Multiport Serial Card


Thread Starter

Ulianov Gil

Hi my name is Ulianov Gil, i'm working in Honeywell Venezuela, and i have a question!

i have a single PC that i need comunicate with 10 Modbus device, so I use a multiport serial card to perform that. The multiport card give me 8 port but i need 10 port , so I have this question:

may i install two multiport card in a single pc?

if the answer if yes!

may it load the PC?

if the answer if no!

what else posibilities i have to perform this activities?

Sorry for my very bad english

and thank you for all

Ulianov Gil

If your PC has a USB port, then get a USB-Serial device. You can get them with multiple serial ports and then there is no card to install and no
worries about interrupts etc.

L. R. Schneider

Is there any chance you can multidrop some of the 10 units on the same serial port? If the Modbus slave addresses are different they can be multidropped however you may have to use RS232/485 converters.

I have used two 8 port serial boards (16 additional ports) under Windows 2000 without a problem. The boards were Rocket Port cards from B & B Electronics ( In addition to the 8 port models they also have 4 port ones available.

L. R. Schneider
Calta Computer Systems Ltd.

Curt Wuollet

Yes you can use more than one.
Will it load the PC? It depends on what type of multiport card. I use Rocketport cards as the Linux support is excellent (Ted T'so works for Comtrol) and the cpu utilization is very low. There are "dumb" cards which are essentially just N serial ports. These are cheap but require quite a bit of CPU time. The more intelligent cards have one or more coprocessors. These cards have been and still are often used with PC *nix of some sort to support lots of users or modems. The quality of the drivers is also very important. I can't help you much with Windows but I have supported 64 users on a 266 mhz. Linux PC without swamping the cpu. Modbus with it's fussy timing might be a different story.



Aaron Gelfand


Depending upon the vendor of the card, you should be able to install more than one serial port expansion card in a computer. Some of the key
factors will be whether the vendor's driver supports more than one instance of the serial port board, and how interrupt sharing on serial
port's is handled. Windows itself has a limitation of no more than 256 com ports.

Aaron Gelfand
National Instruments

Kirk S. Hegwood

Is a USB suitable for industrial applications? The USB hub I have in my office has a tendency to "blip" out occasionally.

Just a thought,

Kirk S. Hegwood
Signing for Hegwood Electric Service, Inc.
[email protected]

Brian Delsey

Consider using an external Ethernet based terminal server for flexibility. See some of the examples from DigiBoard at the following link:


One benefit of this route is the ability to have a backup control PC on the network that can provide quick switchover should your primary PC fail. The devices are simple to use - with the driver installed on your PC, the IP based ports appear as native COM ports on your computer. You can have multiple units on your LAN and thus you have an easy expansion path.

Brian Delsey
IBI Group
Had the same the same problem with one I used, an EdgePort/8 model. When one of the devices on the other end started streaming it knocked the other 7 ports out. It only happenned once in 2 years but it hurt. It's a tough call.

Jay Kirsch