理论描述：

　　Serial polling is a method of obtaining specific information from GPIB devices when they request service. When you conduct a serial poll, the Controller queries each device, searching for the one that asserted the SRQ line. Each device responds to the poll by returning the value of the Status Byte contained in its Status Byte Register (see Figure 1). Device-dependent conditions, such as the presence of available data or an error condition, determine this value. ANSI/IEEE Standard 488.1-1987 specifies that one bit in the Status Byte, the RQS bit (bit 6), be TRUE if the device requested service. The other bits in the Status Byte are left to the instrument manufacturer to define. IEEE 488.1-compatible instruments may have bits that determine if an instrument error has occurred or if the device is conducting a self-test. Because these bit definitions are not consistent among instrument vendors, the method for determining the cause of a service request varies with each device.

Figure 1. Status Byte Register

总结

　　1.serial poll response byte 是由函数"IBRSP"（或ReadStatusByte）获得，不可能通过其他函数（ibsta，此函数返回的是status word）。

　　2.serial poll response byte 是存储在Status Byte Register（一个字节）里面的，不是存储在Status Word（16字节）里面的。

　　3.If Bit 6 (0x40) is set (the RQS bit—bit 6 in the device‘s Status Byte Register—is TRUE),then the device requested service.

　　4.If Bit 6 (0x40) is set, then the SRQ line is asserted to indicate to the Controller that this device is requesting service.

　　5.有两个RQS（一个在Status Word里面定义，一个在serial poll response byte里面定义）。RQS is set in the Status Word whenever the hex 40 bit isasserted in the serial poll response byte of device。

Serial Poll Response Byte