Error 10327: Can't determine definition of operator ""-""

Hi!

I'm getting this error all the time... and I don't know how to solve this because i'm still a rookie e this thing. Can anybody help me with that and tell me what i'm doing wrong?

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Error 10327: Can't determine definition of operator ""-""

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Code:

LIBRARY IEEE;

USE IEEE.STD_LOGIC_1164.all;

USE IEEE.STD_LOGIC_ARITH.all;

USE IEEE.STD_LOGIC_SIGNED.all;

ENTITY Rec_dados IS

PORT( RXD : IN STD_LOGIC;

clock_115KHz : IN STD_LOGIC;

data_ready : OUT STD_LOGIC;

accx_out : OUT STD_LOGIC_VECTOR(15 DOWNTO 0);

accy_out : OUT STD_LOGIC_VECTOR(15 DOWNTO 0);

accz_out : OUT STD_LOGIC_VECTOR(15 DOWNTO 0));

END Rec_dados;

ARCHITECTURE Behavior OF Rec_dados IS

SIGNAL count : STD_LOGIC_VECTOR(6 DOWNTO 0);

SIGNAL accx_out_int, accy_out_int, accz_out_int : STD_LOGIC_VECTOR(15 DOWNTO 0);

BEGIN

PROCESS

BEGIN

WAIT UNTIL clock_115KHz'EVENT and clock_115KHz = '1';

IF count < 32 THEN

count <= count + 1;

END IF;

IF count > 31 AND count < 48 THEN

accx_out_int(47 - count) <= RXD;

count <= count + 1;

END IF;

IF count > 47 AND count < 64 THEN

accy_out_int(63 - count) <= RXD;

count <= count + 1;

END IF;

IF count > 63 AND count < 80 THEN

accz_out_int(79 - count) <= RXD;

count <= count + 1;

END IF;

IF count > 79 THEN

accx_out <= accx_out_int;

accy_out <= accy_out_int;

accz_out <= accz_out_int;

data_ready <= '1';

END IF;

END PROCESS;

END Behavior;

Thanks everybody!