Best serial processing PLC

I guess it depend on what type of "processing" is needed. Take a look at this http://www.selinc.com/SEL-2411/

I second BobB's post on the OMRON CJ Series PLC's. Up to 16 units with 2 ports per unit [32 Ports] can be used. Very flexible using either Transmit/Receive commands OR Protocol Macros [Customizable for each port] called from the PLC program and managed automatically AND independently of program scan. V. Powerful and Fast.
My 2 Cents.

I have maintained systems (not PLC) that use ******* and they were completely trouble free. Here's a link to their industrial stuff.

http://www.*******.com/pub/en/Industrial1

Shawn

Okay.............here's a tiny URL as I cannot, apparently, type the name.

http://tinyurl.com/3oa2y67

I saw these at last year's Automation Fair but haven't had a chance to use any yet: http://www.wrcakron.com/devicenet_gateways.html.

A PLC-5 project I worked on had one- and four-port 1771 modules. These must be the successors.

I would go with CLX platform and Red Lion G3.G3 Data station have 2 RS232 Ports and 1 RS485.
Also they have Ethernet I/P driver so the CLX see the unit as remote IO.
The G3 is very easy to program and you have very good support from RLC.
We use it as remote serial ports for scale and temp controllers.Also for analog IO
(It save connections compering AB cards)
You will need few units but it worth.
My second option is Omron.As long as you know how to program it and not to start from beginning.
It would be cheaper then AB+RLC.

I would recoemnd B&B Electronics. Thye have a large line of serial converters.I would call them and hear what they remcomend. I know you said you did not want to set up an ethernet network but this may help you down the roda and just have ethernet to serial drops at the devices to convert your protocal.

I have used these guys in the past and it was always with good results.

I've had good success with the Prosoft cards that the_plc_kid mentioned. I used two of the MVI56-GSC for RS232 communications from CLX to a product dosing controller several years ago and I'm working on a project now using the MVI56-MCM to get information from a "black box" power monitoring system that has Modbus to CLX.