running rough

[1987mb]

hello , i am new to this forum actually i never joined any forums , but i recently purchased a 1987 mercedes benz 260e and i am having some issues that i would like to get resolved. I am currently seeking any recommendations where i might be able to a diagnostic code reader to address a rough idle problem, this particular car does not have the analog port near the battery side of the engine , but it does have a plug in circular port on the opposite the engine on the drivers side, if anybody has any suggestion or guidance it would be greatly appreciated