Campbell Hausfeld Serial I/O Interface SDM-SIO4 - Manual (Page 29)

Section 4. Programming the SDM-SIO4
4-5
testio [test#]
This is used by Campbell Scientific for test purposes.
Each test command is carried out for 2 seconds. Test# can be in the range 0-255;
only five test numbers are currently valid, as shown below.
testio returns
test 0 DTR,RTS,IO=LOW
test1 DTR=HIGH,RTS,IO=LOW
test2 DTR=LOW,RTS=HIGH,IO=LOW
test3 DTR,RTS=HIGH,IO=LOW
test4 DTR,RTS=LOW,IO=HIGH