Theory of Operation Chapter 4
GPIB-100A User Manual 4-2 © National Instruments Corporation
System Controller Detection
PON initializes FFs U22A and U12A to clear the Remote System Controller (RSC) and Local
System Controller (LSC) signals.
When Interface Clear (IFC) is received from the local side via GPIB transceiver U2B, the LSC FF
is set on the leading edge of IFC and after a delay through U21B/C/D/E, IFC is enabled (U35D) to
the remote unit as XIFC through driver U29A. LSC enables the local unit to transmit Remote
Enable (REN) to the remote unit through driver U8A.
XIFC becomes RIFC on the remote side and is received through U10D and propagated to the
remote GPIB through transceiver U2B, where the bus signal is received back to clock the RSC FF
U22A. RSC enables REN (U32C) to be driven on the remote GPIB through transceiver U2D.
Active Controller Detection
The Remote Active Controller (RAC) and Local Active Controller (LAC) FFs U22B and U12B
remain cleared until either RSC or LSC is set via U41B–that is, until the System Controller has
been located. After a short delay (U21A/F and U31B/F), the Attention (ATN) receiver on the local
side (U32D) is enabled. This delay allows the LAC FF to be set if ATN is already asserted when
IFC occurs.
When ATN is received from the local side via transceiver U2C, the LAC FF is set on the leading
edge of ATN and, after a delay through U11B/C/D/E, ATN is enabled (U32A and U43D) to the
remote unit as XATN through driver U29B. LAC enables the local unit to receive Service Request
(SRQ) from the remote unit through receiver U10B, U32B, and transceiver U2A.
XATN becomes RATN on the remote side and is received through U20B and propagated to the
remote GPIB through transceiver U2C, where the bus signal is received to clock RAC FF U22B.
When RAC is set, drivers U8C/D, which transmit SRQ and parallel poll handshake signal BUS
PP to the local unit, are enabled (that is, toward the Active Controller).
Source Handshake Detection
The Local Source (LS) handshake FF U45A is cleared via U33C on the following events:
• Before the Active Controller is identified (TLC mode only)
• Whenever a change in the state of the local ATN signal is caused by a pulse created via U38D,
U24A/D, and associated RC network.
• While ATN or Data Valid (DAV) is received from the remote unit (U34B).
• During a parallel poll (U46C).
The Remote Source (RS) handshake FF U36B is cleared via U33B on the following events:
• Before the Active Controller is identified (TLC mode (U33A) only).