Fluke NetDAQ 2640A, Service Manual

Page: 86 / 260

NetDAQ
Service Manual
2-44
Temperature sensor transistor A4Q1 outputs a voltage inversely proportional to the
temperature of the input channel terminals. This voltage is 0.6V dc at 25
°
C, increasing
2 mV with each degree decrease in temperature, or decreasing 2 mV with each degree
increase in temperature. For high accuracy, A4Q1 is physically centered within and
thermally linked to the 20 input terminals. Local voltage reference A4VR1 and resistors
A4R1 through A4R3 set the calibrated operating current of the temperature sensor.
Capacitor A4C1 shunts noise and EMI to ground.
A1 Main to A3 A/D Converter Communications 2-76.
The exclusive means of communication between the inguard and outguard is a
bidirectional, asynchronous, optically-isolated serial link. This link operates at a rate of
120,000 baud. The individual bytes are transmitted with eight data bits, one stop bit, and
one even parity bit.
The outguard can send either a reset or a command to the inguard. A reset consists of a
number of consecutive break characters, and causes a complete reset of the inguard
hardware and software. The inguard returns no response to a reset. A command is a
six-byte packet (hereafter referred to as a ’command packet’) that causes the inguard to
perform some action and return one or more six-byte response packets. Transactions
between the outguard and inguard are always initiated by the outguard. The inguard
never sends data across the guard without being asked to do so.
There are two modes of communication between the inguard and outguard:
non-pipelined and pipelined. In the non-pipelined mode, commands and responses are
synchronous, i.e., the outguard waits for the response to a command before sending
another command. In the pipelined mode, the outguard may send a second command
before the first command has completed. The outguard must wait for the response from
the first command before sending a third command.
Special Codes 2-77.
An ACK response packet is arbitrarily defined as the sequence of bytes (42,0,0,0,0,x)
where x is the checksum byte. A NAK response packet is defined as
(255,255,255,255,255,x) where x is the checksum byte. A break is an all-zeros character
without stop bits.
Resets 2-78.
A reset consists of 5 ms of consecutive break characters sent to the inguard. A hardware
circuit on the inguard detects this condition and causes a complete reset of the inguard
subsystem. The inguard sends no response to a reset. After sending a reset, the outguard
must wait a predefined amount of time before attempting further communication with
the inguard. This is the same amount of time it waits after a power-up, approximately 3.5
seconds.