Baycom modem experiments

Hi,
I recommend you to follow those four steps without a computer first. The modem can handle static levels at the data input. Step 3 tests the transmitting section, step 4 the receiving one. If all 4 steps work O.K., the modem should work together with a computer and software immediately.

VBR from Ivan

Many thanks for the advise!

Made some progress. Corrected the errors on the PCB and started testing the modem with an old PC with the original Baycom software (under DOS).
It does key the radio, so at least PTT is working. Will continue tests for audio receive/transmit.

It is hard to write more specific advice when you do not specify your trouble more exactly. I recommend you these steps:
1. Check the Vcc and all wiring.
2. Check CLK at pin 2. If there is no clock, try to change the 33 pF capacitors at pins 15 and 16. The Xtal may be defective as well.
3. Check the output at pin 11. Is it 1200 Hz resp. 2200 Hz according to the TTL level at pin 14?
4. Feedback the attenuated and DC separated signal from pin 11 to pin 4. Does the level at pin 8 follow that at pin 14?

VBR from Ivan

Hello!

VCC levels are ok, I think...
I do get 5V at the output of the 78L05 regulator and on the VCC pins of the two ICs.
Will need to check again, however.

USB to serial converters may work, but I'm trying to troubleshoot one step at a time. I have an old laptop with serial port, which used to run an APRS IGate with the Baycom modem. Trying to use that for the first stage of testing. 

Some of the problems I'm facing are inconsistencies between schematics and PCB foil patterns (can provide more details if you want) and finding an easy way to test. 
Found an old MS DOS boot disk yesterday, which also seems to contain the Baycom software. Will test it on an old laptop at work...

Hi,
the funny (?) fact is that the original MSDOS Baycom software seems to have used DTR instead of TXD and CTS instead of RXD. It probably generated the data programmatically and did not use the UART of the COM: port. Up-to-date solutions (native COM: under WinXP - Win10, USB via FTDI, USB via MCU) must use another software and connect the modem properly. If the USB convertor chip has TTL levels instead of RS232, the inverters connected to pins 8 and 14 may be unnecessary: defined levels L/H are 0V/3V in TTL, but +9V/-9V in RS232.

VBR from Ivan

If you use a cheap RS232 to USB converter (FTDI, etc.) you should be able to use a modern computer, I routinely use Hyperterminal on a windows 7 machine for my PIC programming. Other versions such as Tera Term are available, but I found somewhere to download the original Hyperterminal that used to be included with windows XP. 
Regarding the cheap chinese copies, its often worth taking a gamble when the prices are so low. I think the types which are long obselete are less likely to be fakes than more modern and expensive devices. I recently bought some Motorola PLL synthesizer chips type MC145145, MC145157 and MC145158 very cheaply and all appear to work as expected despite their low cost. I wouldn't expect bargain price RF power transistors to be any good from these suppliers however. 
What exactly are the problems you have been having?

JC

The modem is set to Bell.202 1200/1200 Bd via pins 5, 12 and 13. Therefore mark is 1200 Hz, space is 2200 Hz and you should find 19,11 KHz on CLK.
The pin numbers are valid for a 16 pin case. A 24 pin version exists and is slightly different. See the datasheet.

One more question: is the Vcc O.K.? Some COM: ports do not use the standard -9V/+9V levels (ev. -12V/+12V) and use 0V/+5V or even 0V/+3,3V instead.

VBR from Ivan OK1SIP

Well... There are cheap copies of the chip available on ebay and AliExpress. Have not yet verified whether they actually work or not...
Has anyone here tested them?

Built a Baycom modem years ago. Back then I got the chip from Futurlec (https://futurlec.com). The modem worked and was used with a Linux PC as an APRS iGate that ran for many years.

There's a website ( https://mijndingen.nl/baycom-modem-usb-kiss-interface/ ) which describes how to make the modem work with modern computers by adding a micro controller. Through it would make an interesting club project, but need to make sure the Baycom works first :-/

Don't know about the frequencies, unfortunately...

Is that chip still available? I have been visiting HHH since the late 90's and know that to be an old project. Don't know what the Baycom modem does but perhaps it can be implemented in a PIC or Arduino now that these are cheap and widely available. I know that NR Bardwell closed down quite a few years ago as I live nearby. 

What are the mark and space frequencies for this FSK modem? 

JC

Hello!

Been trying to build a couple of Baycom modems ( http://sm0vpo.altervista.org/use/baycom.htm ).
Thought it would be a nice club project, especially for new hams that want to get started with packet radio.

Unfortunately we ran into a few problems. Has anyone built the one shown in the link above or something similar?
Any assistance would be appreciated :-)

73 de 5B8AP