Spellman 225 Series, Instruction Manual, Page: 10 / 20

225 Series User’s Manual
5
118120-001-Rev. A
A1 GPIB Address Bit
A0 GPIB Address Bit
3.3 GPIB PROTOCOL
The 225 Series implements the following GPIB
functions:
SH1 Source Handshake
AH1 Acceptor Handshake
T5 Basic Talker & Serial Poll L4
Basic Listener
RL1 Remote/Local with Lockout
PPO No parallel poll response
CO Not a controller
SR1 Service Request
DC1 Device Clear
DT1 Device Trigger
Using these GPIB functions, the GPIB controller can
send ASCII command strings to control and monitor the
high voltage power supply.
3.3.1 DATA TRANSFER FUNCTIONS – SH1, T5,
CO, AH1, PPO
These seven data transfer functions are normally
transparent to the user. They permit communication
between the GPIB controller and the 225 Series high
voltage power supplies.
3.3.2 SERVICE REQUEST FUNCTIONS – SR1
SR1 contains two sub-functions: Service Request and
Serial Poll. The service request (SRQ) is the mechanism
by which a power supply can interrupt the GPIB
controller and the serial poll (SPL) is how the GPIB
controller can read the status of the power supply.
3.3.2.1 SERVICE REQUEST (SRQ)
There are three conditions that can make a power supply
issue an SRQ. They are 1) at power-on, 2) in response to
an overload detection and 3) in response to receipt of an
invalid command. The meaning of the serial poll status
byte depends upon what caused the service request. The
power-on byte is true when the power supply is first
turned on AND the GPIB controller has not yet sent a
valid command to the unit. The “normal operation” byte
is true after the GPIB controller has sent its first valid
command to the unit. Any service request can be ignored
without affecting operation of the GPIB interface or of the
power supply.
3.3.2.2 SERIAL POLL (SPL)
When the GPIB controller executes a serial poll, the
power supply sends it one status byte. As mentioned
above, decoding of that byte depends on when it is read.
The figures below show the decoding of the “power-on”
status byte and “normal operation” (not the “power-on”)
status byte.
Figure 3.2
At power-on before any command is sent, the
status byte is:
If SRQ = 1, pole A5 of the GPIB address was set for
“SRQ” at “power-on”.
Bit #7 is at logic 1. That shows this is the status register
contents at power-on, prior to the issuance of a valid
command. If you don’t do a serial poll at power-on, but
simply send any valid command, the status byte obtained
in response to subsequent serial polls will instead be as
seen in Figure 3.3 below.
Bit #5 and Bit# 6 function identically as in the normal
operation status byte of FIGURE 3.3 below:
Figure 3.3
After the first valid command, the status byte
looks like this: