What serial cable do I need?

June 2, 2006 11:20 AM

I know nothing about serial cables, and I'm having trouble figuring out what I need, or what's going on with my receipt printers.

Non-profit retail store. Two computers, one old, one new. Serial cable from the COM port to the receipt printer, network-type cable from the receipt printer to the cash box, which opens when a receipt prints.

When I set up the new computer I just used the existing serial cable set up (from the old computer) for the receipt printer. It has a 25-pin male (into the COM port) to 25-pin female cable going into a 25-pin male to 9-pin female converter. The 9-pin female end of the converter goes into the receipt printer. This works fine, on either computer.

I bought a 25-pin male to 9-pin female cable to get the receipt printer set up on the second cash. It doesn't work on either computer.

Is the cable defective, or is there some cabling voodoo going on with the converter? Something else?

posted by valleys to computers & internet (5 answers total)

You bought a regular serial cable. You need what is called a "null modem" cable. The adaptor on the working cable is probably a null modem adaptor. What this does is switch a couple of the wires around in the cable, so they connect to different places at either end of the cable.

Hard to explain. But anyway, you can either get a 25-pin male to 9pin female NULL MODEM cable, as opposed to a regular serial cable; OR, you can buy a null modem adaptor (there would be several possible adaptors, depending on where you put it - i.e., with your existing cable, you could get a 25-pin female-male adaptor, and put it on one end of the cable, or a 9pin female-male adaptor, and put it on the other).

Probably easiest just to get a 25-pin male to 9pin female NULL MODEM cable (often specifically sold as printer cables, for more money).

I apologize on behalf of the technical world. It's our fault that you bought the wrong cable. On the upside, we did solve many of these problems with USB.
posted by jellicle at 11:53 AM on June 2, 2006

Wow, thanks for the quick response. I'll get to a supply store tomorrow and pick up something. I just looked at the adaptor again - it doesn't identify itself as anything special (i.e. no writing / markings or anything to identify itself as null modem). Hopefully that's what it is.

And yeah - USB has made life a lot easier!
posted by valleys at 12:05 PM on June 2, 2006

OK jellicle ... I owe you a very large beverage of your choice. It worked.

That's (hopefully) the final step in what has been a very long and frustrating upgrade process.
posted by valleys at 5:28 PM on June 2, 2006

The more common setup is to have a regular 25-to-9 adapter and a null cable. Regardless of whether you've got a 9-to-9 cable or a 25-to-25 cable, that's where you'd do the nulling. Something to keep in mind if they don't have a "25-to-9 null adapter".
posted by intermod at 9:05 PM on June 2, 2006

Thanks. I was able to find a 25-to-9 null cable which works and cost me $10. I'll be returning the $25 regular cable my tech guy sold me!
posted by valleys at 8:40 AM on June 3, 2006