The newer IEEE-488 chips emulate either the uPD7210 or the TMS9914 or both,
so their docs might have the info you want. You should be able to find the
following online:
National Instruments TNT4882 Programmer Reference Manual 320724-01
National Instruments NAT7210BPD datasheet
National Instruments NAT9914 Reference Manual (9914 but with 7210 mode)
ines ieee488.2 iGPIB Hardware Manual
Computerboards CB7210.2 data sheet
Also, the source for Linux/NetBSD/FreeBSD IEEE-488 drivers might be helpful.
-- Adam