--- a/doc/RF-cal/Architecture	Sun Apr 23 18:01:17 2017 +0000
+++ b/doc/RF-cal/Architecture	Sun Apr 23 21:03:44 2017 +0000
@@ -49,4 +49,5 @@
 perform the required measurement or signal generation on the given type of test
 equipment.
 
-The details of the TSID socket interface remain to be defined.
+See the Test-system-interface document for the details of the TSID socket
+interface.

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/doc/RF-cal/Test-system-interface	Sun Apr 23 21:03:44 2017 +0000
@@ -0,0 +1,46 @@
+The Test System Interface Daemon (TSID) will provide a connection-oriented
+(SOCK_STREAM) UNIX domain local socket at /tmp/fc_rftest_socket, and will speak
+an ASCII line-based command-response protocol on this socket.  All command and
+response lines will end with \n only (no \r), and each TSID response including
+the initial greeting on connection establishment will begin with a '+' or '-'
+character to indicate OK or error, respectively, like in POP3.
+
+The following commands are currently defined:
+
+vcxo-cal-setup band arfcn
+
+This command is sent to the TSID to prepare the RF test system for the VCXO
+calibration procedure.  The first argument after the command keyword is the
+band to be used (850, 900, 1800 or 1900) and the second argument is the ARFCN
+in that band on which the DUT will be transmitting.
+
+freq-meas hint
+
+This command directs the RF test system to take a frequency offset measurement
+(vcxo-cal-setup must have been performed previously) and report it back to the
+client in the response line.  The hint argument after the freq-meas keyword will
+be sent by the VCXO calibration program (fc-rfcal-vcxo) to tell the RF test
+system what kind of frequency offset measurement is being made, i.e., which step
+in the VCXO calibration sequence is being performed; the hint keywords remain
+to be defined.
+
+signal-gen-setup band
+
+This command is sent to the TSID to prepare the RF test system for tests in
+which the latter acts as a signal generator.  The argument after the command
+keyword is the band number (850, 900, 1800 or 1900), and it selects the
+downlink frequency band in which the signal generator will need to operate.
+
+signal-gen-sine arfcn offset level
+
+This command directs the RF test system to turn on its signal generator and put
+out a pure sine wave at the frequency corresponding to the specified ARFCN (in
+the downlink band previously selected with signal-get-setup) plus the specified
+offset in kHz (-67 or +67 with TI's Rx path calibration procedures), and at the
+specified Tx power level in dBm.
+
+signal-gen-off
+
+Turn off the signal generator.
+
+The commands for Tx power level calibration remain to be defined.