diff --git a/ModbusRTU.d.ts b/ModbusRTU.d.ts
index fd445e3..45aec7b 100644
--- a/ModbusRTU.d.ts
+++ b/ModbusRTU.d.ts
@@ -8,9 +8,9 @@ export class ModbusRTU {
 
   static getPorts(): Promise<PortInfo[]>
 
-  open(callback: Function): void;
-  close(callback: Function): void;
-  destroy(callback: Function): void;
+  open(callback?: Function): void;
+  close(callback?: Function): void;
+  destroy(callback?: Function): void;
 
   writeFC1(address: number, dataAddress: number, length: number, next: NodeStyleCallback<ReadCoilResult>): void;
   writeFC2(address: number, dataAddress: number, length: number, next: NodeStyleCallback<ReadCoilResult>): void;
@@ -161,4 +161,4 @@ export interface TelnetPortOptions {
 
 export interface C701PortOptions {
   port?: number;
-}
\ No newline at end of file
+}