Swordfish

garryp4 · Joined: 21 May 2007 Posts: 47 Location: Loveland, CO USA

Has anyone tried to do 7-bit ascii? I have a project to communicate with an old system that uses it.

Raistlin · Joined: 01 Apr 2008 Posts: 61

7 bit ASCII is easy enough , depends on what your aiming to do 7 bit ASCII is the same as 8bit only without the other codes Ie highest 7 bit ASCII is 01111111
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If you can read this you are too close

Jerry Messina · Joined: 30 Jan 2009 Posts: 99 Location: Frederick, MD

I assume you're talking about the hardware uart and 7-bit mode. Since the uart in the pic only supports 8/9 bit operation (depending on the pic), you'll have to manually manipulate the eighth bit to match what the end system is expecting. You may have to use it for parity, a stop bit, etc...exactly what depends on what you're talking to. I did this with a system using 7O1, and it worked fine.

Jerry

garryp4 · Joined: 21 May 2007 Posts: 47 Location: Loveland, CO USA

Thanks for the quick reply. Its a standard called SDI-12. The communication is 7n1 at 1200 baud. Kind of dates the standard, but have a need none the less. I did look at the code for the usart and suart libraries but do not know assembly. That's why I use swordfish.

What changes did you have to make to talk 7O1?

Thanks.

Jerry Messina · Joined: 30 Jan 2009 Posts: 99 Location: Frederick, MD

To support 7O1 I had to make a number of changes to support dealing with the parity bit and inserting it into the eighth data bit.

For 7N1, things are a bit easier. Since there's no parity, the eighth bit is the stop bit (which is the same as an idle line), or a logic 1.
To transmit a byte, simple OR the ascii data with $80, and when receiving AND it with $7F.

When you transmit, you'll actually be sending a byte that looks like 7N2 since the uart will add a real stop bit, but the receiving system won't care.

When you receive, since the uart requires eight bits, you'll actually get the stop bit in bit 7 (which is why you AND out the msb).

There is one hiccup, and that's if the remote system is capable of sending back-to-back transmissions. For 7N1, a uart has 9 bits to deal with...
<start bit> <7 data bits> <stop bit>. The uart of the pic requires 10 bits...<start bit> <eight data bits> <stop bit>. For this to work, the remote
system must idle the line in between bytes for at least one bit time so that the pic's uart will see the idle line as a stop bit. If it doesn't do this,
then you can't use the hardware uart, and would have to resort to using a modified software uart that would deal with 7 bits.

Hopefully, it's an old, slow system and you'll have the extra bit time you need.

Jerry

nigle · Joined: 12 Aug 2008 Posts: 36 Location: West London

Jerry Messina · Joined: 30 Jan 2009 Posts: 99 Location: Frederick, MD

Good catch nigle.

Here's some bits of code for dealing with transmitting 7E1...

garryp4 · Joined: 21 May 2007 Posts: 47 Location: Loveland, CO USA

Thanks for the replies. And Nigle, you are correct. I want 7E1 not 7N1. Think I am so used to 8N1 and failed to check what I typed.

So, I came up with