[time-nuts] Motorola MC68HC11 Crystal

[Joe Leikhim]

Hi Bob; Jim;

After I wrote that I did take a look at RS232 serial specs and yes, they are quite permissive.
The programmer I have is simply an IBM PS2/E "pizza box" which is a 486 machine running DOS.
Motorola was not very competent in their design of radio service software and as such,
the speed of the PC processor can affect the serial connection. That said, I use this same computer,
cables and RIB (RS232 to TTL) interface box on many of the same radios without a problem.
It is so critical that I have a back up PS2/E because no modern computer will do.

The radio service manual points to the crystal in the event of serial communications error.
I would not be surprised if the designers of the software were targeting a much tighter
timing window than sensible industry standards.

So I am back to the question as to my part substitution being proper as they are a common
failure component because these radios, though built to MIL-810D standards, get dropped often.

I have not as yet encountered "birdies" because I have been unable to load the frequencies
I wish to use in these few "broken" radios. Generally, when working properly, these radios
normally don't suffer from RF design problems as they were top tier radios at the time. In fact,
the portable products made since are inferior.

This is the work zone:

http://www.leikhim.com/attachments/Image/P1130492a.JPG

Joe K4SAT

"Async communications will usually tolerate an timing error of about 0.5 bit
time in 10 bits or about 5%. So if your oscillator is within 1% (10,000
ppm) you should be OK in that regard.

I suspect your serial communication problem is coming from someplace else.
Maybe the radio needs real RS-232 voltages, or you have a bad interface
chip or power supply for the RS-232, or you need a null modem cable that
swaps the Rx and Tx data lines."

"Hi

First off, the programmer will work with any of the crystals you are talking about.
You should be fine at 0.1% which would be +/- 7.4 KHz relative to 7.3728 MHz.
Indeed you should find that serial com will work fine at offsets even greater than this.

In terms of birdies - does the radio work? If not is it an IF issue or a front end problem?

If none of the replacement crystals oscillate, that’s probably a good sign that the problem
is not the crystal …..

Bob"

-- 
Joe Leikhim


Leikhim and Associates

Communications Consultants

Oviedo, Florida

JLeikhim at Leikhim.com

407-982-0446

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