6502.org

Which is easier to interface to, a 6850 or an 8251?

My choice would be using the 8250/16450/16550. I have lots of the first two by scrapping old PC's and PC-cards. Which also means I have the needed crystals.

Easy to interface with a 6502 bus. One half of a 74LS139 will do to generate the needed typical Intel bus signals. And the other half can be used for addressing the IC.

Very easy to prgram.

Could you post some schematics and code for how to do that? The last few times I tried to use an 8250, I couldn't get it to work properly. Maybe I was doing something wrong, but anything has to be easier than using a 6402.

Hi,

I remember that 8250's have pretty slow access times.
Watch your bus timing parameters when connecting.

Bye,

Paul

I've had success interfacing and programming a 6850 with a 6502. It was very straightforward. I could show you what I did if you're interested.

That would be great. I am very interested to see how to interface to and program a 6850.

Are you able to get hold of a 6551? It's similar to the 6850 but has internal crystal oscillator (just add the crystal and two capacitors on the outside) and internal baud rate generator that allows software selection of 16 standard baud rates from 50bps to 19,200bps on a standard 1.8432MHz crystal. Other baud rates can still be achieved by using the 16x external clock, like if you wanted the 31.25kbps rate for MIDI, or other rate up to about 250kbps.

There have not been any companies making them in the last couple of years but WDC will be selling them again sometime soon. ("Soon" is a questionable term with WDC. They said they'd have them out a year ago and still don't, but at least they have a datasheet now.)

No, unfortunately I wasn't able to find one when I went to get components last week. They seemed to have eliminated a large ammount of their ic's since I last went there. All I managed to find from the 65xx family was a handful of 6529's and a couple of 6530's. The only UART's I found were 8250's, 6850's, 6402's, and 8251's.

Even Jameco ( www.jameco.com ) has 6551's for $4 (although they only have them in 1MHz and apparently no CMOS). Jameco is one of the good distributors for hobbyists to cozy up to. The 6551 is at
http://www.jameco.com/webapp/wcs/stores ... ctId=43318

I don't mind having to divide the clock externally, I can just connect the oscillator to the clock input on a counter and divide it down to the frequency that I want. To get components I go to a liquidation outlet that has a whole floor full of electronics and get parts for much cheaper than anywhere else. I'd rather work with what I have, and if a 6850 is the easiest option, then that's what I'll use.

My project is a little old, and very basic (it was just a hobby board), but it interfaces a 6502 with a 6850. You can't beat the 6850 for simplicity, but the 6551 is definitely better.

Here's the link to the project: http://chidesters.org/scott/meadow/Mead ... r_Doc.html

(P.S. to the moderator: I don't know if you want to add this to the projects page but you're welcome to).

Tukasa, what I'm saying is that the 6551 lets you eliminate the additional cost, board real estate, and assembly labor of the external oscillator and the counter, along with their sockets. The cost of wire-wrap sockets for those extra parts required by the 6850 may be more than the price of the parts themselves, and then the 6850 will still still leave you locked into one data rate unless you add even more parts. OTOH, if you're sure you only need one speed, and you already have the oscillator and a lifetime supply of sockets, then the 6850 will be fine.