Using the single supply MAX202 and MAX203 IC's
 Using the 1488 and 1489 IC's (dual supply required)
 TTL to RS232 using a MAX202 for the AEA CP-1
 TTL to RS-232 using a LTC1383 for the AEA CP-100
 Pinout requirements for various programs
 What are all those serial port pins for?

TTL to RS-232 Interface Using a MAXIM RS-232 Transceiver IC
If you are trying to upgrade an old TTL modem you used before the days of the IBM Clone, here is a great way to do it using a Maxim IC. They require only a single, 5 volt, supply for operation and make +10 and -10 volts (good enough) for the RS-232 output. The MAX 202E (left picture), requires four external capacitors which are needed for the + and - 10 Volt generator. The MAX203E (right picture), does not require any external charge pump components. It also costs twice as much as the MAX202E and has 20 pins.

The MAXIM IC's discussed here do invert data. You need to check to see if this will be a problem with your TTL modem and your program requirements. If so, just use a transistor to invert data on the TTL side of the MAXIM IC. If your modem is purely for packet, then you do not need to worry about the transmit and receive lines. However you need to check the 'Push to Talk' circuit. If your modem is for RTTY and the 'Tor modes then you will need to check the transmit, receive, and push to talk data lines to make sure everything is 'right side up' and that your rig is not keyed while you are supposed to be in receive. When I made my interface for the AEA CP-1, I ended up using one transistor to invert the transmit data line to meet the requirements of my programs.

Note: Sometimes the Maxim chips will go 'jittery'. You may need to add pull down resistors as need on the TTL inputs to keep the RS-232 output from changing states on its own.

I have also noticed that the RS-232 outputs can have a lot of 'trash' on them. So I used a .01 capacitor and it is effective.

TTL to RS-232 Interface for the AEA CP-1

If you have an old Advanced Electronic Applications AEA CP-1 modem that needs RS-232 to work with a PC, you can take advantage of the work I have done on mine. I built my TTL to RS-232 interface around the MAX202 simply because I had them. The programs I use, and the design of the AEA CP-1, requires that the transmit data line be non-inverting. Being the Maxim IC inverts, I had to add the PNP transistor, shown above. The data receive and push to talk lines, again due to program requirements and the design of the CP-1, needed to be inverted so I did not need to add any extra components to those lines. See RS-232 interface for the CP-1 for more! Want to know more about the AEA CP-1? Need Tips, and modifications for the CP-1?

TTL to RS-232 Interface for the AEA CP-100 Using LTC1383 RS-232 Transceiver IC

If you need to get your Advanced Electronic Applications AEA CP-100 modernized in order to work with a PC, here's the way to do it! I decided to give the LTC1383 IC a try when I ran out of MAX202's. It is pin for pin with the MAX202. In comparing the data sheets between the MAX202 and LTC1383 there is only one small difference. On the MAX202, pin 2, there is a .1uF capacitor going to the +5v source. With the LTC1383, pin 2 has the .1uF capacitor going to ground. Download the LTC1383 data sheet. This IC is so easy to install in the AEA CP-100 that you can do all the work from the component side of the CP-100 board! See RS-232 interface for the CP-100 for more! Want to know more about the AEA CP-100? Need Tips, and modifications for the CP-100?

TTL to RS-232 interface using 1488 and 1489 IC's

Download the 1488 and 1489 data sheets.

The 1488 Quad Line Driver and the 1489 Quad Line Receiver IC's are very outdated today as far as building a TTL to RS-232 interface. One drawback would be that it requires a dual supply voltage, +12 volts and -12 volts, and you only have one supply voltage available. This is not a problem if you have an AEA CP-1 as it uses a dual voltage supply for all those op-amps. AEA came out with a bulletin regarding RS-232 interface for the AEA CP-1 using the 1488 and 1489. If you want to add RS-232 using the bulletin, you may want to also look over the AEA CP-1 page for more details and goodies.

I must point out that if you are using the AEA service bulletin, it does not cover what to to for the push-to-talk. Be sure to follow the schematic for the CP-1 and install components Q12, D29, R123 and R124 on the CP-1 board. This push-to-talk circuit meets the requirements of the programs mentioned on this web site.

Technical Data

Where to download MAXIM data sheets + FREE samples!

Download LTC1383 IC data sheets

Download 1488 and 1489 data sheets

Where to purchase the IC's

CP-1 page with links to mods and RS-232 interfacing

CP-100 page with links to mods and RS-232 interfacing

DOS programs for the modems mentioned here

Notice...
You are encouraged to check all of the pages on this web site for the information you seek, starting at the main page. Each page is possibly related to another. The information on each page is "as-is", try at your own risk, and so on. We can help you out some what, as time permits. Try to stay on topic. Please, do not ask to "engineer" or "re-engineer" a design to suit your special project. We do not sell parts or kits of any kind. All said, we are open to any suggestions on adding more material, as long as it is on topic with a particular web page of the Technicothica web site. An example of things that are not "on topic" are: Support for APRS, EMWIN, TNC's and sound card stuff.    ~Thanks!