Serial to USB cable - varying results

[brid0030]

I would like to use a Serial to USB cable to communicate with a PiC16F688 which is running through an RS232 type level converter (actually an ICL2323). If I do straight serial communication with a com port on an older PC, everything works fine all the time. I can also communicate with a palm pilot, no problem. So I've established that straight serial works fine. But when I try serial to USB cables, the results vary from device to device. My first attempts failed utterly--I only got some periodic gibberish on the PC. A colleauge working with the same PIC-driven device also failed to establish communication, but then she tried a different (newer) PC and got it to work. I'm at a loss to see why there should be these discrepancies--serial to usb ought to be pretty simple right? Anyway, I was wondering if you had any insights because I have several people who will be using these devices, and I need to be able to recommend communication setups that work.

One possible clue is flow control. I tried setting up serial communication between a palm pilot and the PC using the serial to USB cable, and I could not get it to work until I messed around with the flow controls. Flow control is not necessary when doing straight serial communication between the same palm and PC.

[mackrackit]

The converters I find most reliable are
U209-000-R from Tripp Lite. They seem to be pretty close to RS232.
Belkins are not reliable, does not have the voltage swing needed by some things.

[BrianT]

Bootloaders seem very time sensitive to the variable delays when using USB to RS232 converters. Colleagues and I have tried at least half a dozen different looking adapters with netbooks, laptops notebooks and desk PCs. NONE gave 100% reliable results. Some mostly work, some never work. They are all Chinese and are generally fine for dumb ASCII communications but whenever accurate timing between sending a character and getting a reply is needed then it's back to an old PC with dedicated RS-232 DB9 connectors.

It was my design practice to always fit an RS-232/TTL port to every design and use the MCSP bootloader for field bugfixes. I have abandoned that now and use the MeLabs U2 USB programmer for all code programming and updates. I have requested Mecanique to have a look at relaxing the timing on the MCSP bootloader but that message sank without trace.

HTH
Brian

[Dave]

brid0030, I only use the products from FTDI and have had no issues using them with bootloaders or normal programming serial communications. We deal with lot's of different inferfaces here at work and the FTDI seems to be one of the best I have found. The bootloading seems to be a bit slower but never fails....

Dave Purola,
N8NTA

[Charles Linquis]

MCLoader does have some issues - especially with USB-serial. I made it work much better by disassembling it, adding a 16 bit delay after the 1st jump vector and before the loader actually started executing and then re-assembling it. I now have something that works all the time. I use only a two-wire connection (I don't control MCLR). It still loads slowly, but at least it always gets started when I turn power on.

[brid0030]

Well, I'm not attempting bootloading, just serin and serout. I'm just gonna buy a few cables and see what works. The option I will put forth to users first is to get a PC card with a com port....or just dust off that old laptop from 6 years ago.