MCS260B
CORNERSTONE 260B MONOCHROMATORS
44
15.2 TROUBLESHOOTING RS-232
One of the most common problems when first connecting a PC to an RS-232 device is that the
cable may swap the signals. The cable should be the straight-through type, and i
f you aren’t sure
that it is, there are a couple of things you can try.
The simplest is to connect an ohmmeter between pin 3 on the PC end of the cable and pin 3 on
the device end. There should be no resistance between them. The same goes for pin 2. And just
to be sure, also check pin 5 because sometimes there is a break in the cable.
The nomenclature for RS-232 has suffered a great deal of re-
naming and the “standard” has
become non-standard, but most PCs now transmit signals on pin 3 of their DB-9 connector and
receive signals from pin 2. The Transmit signal (Tx) is held at roughly -9V when not transmitting,
and the Receive signal (Rx) is floating. So another troubleshooting method is to first connect one
end of the cable to the PC, and connect a voltmeter between pins 3 and 5 on the other end. You
should see some negative voltage between pin 3 and pin 5, and close to 0V between pin 2 and
pin 5.
Figure 36: DB-9 connector.
The CS260B receives on its pin 3 (device Rx) and transmits on its pin 2 (device Tx). The PC
transmits to the CS260B
’s Receive pin and receives from the CS260B’s Transmit pin.
The CS260B implements two RS-232 communications modes that may be useful when problems
arise. They are “echo” and “handshake”. See the following sections for more information about
them.
One of the most useful tools is called a “sniffer”. It monitors and displays traffic on the transmit
and receive lines so that you can verify that the PC is sending what you want it to and examine
what the device is sending. The EZ-Tap from Stratus Engineering is a good example.
The “activity” LED on the CS260B panel blinks rapidly when the CS260B receives a command on
the RS-232 or USB. This is the small light on the left side of the panel. It normally blinks at a 1 Hz
rate to show that the unit is powered up and ready. It blinks quickly when the grating or filter
wheel are moving, or when a command is received on the USB or RS-232 port. Make sure it
blinks quickly a couple of times when you send a command.
Another common problem is using the wrong com port identifier when using a USB-to-Serial
converter. On Windows machines, you can use the Devices and Printers screen from the startup
menu to
find a converter’s COM number.
DB-9 connector at end of cable. Other end is connected to PC.
6
5
1
9
V from pin 3 (PC Tx) to pin 5 (gnd) ~ -9V
