[Coco] Max baud rate for 68681 in MM/1?

Dave Philipsen dave at davebiz.com
Mon Feb 1 01:01:40 EST 2016

Interestingly, the closest thing I can find to the Signetics part now is 
the Philips SCC68681.  I believe that Philips bought Signetics some time 
ago so I can only make an assumption that the Philips part is as good as 
or better than the Signetics part when it comes to the higher 
communication speeds.  I just now found the Philips datasheet on the 
internet and instantly recognized it as the document I used when I wrote 
the code for the 68C681 some 15-20 years ago.  The reason I used the 
Philips document instead of the Exar one was that it just seemed so much 
more descriptive and complete especially when it came to the 9-bit 
address recognition mode which I ended up using in my design.  However, 
I remember making the decision to use the Exar part because I had more 
confidence in its ability to handle higher bit rates.  I never did test 
the Signetics part but I believe it has the same pinout.

That being said, if you find out that indeed the Signetics part is 
inferior (as the Philips datasheet seems to imply in regard to max clock 
speed) and you want to try the Exar part (which I believe is still in 
production or at least is still available) I have a good number of brand 
new ones stashed away that I would let go at half of the going retail price.

In fact, I'm starting to now remember that it was my belief that the 
Exar was rated at a maximum clock of 7.372 MHz simply because that 
frequency was a good number to derive all of the most common bit rates.  
In reality, I remember testing them at 8 MHz and they performed just 
fine.  If so, they could very likely work at 500Kbps.

I'm also remembering that in my design each one of the almost 400 
locations that received the equipment had four 68C681s in the design 
each running at 250 Kbps.  So I guess I bought more than 1600 of them 
over a span of maybe 5-6 years.


On 1/31/2016 10:35 PM, Joel Ewy wrote:
> On 01/31/2016 09:38 PM, Dave Philipsen wrote:
>> I'm not sure if Motorola (Freescale) even produces the 68C681 anymore 
>> but the ones I used were the Exar 44PLCC parts.  The datasheet is a 
>> little misleading when it says that maximum bit rate with a 16x clock 
>> is 125 Kb/s because later the sheet says the maximum clock frequency 
>> is 7.372 MHz which would yield a bit rate of 460.75 Kb/s. I would 
>> hazard a guess and say that the Exar 68C681 is quite capable of that 
>> speed and I can say for sure that none of the parts I bought ever 
>> failed to work with a 4 MHz clock and 250 Kb/s bit rate.
> FWIW, I still have 17 Signetics 68681C1A44 parts that were intended 
> for MM/1 I/O boards in the '90s, most of which never got assembled. I 
> assembled boards, but never looked too deeply into the specifics of 
> how that chip was used there.  In those days, 34,800 would have been 
> plenty adequate, since a 14.4 modem would have been the fastest thing 
> I would have been able to hook up to it.  I guess I need to delve into 
> the schematics and data sheets. I was hoping someone on the list might 
> just have an answer.
>> Dave
>> //
>> On 1/31/2016 9:22 PM, Dave Philipsen wrote:
>>> And any form of 68000 CPU should have no problem servicing 
>>> interrupts at 1Mbps.  The 68C681 has a receive buffer so you can 
>>> receive many bytes while generating only one interrupt and even if 
>>> it did generate an interrupt for every byte that's only 100k 
>>> interrupts per second which I believe is quite doable with a 68000. 
>>> Without looking at the technical info on the 6809 I would guess a 2 
>>> MHz 6809 could do it too although it might tax it a bit.
>>> Dave
>>> On 1/31/2016 9:16 PM, Dave Philipsen wrote:
>>>> I have personally used a 68C681 UART in a 68HC11-based system that 
>>>> transmits DMX512 theatrical standard lighting data at 250 kbps 
>>>> (RS422/485).  It is tried and proven and currently running in about 
>>>> 400 different locations in the US and a few other countries.  It is 
>>>> using a 4 MHz clock as the BRG.
>>>> Dave
>>>> On 1/31/2016 3:33 PM, Joel Ewy wrote:
>>>>> I read ambiguous information about the 68681 on the MM/1's I/O 
>>>>> board.  Does anybody have a definitive word?  Page 2-5 of the MM/1 
>>>>> technical manual claims that it can be run at "1 megabytes per 
>>>>> second" (seriously 8M bits/sec?), but the table on page 9-2 and 
>>>>> the text on 9-2 state that the maximum baud rate is 34,800.  Is 
>>>>> this a pragmatic maximum based on the CPU's ability to service the 
>>>>> DUART's on-chip buffers without interrupt-masking trickery, is it 
>>>>> the real hard maximum for the 68681, or is it a hard maximum 
>>>>> imposed by the MM/1's hardware? Anybody have any insight?
>>>>> JCE
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