lime Green

Stereo, Headphones




Table 77.  K3 audio inputs and outputs.

K3 Jack

Jack Type



3.5 mm Stereo (1/8” tip-ring-sleeve)



3.5 mm Stereo (1/8” tip-ring-sleeve)



3.5 mm Stereo (1/8” tip-ring-sleeve)



3.5 mm Stereo (1/8” tip-ring-sleeve)



3.5 mm Stereo (1/8” tip-ring-sleeve)




Figure 724.  Typical K3 to computer cabling.


7.6    ERR  IO3 KIO3 not Responding

. . .

[1]  See Appendix B for an in-depth discussion about RS232 serial interfaces and troubleshooting information.

[2]  The DE9S is a nine-pin socket connector, previously called a female connector. The DE9P is a plug connector, previously called a male connector. These connectors are often mistakenly called DB9 connectors. The letters E and B refer to the shell size and E is the correct nomenclature for the 9-pin connector. For more information, see http://en.wikipedia.org/wiki/D-subminiature.

[3]   Radio Shack #276-1501

[4]   Band A, B, C, D is the terminology used for a variety of band decoders used for automatic antenna switching etc.

[5]   KIO3 Remote I/O Board Upgrade (REMIOUPGD).