HP 3457A Service Manual page 183

PLUG-IN MODULES (44491, etc)
RELAY CONTROL (RL) -- A2Q121, A2U121
MISCELLANEOUS LOGIC CONTROL (ML) -- A2U122, A2U123
AC -- A3U315, A3U316
INPUT HYBRID (IH) -- A2UI101 AnswN
8-39. Cross-Isolation Link
In order to make precision floating measurements, the impedance from isolated assembly A2 to earth
ground should be large. Only two electrical paths need to be high impedance for this to occur, the
floating common power supply and the communications path between the A2 and Al MPU's. The
power supply isolation occurs inside the power transformer. The communications link isolation is
achieved through opto-isolators U509 and U510.
Communications between the Al and A2 assemblies is accomplished through a full duplex UART link
on each assembly and the opto-isolators. The link operates at a 187KHz rate. Data is received and
transmitted in || bit frames consisting of one start bit. eight data bits, one even parity bit and one stop
bit.
MPU A2U501 will be RESET when all zeros have been sent over the communications link for a period
greater than 20mS.
8-40. Main Controller (03457-66501)
Refer to schematic Al/1 for the following discussion,
The Main Control Assembly manages the overall operation of the instrument. This includes managing
the measurement process, performing mathematical calculations, storing data, and communicating with
the front panel and with other instruments connected to the interface bus.
The Main Controller consists of microprocessor US01, two ROM's (U5S02 and U503) and two RAM's
(U506 and USI1), The ROM's contain the operating routines for the microprocessor while the RAM's
provide temporary memory. RAM USII contains the calibration and auto calibration constants, the
HP-IB address of the instrument, and instrument configuration states selected by the user.
8-41. Reading instructions from ROM U503.
To read instructions from ROM U503, the microprocessor:
© Sets the read/write (R/W) output (USOL, pin 32) high.
© Sets the address code, of the memory location to be read, onto address lines AO through
Al2.
@ Sets address lines Al3 and Al4 high and sets AIS low to enable ROM U503.
The positive portion of timing signal €+a (U597a, pin 1) and the high r/W signal sets the output of gate
U598d (RD) low to activate the output of ROM U503. The microprocessor then reads the information
from data bus "A", lines DO through D7.
8-42. Reading instructions from ROM U502.
To read instructions from ROM U502, the microprocessor:
HP 3457A Multimeter
8-18