SRS Labs SR530, Lock-In Amplifier Serial Polls and Service Requests, GPIB with RS232 Echo Mode

DC1	Device Clear capability
RL0	REN,LLO, GTL not implemented.
	'I' command sets Remote-Local.

SR530 Response to GPIB Commands

Mnemonic	Command	Response
DCL	Device Clear	Same as Z command
SDC	Selected	Same as Z command
	Device Clear
SPE	Serial Poll	Send Status Byte,
	Enable	& clear status byte

Because the SR530 can be controlled by an RS232 interface as well as the GPIB, the remote- local functions are not standard. There is no local with lock out state. When in the local state, remote commands are processed, even without the REN command being issued. This is because the RS232 interface has no provision for bus commands and remote commands over the RS232 interface would never be enabled.

Serial Polls and Service Requests

The status byte sent by the SR530 when it is serial polled is the same status byte which is read using the Y command (except for bit 6, SRQ). Ofcourse, when the SR530 is serial polled, it does not encode the status byte as a decimal number. The SR530 can be programmed to generate a service request (SRQ) to the GPIB controller every time a given status condition occurs. This is done using the V{n} command. The mask byte, n (0-255), is the SRQ mask byte. The mask byte is always logically ANDED with the status byte. If the result is non-zero, the SR530 generates an SRQ and leaves the status byte unchanged until the controller performs a serial poll to determine the cause of the service request. When the unit has been serial polled, the status byte is reset to reflect all of the status conditions which have occurred since the SRQ was generated.

For example, if we want to generate an SRQ whenever there is an overload or unlock condition, we need an SRQ mask byte equal to 00011000 binary, or 24 decimal ("V24" command). The byte 00011000 binary corresponds to the status byte

with the 'no reference' and 'unlock' status bits set. If an overload occurs, then an SRQ will be generated. The serial poll will return a status byte showing SRQ and overload. If an unlock condition occurs before the serial poll is concluded, another SRQ will be generated as soon as the serial poll is finished. A second serial poll will reflect the unlock condition.

Any SRQ generated by the 'no reference, 'unlock', 'overload', and 'auto over-range' conditions will also reset the corresponding bit in the SRQ mask byte. This is to prevent a constant error condition (such as no reference applied to the input) from continually interrupting the controller. When such an SRQ occurs, the controller should change

some parameter so as to solve the problem, and then re-enable the SRQ mask bit again using the V command.

GPIB with RS232 Echo Mode

It is sometimes useful when debugging a GPIB system to have some way of monitoring exactly what is going back and forth over the bus. The SR530 has the capability to echo all characters sent and received over the GPIB to its RS232 port. This mode of operation is enabled by setting switch 6 of SW1 to the DOWN position. The baud rate, stop bits, and parity of the RS232 port are still set by SW2. Of course, the RS232 port operates at much lower speeds than the GPIB and will slow down the GPIB data rate in this mode. (Use the W0 command to allow the RS232 interface to run at full speed, otherwise, the GPIB transactions may take so long that the controller can hang.) During actual use, this mode should be disabled.

The SR530 with BOTH Interfaces

If both interfaces are connected, commands may be received from either interface. Responses are always sent to the source of the request (except in GPIB echo mode). It is unwise to send commands from the two interfaces at the same time since the characters from different sources can become interleaved on the command queue and result in 'unrecognized command' errors.

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