Share a com port

[geoffl]

Hi,
I share a printer on a my network. Is it possible to share a com port?
I have 2 computers  running  a cash register program and want both of them to open the cash draw which is connected to a com port.
Any help greatly appreciated.
Geoffl

[DaveLembke]

Please share what type of POS systems you are using. There are many types.

 I use to service POS systems. The ones I serviced NCR and Fujitsu's had ports that looked like serial ports since they used 9 pin D-subs, but these ports sent 24VDC to the cash drawers solenoids to pop the drawers and were not serial communications! If one was to plug a serial device that is RS-232 into this port and the cash drawer was triggered to pop open it wouldn't be good sending 24VDC onto a 5 volt RS-232 serial circuit.

Knowing what you have for a system will better determine your options as for if its not serial communications, but instead a 24VDC output as is more common with  POS systems, you will need to create a Y Cable with Diodes added to prevent blowing the drivers out of each of the POS computer systems. If it is a real serial connection there are adapters out there that can share a single serial communication device with 2 devices sending serial communications. B & B Electronics has a bunch of solutions like this. http://www.bb-elec.com/subcategory.asp?subcategoryid=93

I also custom built an optoisolated serial splitter once that allowed me to download to 4 PLC's at the same time from a single serial connection. One of the PLC's kept the TX/RX handshake with the computer, and the 3 others RX (listened to) the information, but the TX lines back to the computer were open ( not reporting back) at 9600 baud on RS-232 this worked really well for the Allen Bradley PLC's that I serviced for Rockwell Automation. Was a great job there until laid off in 2001 after almost 7 years working as an ET and my job went to Mexico and I decided to stay in the USA even though given the option to relocate to Mexico and lead the service department down there. I took option B which was teach others how to do my job and take a $5000 bonus and severance package

[geoffl]

Hi,
Thanks for your reply. The POS system we wrote in an old MS DOS data base language.
It runs under NTVDM in XP. I have one desktop with a COM port. A device plugs into this. I send 'echo "UUUU" > com1:'  and this device triggers the cashdraw.
The second machine is a laptop without a com port. I have a usb device from the same manufacturer which makes a virtual com port. Don't know why but this device shows up as com4. Echo UUUU > com4: and it triggers the drawer. I asked the manufacturer if I could use some sort of splitter to use the two triggers on one drawer.  He was most unimpressed and thought one trigger current will feed back and damage the other trigger. He suggests 'using a share of the resource over the network. Which leads to this question.
Geoffl

[Geek-9pm]

For this kind of hardware hacking, make sure all equipment has a common ground. Do not depend of just the power plug. Run a earth (ground) shield wire to all equipment.  Otherwise, small leakage currents and burn out serial ports. A shield wire is normal practice  in serial port wiring. But run an additional wire to be sure. A flexible #18 is good enough for this. It ties the chassis of all units together. It can be a insulated wire, exposed where needed. Put it under a screw.

[DaveLembke]

 

He was most unimpressed and thought one trigger current will feed back and damage the other trigger

This is why if this were NOT a serial connection you would want diodes on a custom made Y cable.

But since you are using Serial Communications direct from PC and NOT a 24VDC trigger off of a piece of Point of Sale Hardware, you are dealing with serial and I would buy a splitter from B & B Electronics or another merchant. This device has a common shield which Geek-9PM pointed out. Working with PLC's and industrial electronics I have seen what happens when a common shield is not used.

From my past experience when you ask for manufacturers to OK running their hardware differently, they generally avoid approving it and can only guarantee proper operation if you use the equipment as originally intended. For liability purposes  99.9% of the time they are not very helpful, however I was able to get some schematics once for a section of a Point of Sale system once to make a hardware alteration without trial and error. I was given the schematics with the statement of here is our circuitry and whatever alterations you make with it we are not liable for as well as may void the warranty.

[geoffl]

Hi,
Thank you so much for your interest.
Would the shield on the device from B&B be enough. Geek-9pm  seemed to be saying it should be connected to the computer as well.
From your answer I guess you can not  share a com port.
I am sure you did the right thing staying in the US.
Regards
Geoffl

[Geek-9pm]

Hi,
Thank you so much for your interest.
Would the shield on the device from B&B be enough. Geek-9pm  seemed to be saying it should be connected to the computer as well.
From your answer I guess you can not  share a com port.
I am sure you did the right thing staying in the US.
Regards
Geoffl

I did not mean to discourage you. Having a common ground is standard practice in many industrial applications. POS installations can vary widely from one store to another. If all the equipment already has a common grounding system, ignore what In aid.
My remarks are  for amateurs doing a ad-hoc install. An install that has not been mapped out ahead of time.
For the record, serial chips used in PCs do NOT withstand any kind of abuse. The fail when exposed to voltages encountered with ungrounded equipment. Putting in a around wire is trivial as compared to the rest of then install. Traditional serial connectors are seldom wire  for 'hot swap' usage.
Also, using ungrounded equipment in a retail store violates most fire and safety codes. My issue is that the job is gang done by amateurs who give little thought as to the safety risk of  doing an merriment in a place of business.

Do I need to paint a picture? At the least, you will fry a laptop computer. At the other extreme people could be killed.
This may sound harsh. It is not harsh. It is a serious warming about doing hardware hacks in a place of business.