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KB80313
Slow baud rates affect communication between SNAP-PAC-S controllers and serial I/O units
Revision:  4.0
Published:  12/5/2008
Applies To
Firmware for:
SNAP-PAC-S1
SNAP-PAC-S2
B3000
B3000-B
Also affects:
Versions
Problem was discovered in version:  
R8.2a
Problem is fixed in version:  
R8.3a

SYMPTOMS

SNAP-PAC-S controller serial ports cannot communicate at slower baud rates on 2-wire, half-duplex, RS-485 links such as when communicating to a SNAP-PAC-SB1, SNAP-PAC-SB2, or the B3000-B brains. This occurs when attempting to transmit from the serial port immediately after receiving characters. Communication works correctly at 38400 baud or faster. All four serial ports are affected.

On the SNAP-PAC-S controller, you may see one green LED flash (transmit transaction), then one red flash (receive transaction), or you may see one green, one red, and then one more green flash.

WORKAROUND

The workaround on a SNAP-PAC-S2 for this problem is to use 4-wire mode instead of 2-wire mode when communicating at slower baud rates. This work around does not apply to the SNAP-PAC-S1.

RESOLUTION

Opto 22 has resolved this issue.

In addition to downloading the updated controller firmware, you will need to enter a Turn Around Delay for the controller serial port you are using in PAC Manager. 

Follow these instructions to set the Turn Around Delay in PAC Manager for serial ports that are operating at a baud rate slower than 38.4Kbd.

  1. Start PAC Manager and select Tools > Inspect.
  2. Enter the SNAP PAC S IP address and click Status Read.
  3. Click Status Write, and scroll through the Status Write fields until you see the Turn-Around Delay for Port settings.
  4. Click the Value field to set a value in milliseconds based on the minimum recommendations below. (In general, as the baud rate decreases, the Turn-Around Delay increases.) The default value is 0 and can be used for baud rates of 38.4 Kbd and higher.
  5. 4 ms for 19200 baud

    4 ms for 9600 baud

    6 ms for 4800 baud

    10 ms for 2400 baud

    18 ms for 1200 baud

  6. Click the Apply button.
  7. Highlight the Store configuration to flash command and click the Send Command button.
  8. You may also need to increase the I/O Unit timeout (PAC Control configure mode, Configure > I/O, select your I/O unit and click Modify).

Relevant Downloads
No relevant downloads have been specified.
Questions? Contact Opto 22 Product Support.
Phone: 800-835-6786 or 951-695-3080
Email: support@opto22.com

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This Opto 22 Knowledge Base ('OptoKB') article is intended to provide general technical information on a particular subject or subjects and is not an exhaustive treatment of such subjects. Accordingly, the information in this OptoKB article is not intended to constitute application, design, software, or other professional engineering advice or services. Opto 22 may modify the OptoKB articles at any time. Before making any decision or taking any action which might affect your equipment, you should consult a qualified professional.

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