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

Joel Ewy jcewy at swbell.net
Mon Feb 1 00:46:18 EST 2016

On 01/31/2016 11:33 PM, Dave Philipsen wrote:
> In my opinion, the biggest factors in determining whether the MM/1 
> could do this are:  1) What kind of clocks feed the 68C681 chip(s) in 
> the MM/1.  2) What kind of driver/transceiver chips are downstream of 
> the 68C681.
The crystal connected to it is 3.6864MHz, and the level converters are 


> Perhaps if the proper clock is available to the 68C681 the RxD and TxD 
> signals could be intercepted before the driver chips and you could 
> coerce a pretty fast data rate out of it.  The RS232 interface is 
> inherently weak when it comes to running at higher speeds.  You are 
> much better off using RS422/485 or just straight TTL data if the 
> distance is short.
> Dave
> On 1/31/2016 10:26 PM, Joel Ewy wrote:
>> On 01/31/2016 08:44 PM, Kip Koon wrote:
>>> Hi Joel,
>>> This is a link for the MC68681 datasheet.
>>> < 
>>> http://pdf1.alldatasheet.com/datasheet-pdf/view/4164/MOTOROLA/MC68681.html>
>>> The maximum speed is 1Mbps.  That should have been a lower case 'b'.
>> That's kinda what I was thinking.  The question remains as to whether 
>> the MM/1 can run the 68681 at a higher baud rate than 34,800, and if 
>> so, is it likely to lose characters.  Part of why I'm asking the 
>> question is that I'm wondering about the feasibility of doing 
>> Drivewire over the 68681 serial ports.  On the CoCo, 115Kbps is 
>> achieved through great trickery and masking interrupts for however 
>> long it's necessary.  This seems to work acceptably well in 
>> NitrOS-9.  The ordinary floppy driver halted the CPU for a while, 
>> waiting for a sector.  The major side-effect of this was that the 
>> software clock would lose time, hence the desirability of no-halt 
>> FDCs, like Disto's.  Drivewire largely makes up for this problem by 
>> giving the CoCo access to the DW server's clock.  The MM/1 has a 
>> DS1287 on the I/O board (with the 68681) which probably doesn't have 
>> a very good battery anymore after 20 years.  I don't know how often 
>> the clock module reads from the RTC -- is it only at boot time?
>> My other thought is that there are some fairly inexpensive 
>> serial<->ethernet adapters out there with hardware TCP/IP. But it 
>> would sure be nice to have just a little more speed than 34,800bps.
>> JCE
>>> Kip Koon
>>> computerdoc at sc.rr.com
>>> http://www.cocopedia.com/wiki/index.php/Kip_Koon
>>>> -----Original Message-----
>>>> From: Coco [mailto:coco-bounces at maltedmedia.com] On Behalf Of Joel Ewy
>>>> Sent: Sunday, January 31, 2016 4:34 PM
>>>> To: coco at maltedmedia.com
>>>> Subject: [Coco] Max baud rate for 68681 in MM/1?
>>>> 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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