RS232-RS485 Converters for MB Poll


Thread Starter

D. Freeman

Does anyone know what type of RS232-RS485 port powered converter is required for MB Poll?
Do you think my logic is flawed? Will this ever end?

I'm having trouble making connection (as in can't) with either of Whittecom's (AKA- MB Poll or MB Active X. A few months ago I purchased the software and it worked fine with the hardware I had.... once I learned how to set it up in my lil' spreadsheet......

Until recently, when we (one of my employees) experienced a "pop" noise when we plugged the laptop into the thermostat (his words, and account of the incident) like we would normally do. I know this is not a software issue.... or I am reasonably sure it's not a software issue, having loaded the demo versions onto a different machine (I do hate that 4 minute operational time limit).

But being a humble Welder from Texas (regardless of what the title in my signature says), I'm not that smart and so consequently, when I determined (or at least have reasonable suspicion for a welder) the Converter I had was bad (JaRa Model 2102E), I looked it up on the internet.... only to find the only websites to be found are written in Chinese. Not having the time, or mental facilities, to learn Chinese, and regardless of the endless links I clicked in Chinese, blindly trying to find a schematic, I have not been able to find any answers to my questions.

Finally getting to the point.... what type of RS232-RS485 Port Powered Converter do I need to use?? Does the Software(Driver) require handshaking? (That apparently has something more to do with communication than it does just being cordial.) So you don't think I'm a complete imbecile, I'll explain what type of hardware I had that was working, so you the Samurai of MODBUS, may be able to point me in the right direction.

HP Pavilion ze4805us, AMD Athlon XP-M 2800+ 2.13GHz Processor, 256MB RAM, 60GB Hard Drive, 15-inch XGA TFT Display, DVD / CD-RW Combo Drive, Windows XP Home Edition

JaRa Model 2102E RS232-RS485
(This particular converter is supplied with the RS-485 Pinout @ 5=G, 2=SD/RC+, 3=SD/RC- (I know this much from the endless hours spent (Man Mountain Dew is making a killing) looking into wiring diagrams, and my trusty lil' multi-tester (the only tool besides I've got besides the hammer and the welder(yall outa try to solder up a DB9 with a Lincoln SA-200) and finding out I had a 2 wire (why do they call it a 2 wire when I have 3? or for that matter a 4 wire when you've got 5?(yea I know the 4 wire is technically a RS-422)) configuration (AKA - "Half-Duplex))))))

But after trying 2 other models of converters, neither with handshaking capabilites,(I know, your thinking why did the moron buy the same type, but in my ignorance, the first local available RS232-RS485 was a DB25, so I sent off for a DB9, I didn't learn of this possibly necessary feature until $300 to $400 later. You know "buy 2 so I'll have a spare when it works) I still could not make a connection. I get a Time out error. Every stinkin time.... time out...... time out....

DB9 X RJ-11 (6 Pole) 36 Inches

TSTAT5 Zone Controller (2 wire RS485 linkable Thermostat w/ programming features developed for my application (Man that ain't happnin' again the current tally on my $4700 thermostat estimate is in the neighborhood of $35,000 w/labor, and I'm certainly not finished.))
I concur with Mr. -CH (Switzerland ? ?)

B&B Electronics 485SD9TB is the easiest non-isolated 232-485 converter I've run into.

I do spend the $15 and buy their 12Vdc power supply, because I have had more PC's provide inadequate port power than not.


Matthew Hyatt

For all of that I could have put in a simple $150 temp controller w/ readout and control capabilities. Seems a like a lot of wasted money for a $4700 thermostat which has now cost you over $45000 - the annual salary of a decent, hard working techie who could help solve your techie problems.

The B&B product is great, usually you have turn off the flow control settings for your com port when using these devices and they typically are setup to do some internal delays based upon sending or receiving data so no "handshaking" is required. I done many with just a 3 wire RS-232 cable to the converter. (octogon Systems makes a really nice unit as well!)


Automation Linse

It is impossible to find a 100% predictable "RS-232 port-powered RS-485 converter" for the simple fact that:
1) EIA/RS-232 allows a chip to only drive 1mA of "power"
2) EIA/RS-485 requires all chips to be able to drive 65mA

No blood from a turnip; a 1mA source cannot drive 65mA. (Note: there is a reason most vendors with RS-232 ports put a "you void warrentee if you use port-powered devices" in their manuals ...)

Most PC will put out from 5-12mA of power on the RS-232 port, so it really comes down to how much of a load is on your RS-485? How many slaves are there? Can a strong enough RS-485 signal be created given the limited power sucked from the RS-232 port? If you use a passive capacitor-coupled terminator you may have better luck.

However, for professional/commercial use I'd suggest:
A) using only power RS-232/485 for fixed "field systems" - it will give you 100% repeatable performance 365 days a year.
B) using port-power only for temporary use - testing, etc.

best regards
- LynnL,