What's on this page
Two topics are covered on this page. The first is how to identify whether or not the RS-232 option for serial port communications with a PC has been installed or not. If not installed there are two options, one being easy and cheap, covered here on installing RS-232. The second topic covers the need to make a serial cable for the CP-100, only if the RS-232 option has been factory installed, to talk to a PC.

Factory installed RS-232 option in the AEA CP-100 modem

Here is how to tell if the RS-232 option has been installed. Open the CP-100 so that the rear of the unit is facing you. In the right hand corner closest to you, the female DB-25, as well as Q18 through Q21, U11 and U12 should be present. Those are the easy landmarks. There are over thirty more components involved for RS-232. In the picture to the right, you will see the general area outlined in yellow where the RS-232 components are. Note the area left of the yellow outline in the picture. There are two six pin sockets and places where other components would go. This is for the RTTY loop option, not RS-232. Whether or not the RS-232 option is installed, this page will help you get your CP-100 ready to use with DOS modem programs. What if you want to use Windows? See CP-100 main page.

Two options for adding RS-232 capability to the AEA CP-100

First option
Install the nearly 40 components as per CP-100 schematic. This is the least popular option, mainly due to cost, if you do not have most of the parts on hand. Good luck on finding that special female DB-25 connector. All of the parts used to be available at Radio Shack, but not anymore. If you restore the CP-100 with the RS-232 option as AEA intended, you will need to make a special serial cable as the CP-100 is not pin for pin compatible with a PC's serial port when using typical DOS modem programs. See details.

Second option
Build an interface using a TTL to RS-232 interface IC. They are simple to use, require little or no external parts, and cheap. Buy any PC board mount DB-25 female connector that will fit in the case and extend out the back properly. With a simple trick you can make it work. If you go this route you can wire the female DB-25 in a way that the CP-100 will be pin for pin compatible with a PC's serial port. This means you can use a standard, off the shelf, serial cable. What will actually determine the DB-25 wiring will be what programs you use for the CP-100. On this page I am assuming you are going to use a DOS program such a Terman ( AN-93 ), HamComm, Baycom, or Eskay Packet (SP). All these programs use the same pinouts.

A Simple RS-232 Serial Communications Interface for the CP-100

Amazingly, you could almost build this interface just by looking at the picture on the right! But see the schematic below anyway. This time I built the interface using an LTC1383. It is compatible with the popular MAX232 or MAX202 IC. Note how I turned the PC board mount female DB-25 connector so that the pins are facing up. It was not possible to place the pins in the board holes and have the connector extend out past the circuit board for the serial cable to connect to. The connector fits the mounting holes on both sides of the pins just fine. Being the pins are face up just makes soldering easier. There was a small lip on the top of the connector that made mounting upside down uneven. Just slip a piece of old perf board under part of the DB-25 connector to even it up. Note how I just soldered wires from the rear of connector J6 (TTL connector) to the RS-232 board. I did this so I would not have to do any work from the foil side of the main board.

Better, faster, and cheaper way to make RS-232 for the AEA CP-100 using a LTC1383 or MAX202
(A continuation from above) The LTC1383 and MAX202 IC's are TTL to RS-232 transceivers that require only a single +5 Volt power source. They make their own voltage for the required RS-232 levels. See schematic below. For data sheets on the LTC1383 and MAX202 see the data sheets section. To find out where you can obtain either of these IC's, see the Component Resources page.

An observation on the LTC1383 versus Maxim RS-232 transceiver IC compatibility
The data sheets for the Maxim IC's and the LTC1383 shows one minor difference. On the LTC1383 they have the capacitor on pin 2 going to ground as opposed to to going to +5 Volts with the Maxim IC's. The LTC1383 data sheet says that it is pin for pin compatible with the MAX232A. I tried it with the LTC1383 I installed and it is true. Putting the capacitor on pin 2 to +5 Volts or to ground makes no difference in operation. The only advantage of putting the capacitor to +5 Volts would be that you could use a lower voltage capacitor. I like overrated components so it did not matter. I used a 35 volt capacitor.

Don't take power from the CP-100 +5 Volt regulator
If you are thinking of robbing power from the 7805 (5 Volt) regulator that's on the CP-100 board, to power the RS-232 IC, look at the CP-100 schematic again! They put a zener diode on the reference leg of the 7805, bumping the output to +10 Volts. This may hurt the single +5 Volt supply RS-232 transceiver IC's. In the schematic below a 7805 regulator is provided for a +5 Volt regulated source.

Schematic of a Simple RS-232 Serial Communications Interface for the CP-100

If you use this schematic and wire the RS-232 into the CP-100 as shown, you may use a standard, off the shelf (shielded) serial port cable. As usual, I used what parts I had on hand. Use what ever you want for a +5 Volt stable power source for the LTC1383 TTL to RS-232 transceiver IC. If using a 7805 +5 Volt regulator, definitely use bypass capacitors on the input and output of the 7805 as shown. All solder work can be done from the component side of the board making this an easy project. Before you solder to the back of the TTL connector, try to pull the white insulator towards the back more in order to better expose the TTL connector pins. Or, simply desolder the TTL connector, J6, and solder the wires to the CP-100 main board holes.

If the RS-232 factory option is installed
When you have confirmed that the CP-100 does have the RS-232 option installed from factory, there is only one thing left to do. Connect the CP-100 to a PC via a proper serial cable. NOTE: Assuming the RS-232 was installed at the factory, both the AEA CP-100 and CP-1 are not pin for pin compatible with a standard serial cable to the PC if you plan to use DOS modem programs like AN-93 (Terman), SP, BayCom, HamComm, and BMK Multy. Again, it is the unconventional way the modem programs use the serial port. Two choices exist to resolve this issue.

Rewire serial port on modem
One choice is to go inside the modem and rewire the data lines going to the DB-25 connector on the board so that any standard, off the shelf, serial cable can be used. You would only be changing a few lines. You may want to refer to the charts below and CP-100 schematic to help you out.

Make your own serial cable
The second choice is to just make your own serial cable. Below is a chart to help you do this. It will work for either the CP-1 or CP-100.

Technical Data

Download CP-100 instruction manual with schematic 1.68MB PDF

Download Maxim and LTC1383 IC data sheets

Where to purchase the IC's

AEA CP-100 main page

CP-100 tips, tricks, and modifications

How to make other TTL to RS-232 interfaces

DOS programs for the AEA-CP-100

Oscilloscope Tuning
	The AEA CP-100 and CP-1 has Oscilloscope output for accurate tuning. My favorite feature!

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Page Added to Site 01/01/04

Updated 01 January 2004

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