American Magnetics 135 & 136 Liquid Helium Level Instruments (CE-Marked) - Manual (Page 33)

Rev. 3 21
4 Remote Interface Reference
4.1 Serial Communication/Data Logger Option
The serial communication/data logger option provides a 25-pin D-type
connector on the rear panel of the instrument for serial communications
and data logger function.
4.1.1 Serial port connector and cabling
An IBM-compatible computer’s serial port can be directly connected to the
Model 135/136 via a standard PC modem cable. Refer to your computer’s
documentation to determine which serial ports are available on your
computer and the required connector type. The cable to connect two DB25
connectors is wired directly, i.e. pin 1 to pin 1, pin 2 to pin 2, etc. If a DB9
connector is required at the computer interface, the connector translation
is provided in the Appendix.
The Model 135/136 uses only three wires of the rear-panel DB25
connector: pin 2 (receive), pin 3 (transmit), and pin 7 (common). There is
no software or hardware handshaking. The Model 135/136 is classified as a
DCE (Data Communication Equipment) device since it transmits data on
pin 3 and receives data on pin 2. The instrument to which the Model 135/
136 is attached must do the opposite, i.e., transmit on pin 2 and receive on
pin 3 (the requirements for a DTE, or Data Terminal Equipment device). If
a serial-to-parallel converter is used, it must be capable of receiving data
on pin 3 or the cable connected to the Model 135/136 must interchange the
wires between pins 2 and 3.
Optional RS-422 connector pinout is provided on page 58.
4.1.2 Command/return termination characters
All commands are transmitted and received as ASCII values and are case
insensitive. The Model 135/136 always transmits <CR><LF> (i.e. a
carriage return followed by a linefeed) at the end of an RS-232
transmission. The Model 135/136 can accept <CR>, <LF>, <CR><LF>, or
<LF><CR> as termination characters from an external computer.
The simplest method for communicating with the Model 135/136 via
RS-232 is by using the interactive mode of a commercially available
terminal emulation program. The Model 135/136 transmits and receives
information at various baud rates and uses 8 data bits, no parity, and 1
stop bit. When the Model 135/136 receives a terminated ASCII string, it
always sends back a reply as soon as the string is processed. When sending
commands to the Model 135/136, you must wait for the reply from the
Model 135/136 before sending another command even if the reply consists