The SPring-8 Compact SASE Source (SCSS) project aims to build an X-ray SASE-FEL. The coherent X-ray generation requests a revolutionary improvement on an accelerator performance. To improve the performance, we developed an A/D and a D/A VME board for high-speed signal control. These are key components for the stable accelerator operations. The boards control the acceleration gradient variation caused by the instability of the klystron output RF power. The A/D board with a down converting mixer and an IQ demodulator was designed for an RF-detector of phase/amplitude signals that are generated with the klystrons. The board is used for the readout of signals from RF beam position monitors as well. An analog-to-digital converter is running at 238MHz clock, which is the sub-harmonic of C-band main acceleration frequency (5712MHz). It has 4Mbytes (2k word * 1k samples) memory for each of four inputs. An effective number of bits (ENOB) and usable dynamic range are better than 10.7bits. In addition, the A/D board has a fault detection capability to compare an observed waveform with a reference waveform stored in the memory. The D/A board is connected to an IQ modulator and an up-converting mixer. It was designed to form the RF modulated phase/amplitude signal to the klystron input. A digital-to-analog converter is running at 238MHz clock, and it has 4MBytes(2k word * 1k samples) memory for each of four outputs. A group delay time is within 0.5nsec, and an ENOB is better than 11bits. The D/A board has 32 taps FIR filters to control a rise time. This paper gives an overview of the low level RF control system, and also describes the function and the performance of the A/D and D/A board.