A.1. Response FIFO Data Format
A.1.1. LATp Overview
A brief overview of LATp is presented next. Read LAT Communications for more details.
In LATp data is transmitted and received in the form of a packet, which consists of 1 control cell followed by 0 or more data cells. A packet always has a control cell.
Preceding every cell on the wire is a two-bit field called the delineator. If the MSB of the delineator is set, a cell is assumed to immediately follow the delineator. The LSB of the delineator indicates the type of the following cell – control cell (1), data cell (0).
Cells are 128 bits long regardless of type. Following every cell is a truncate bit (see Flow Control, Section A.1.3 ) and a parity bit, containing the odd parity of the preceding 129 bits.
Control cells consist of a 16-bit header, one bit of which is the odd parity of the header, followed by 112 bits (seven 16-bit words) of payload.
A packet may optionally have data cells following the control cell. Data cells consist of 128 bits (eight 16-bit words) of payload.
An isolated LATp packet consisting of 1 control cell and 1 data cell is shown in Figure A-1. The following sections describe the parity and 16-bit word mapping depicted in Figure A-1 as it would appear in the 18-bit wide response FIFO.
 | An isolated LATp packet ( no other packets immediately following ) must be followed by a word of all zeros in the response FIFO. In particular the Cell Announce and Cell Type bit fields must be set to zero. This indicates that the packet is complete. In the case of back to back packets the word of all zeros would not be needed as the Cell Announce and Cell Type bit fields of the next packet would both be set indicating the start of a new packet. Again, see LAT Communications for examples of back to back packets. |
A.1.2. Mapping LATp to Response FIFO
LATp is a bit serial wire protocol that must be mapped into multiple 18-bit chunks to fit in the response FIFO of the VME LAT COMM I/O Board.
It is straight forward to store a 128-bit cell as eight 16-bit words into the 16 LSB of the FIFO. The two MSB of an 18-bit FIFO word can also be used to store the two delineator bits, truncate bit and cell parity error bit as follows:
The two-bit delineator will accompany the first 16-bit word of a cell in the response FIFO as the two MSB. This is indicated in Figure A-1 as the Cell Announce and Cell Type bit fields.
The truncate bit and the cell parity error bit will occupy the two MSB of the 18-bit FIFO word immediately following the last (eighth) 16-bit word of a cell in the response FIFO. The 16 LSB of this FIFO word are unspecified. Refer to Figure A-1 for details.
| The two MSB of the 18-bit FIFO word are unspecified, except for:
|
A.1.3. Flow Control
LATp has the concept of flow control between transmitters and receivers – for complete details read LAT Communications. To summarize the truncate bit ( see Figure A-1 ) is set by the transmitter when the following conditions are all true
The transmitter detects busy while transmitting a cell AND
The cell is not the last cell in the packet.
As a consequence in the command response protocol we will never see the truncate bit set because the response packet consists of only 1 control cell. During event taking, however, we may see a set truncate bit because event packets consist of multiple cells.
| Sending data with flow control requires two wires, one for the transmitter to send data and one for the receiver to assert busy. |