8.6 Troubleshooting - RS232C
This section provides guidelines for resolving communication problems that may occur when
using the PGUI32 software under Windows .
You may encounter problems when using the serial interface with the PGUI32 program that
is supplied with the interface option for this source.
Symptoms:
1. Unable to verify connection to the power source through RS232C interface. Time-out
occurs and dialog box appears that asks if you want to switch to simulation mode. An
error message (ERR -100) may appear on the front panel 7 segment LED‟s of the power
source.
- Or -
2. Verification is successful but slewing of voltage, frequency or current limit results in an
ERR -100.
Things to check first:
1. Is the PC COM port you selected to use really available for this purpose? On older PC‟s
COM port interrupts are often shared between COM1 and COM3 as well as COM2 and
COM4. If any of these other com ports is in use, it may prevent you from using the COM
port that shares the same interrupt.
2. Did you use the RS232C cable that was supplied with the P/RP AC source? This is not a
null-modem cable and cannot easily be obtained at a computer store. If you do not have
the original cable, consult the PGUI32 on line help under Interface settings for a wiring
diagram.
3. Did you connect the cable in the right direction? This cable is unidirectional and should
be marked with a SOURCE and a PC label. Make sure you connect the SOURCE side of
the cable to the back of the AC source and the PC side of the cable to the PC port.
Resolution for Symptom 1
The P and RP Series of AC sources require hardware handshaking to control data flow from
the PC to the AC source. After receiving a command, the AC source asserts the DTR line to
hold off further communication from the PC until the command just sent has been processed.
Under Windows 95, it is possible to use a 16 byte transmit buffer if a 16550 UART is present.
This FIFO exceeds the length of most AC source commands. When enabled, this mode
may result in more than one command being placed in the UART output buffer. If this
happens, the handshake from the AC source will not prevent the content of this buffer being
sent to the AC source and thus will result in more than one command being sent to the AC
source.
To resolve this problem, the UART FIFO length needs to be reduced or disabled altogether.
The procedure to disabled the FIFO mode is outlined in the next paragraph.
Note: If turning off the UART FIFO conflicts with other applications that need to run on the
same PC using the same COM port, this solution may not be acceptable. In this case, skip
this step and proceed to the second symptom resolution which relies on a software
implemented hold-off instead of turning off the FIFO buffer. If you are using Windows 3.1 and
experience the same problem, the latter method is the only resolution available to you so you
should skip the next paragraph as well.
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