From: mwr@tridom.uucp (Mark Reardon)
Newsgroups: comp.sys.m68k
Subject: Re: RS-422 in 68000 system
Date: 9 Apr 92 13:54:48 GMT
Reply-To: mwr@tridom.uucp (Mark Reardon)
Organization: AT&T Tridom; Marietta, Georgia

In article <ADAMS.92Apr8145405@PDV2.pdv2.fmr.maschinenbau.th-darmstadt.de>, adams@pdv2.fmr.maschinenbau.th-darmstadt.de (Adams) writes:
|> In article <73157@netnews.upenn.edu> green@eniac.seas.upenn.edu (Bradley Green ) writes:
|> 
|> >   I'm trying to build an extremely inexpensive (to produce) single-board
|> >   computer based on an 8 mHz 68000.  I need to get an RS-422 serial interface
|> >   into this system, running at ~232Kbaud (AppleTalk speed, for interfacing with
|> >   Mac's).  My question is this: What hardware (UART chip) will most easily
|> >   work in this system, with the least amount of 'glue' chips?  Also, if
|> >   anyone knows of a site where I could get a pre-rolled interrupt-service
|> >   routine for this interface, it would be very helpful.
|> 
|> Fine, how about using Motorola's integrated micro controllers 683xx?
|> There is at least one composed of a 68k kernel and an intelligent SCC
|> (Serial Communication Controller). I would not be astonished if
|> it could emulate LocalTalk (physical Layer of AppleTalk over twisted pair)
|> in microcode....
|> 
|> Get the manual of the 68332 and save yourself money and headache.
|> 
|> best regards, adams

This is a followup to a previous post that may have gotten lost.

One cheap solution to serial I/O on a 68000 is to instead use a 68302.  This part has a 68000 processor, three timers, two parallel ports, chip selects, Internal RAM, a spare DMA, and three serializers.  The serializers each have bi-directional DMA with up to 8 transmit and receive buffers, and a RISC processor for controlling the ports.  The ports can be run in async, hdlc, bisync, and ddcmp modes.  This part allows the ports to be used together to create an ISDN basic rate interface or separately for oth



er applications.  

Motorola also has a good set of application notes on differing subjects.  These include interfacing to dynamic memory using the 68302 to do refreshing, and interfacing to LocalTalk networks.  This part is handled out of Motorola's Austin, TX. office.  They have been quite helpful.  Pricing of course depends on quanity and with this part, time.  The prices seem to be falling as the part becomes more widely used.  In addition, the new 20 and 25 Meg parts should help keep the cost of the 16 meg part down.

On page D-82 of the MC68302 User's Manual, Rev 2 is a discussion on how to make an Appletalk interface for a 68302 using the MC68195 LocalTalk Adaptor (LA).  The LA board layout is included.  This layout calls for a 26LS32 and 26LS30, two mini DIN 8 connectors, a crystal (2.304 Mhz) and some resistors and caps.  This design leaves on of the serializers unused so it can be used for either async or sync connection.

I've probably said more than I should already.  To save bandwidth, call moto for more information.  BTW, my only affiliation with motorola is that of a satisfied customer of 68000 parts since my first 500Khz cpu.

Mark Reardon (mwr@eng.tridom.com)
Organization: Masters student, Information Networking Institute, Carnegie Mellon, Pittsburgh, PA