To: File file: vlba4_training_exercise1

From: Jon Hagen

Date: April 4, 2002

Subject: BBC/formatter/decoder test: using the decoder to check data from the formatter.

Reference: See the SNAP command manual for details of the SNAP commands used here.

The formatter does not produce output data unless its output drivers are enabled. This is done with the SNAP command tracks. Example: "tracks = v1,v2,v3,v4" enables the four VLBA track groups in the formatter.

Formatted data can reach the decoder only via the bypass connection that is made through the Monitor Module in the recorder rack (see Figure 1). The data proceeds on ribbon cables from the formatter to the recorder rack write modules. Two selected one-bit write signals (from two write heads selected via" repro = .....") are sent as "bypass" signals to the monitor module. The monitor module also receives two signals selected from the read module(s). Another field in the "repro= ...." command tells the monitor module which pair of signals, "bypass" or "read head", to send to the decoder. (Note that the selected signals can be either sign or magnitude bits or one of each).

The selected bypass signals will, however, not get to the monitor module unless the write modules are enabled. This enabling can be done with the SNAP command "enable=...."

Example1 : "enable=g0,g1,g2,g3" enables all four head groups in Headstack 1.

Example 2: "enable=g0,g7" enables Group 1 in Headstack 1 and Group 3 in Headstack 2.

Example Test Procedure

Suppose we want the A channel of the decoder to monitor BBC3 USB SIGN and the B channel of the decoder to monitor BBC7 LSB MAG. We will assign these two bits to heads 24 and 25 of Headstack 1. (It might be that we are more interested in testing these particular data paths than in testing the BBCs).

0. Get the field system started; at Arecibo, our user name is oper and our password is vlba4oper Start the field system with the command fs .

1. Assign the BBC data to selected paths using the SNAP command trackform:

trackform=24,3us,25,7lm (note, "24,3us" rather than "24,03us"; leading zeros are not accepted)

2. Enable the formatter output drivers using the SNAP command tracks:

tracks=v0,v1,v2,v3

This enables all the formatter signal groups destined for Headstack 1. (We could have enabled only the formatter group(s) that include the signals destined for heads 24 and 25).

3. Tell the formatter to act on the two preceding commands:

form=m

4. Enable the write drivers in the recorder rack

enable=g0,g1,g2,g3

This enables all the heads in Headstack 1. (We could have enabled only the write group(s) that include heads 24 and 25).

5. Select the head signals for the two decoder channels, A and B.

repro= byp,24,25,byp

The decoder should now be displaying the time, aux data, errs (none), and pcal (pcal always displays an answer, even when no tone is present). Check that the aux data is correct:

6. If you don't want to get up to look at the decoder display, you can read it using

decode4=aux

The screen will display the aux data for both decoder channels. The format is explained in Alan R. Whitney's memo, "Mark IIIA/IV/VLBA Tape Formats, Recording Modes and Compatibility." When this procedure was performed, the aux data displayed were as follows:

aaaa bbbb rrss ttuu

Decoder Channel A aux data 0000 0000 2402 006D

Decoder Channel B aux data 0000 0000 2536 006D

The last two hex characters, uu, show the system ID, set by hard wiring in the DAS rack. Arecibo's ID number is 109 (= Hex 6D).

The bits in the two zeros, tt, preceding the 6Ds show, from left to right, that there were no time sync errors, no internal clock errors, no processor time-out errors, no communications errors, unused, unused, that barrel roll was not enabled, and that the sequence was not suspended for command processing.

The rrss field for the A Channel indicates that the A Channel was getting data from head stack 1, head 24 and that that head was getting the sign bit of the USB signal from BBC3, just as had been commanded. Likewise, the rrss field for the B Channel indicates headstack 1, head 25, and magnitude bit from the lower sideband of BBC7.

The aaaa and bbbb fields indicate the positions, in microns, of Headstack 1 and Headstack 2. Here, both heads were found parked at the zero position.

The head positioners can be tested as follows:

Define a couple of headstack positions ('passes") using

tapeform=12,−150, 6,120

This assigns arbitrarily chosen pass numbers 12 and 6 to the respective positions -150 microns and 120 microns. (These positions are relative to an offset position stored the file head.ctl).

Command head 1 to the pass 12 position and head 2 to the pass 6 position using

pass=12,6

The head position fields in the aux data from the decoder will read 4150 and 0120, BCD representations of -150 and 120. The aux data is now

aaaa bbbb rrss ttuu

Decoder Channel A aux data 4150 0120 2402 006D

Decoder Channel B aux data 4150 0120 2536 006D

Figure 1 VLBA4 Data Paths