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USING COMTEST TO CHECK OPERATION
1. Make the Test Connections for Loopback and the required setting to Enable RX or Echo.
You should already have the Echo On or RX Enabled (422 mode) for 4-wire operation.
2. Start ComTest from your Programs menu under B&B Electronics.
3. Select the Com port number you want to check. ComTest will only show Windows based
Com ports that are not in use. If another program is using the port, it will not be available.
Please close down any other programs that may be accessing the port. If another program is
using the same IRQ number that port is using, you will not be able to use both ports at once
unless both devices are PCI or USB based.
4. Accept the default baud rate, data bits, parity, stop bits.
5. If you are checking a RTS Controlled Converter or port: Select Options, then RTS (check
marked) to set the RTS indicator red. If using a Port Powered Converter requiring DTR Hi for
power, make sure the DTR indicator is red, DTR (check marked).
6. Type some characters in the upper window. The same characters should appear in the lower
window. Note that the enter key value will be shown in brackets as <0D>
If data is shown in the lower window when typed in the upper, the Converter or port is working.
Actual signal levels can be checked with a oscilloscope. With no termination, the levels must
exceed 200 mV at the receiver, 4 to 4.5 Volts difference between the TD(A) and TD(B) lines is
typical when Transmitting.. Due to Receiver Biasing (refer to the RS-422/RS-485 Application
Note in our Technical Library), the normal DC voltage between RD(A) and RD(B) is about 4 volts
(without 120 ohms termination or any devices connected), RD(B) is positive relative to RD(A) or
signal ground/common. With a dual trace scope in DC coupling mode, with one channel on
TD(A), and the other channel on TD(B), the two lines can be seen as opposite levels, when A is
high, B is low and vice versa. The receive lines should have similar voltages input.
Note:
1. RTS Control units require RTS hardware control software to operate properly. RTS control
timing can be critical for half duplex RS-485 control, and can be erratic under windows.
2. SD Control units require a matching baud rate setting if preset for 9600 baud. Resistors
and/or capacitor values may need to be changed. In 4-wire mode, the SD timing needs to be
altered for slower baud rates, but not usually for faster baud rates. If transmit is set for RS-
422 mode (single master 4-wire RS-485), the baud rate makes no difference since the
transmitter is always active.
3. Many Common problems are caused by wiring custom cables for the RS-232 side, especially
DB9 to DB25 and not understanding DTE/DCE devices. Usually Rx and Tx are wrong. Refer
to our web site Support, Technical Library for DTE/DCE Illustrations.
ComTest features listed on next page.
4-WIRE RS-485 TESTING
LOOPBACK CHECKS
(14 Seiten)
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